Copyright (c) 2004 Cloudy Nights Telescope Reviews
100mm Binoculars What Can You See
by Ed Zarenski Not all binoculars are created equal. There are wide-field binoculars and there are high power binoculars. Some have fine sharpness of image and some not so sharp. Some show slight flares in the image, some show a little astigmatism, some show a slightly bloated image in one or both sides of the binocular. Some are more difficult than others to bring to a fine pinpoint focus. Even with some of these deficiencies present in various of the binoculars used in this comparison, these results, achieved over a long period of months of use, will show some very good performance can be achieved. Some have better coatings and contrast and certainly all are not equal in mechanical functions or quality of components. But, what is most noticed is this; regardless of what slight imperfections appear in the image, there is a dramatic increase in what can be seen with larger binoculars. There is no question a higher quality binocular will step up the ladder when compared to its closest companion sizes. But that can take you only so far. This does not mean to indicate the largest binoculars are the best binoculars. Nor is it intended to mean the best binocular in the middle sizes is the equal of the largest. In many ways, largest may not always be best, and this can sometimes be experienced when attempting wide field view observations that might require 4 to 6 just to encompass the whole view. This type of wide-field view cannot be obtained with a 2.5 Tfov 22x100mm or 25x100mm binocular.
While this report discusses some of the relevant aspects of binoculars, such as quality of coatings and size of field of view, mostly it concentrates on a comparison of the visual capabilities of the optics of several 100mm binoculars as compared to each other and as compared to several other well-known models of other sizes. Hopefully, this will provide the reader with useful information about the performance that can be expected from some of these 100mm binoculars and how that might compare to some other select binoculars. Binoculars included in this report: Primary comparison is the Celestron Skymaster 25x100 and Oberwerk Giant 22x100. Also used for many comparisons are Oberwerk BT100 and Oberwerk Standard 20x80. Among other binoculars used for performance indications are references to Fujinon FMTSX 16x70, Oberwerk 15x70 and Nikon SE 12x50. Clear Skies, and if not CloudyNights Edz Oct 2004
Oberwerk Giant 22x100 vs Celestron Skymaster 25x100 As far as the ability to see, 22x100s and 25x100s are a step above every 20x80 or smaller binoculars I've ever used. Literally, they see more. They see fainter objects and image scale is larger. The Pleiades fills the field of view with over 200 stars. Fainter deep sky objects are seen and closer doubles are split. Objects like the double cluster are simply stunning. But don't expect they are performing to the full potential of a 100mm binocular. They are not. I have highlighted some test data that will show quality smaller binoculars perform to a higher level relative to their size. While the Celestron cannot reach a fine pinpoint image, the Oberwerk has an optical alignment issue and both of these have some chromatic aberration, still these two 100mm binoculars see more than either my 20x80 or my 16x70 binoculars. The Celestron, a waterproof sealed binocular, seemed to always show fainter extended objects easier. The Oberwerk have a slightly wider field of view and seemed to have better sharpness across the field and better onaxis resolution. For an equal size 100mm comparison, if you were able to spend one night out doing side-by-sides with a higher quality binocular like the Oberwerk BT100 you would easily observe some of the higher performance abilities the BT provides. But then, the Oberwerk 22x100 at $395 and the Celestron 25x100 at $349 cost less than the Fujinon 16x70 and only about 25% of the BT100.
Oberwerk Standard 20x80 Compared to my previous 20x80s, the Oberwerk 20x80 Standards have a far sharper image much further out from center, have very little CA, see deeper in magnitude, have excellent coatings that show almost no reflections, and see fainter diffuse objects. The 20x80 Standards weigh less, have much better eye relief than most binocs and for me the
right diopter adjustment has plenty of room for adjustment. The 20x80s have a sharper field of view than both of these 100mm binoculars above. The sample of 20x80 Standards that I used had a spiked flare around bright stars. This was not present in some other samples. The spikes were not seen on stars mag 5-6 or fainter. Even with the spikes present in the image on bright stars, these are the results I achieved. Other than that, these were the best 20x80s I've ever used. However, I still see significantly more with Oberwerk 22x100s and even more again with Celestron 25x100s. Coatings were very good in both 100s, but I think the 20x80s coatings were better.

Oberwerk BT100 Binocular Telescope The BT100 is a different animal altogether than any other 5# to 12# binocular in the 80mm to 100mm range. The BT100, at 26# (12kg), is heavier than my largest telescope, a 1200mm f8 6" refractor. The BT100 cannot be picked up while mounted on its tripod. You must first set out the custom tripod then attach the BT100s. You would not move it while mounted. You must first take the binocs off the tripod. The BT100 is a straight thru viewing binocular. The standard tripod, a specially modified heavy-duty wood surveyor's tripod results in the need to sit in a chair under the binocs to view at any altitude. Actually, Ive found for most altitudes this can be very comfortable. Even though this BT100 shows a bit of astigmatism in one barrel, the views are simply stunning. It comes standard with two sets of WA eyepieces, a 24.5mm = 2.5 Tfov at 25x and a 10mm = 1 Tfov at 62x. My favorite eyepieces in the BT100 so far are 26mm and 20mm Televue plossls.
Mounting Considerations for 100mm Binoculars If you've never seen or held or looked thru a pair of 100mm binoculars like the Oberwerk 22x100 or the Celestron 25x100, when you first do get the chance, you will find these are dramatically different than anything else smaller you've used. These are big, real big. And they show it. They require a substantial mount, not just any tripod but a heavy-duty tripod with a heavy-duty head. These binocs weigh about 9-10#. Most light or basic parallelogram mounts groan under the weight. The best mount I have used for both of these binoculars is a sturdy Bogen 3246 tripod with a heavy duty Bogen 501 head. That combination setup will easily hold either of these 100mm binoculars. I use it to hold all of my center post design binocs 8# -10#. Purchased based on a CN recommendation, its the heaviest duty tripod I own. It has smooth fluid motion and the elevator crank is great. Its pretty tall, not as tall as the Bogen 3211/3130 (my 3246 is an older model, the newer model is taller). With 22x100s mounted to the 3246/501 combo, elevator shaft fully extended and with binoculars pointed at an altitude of 60, the eyepieces are 68 above the ground. I can stand comfortably behind the eyepieces to view. I'm 6'1" and maybe need to scrunch down 3
4 inches when looking at zenith. The 3211/3130 legs/head combination is not up to the load of these big 100mm binoculars. The 3211/3130 is about $150 value. The 3246/501 is about $350. The mounting post on the Celestron 25x100 is a polished chrome post without base plate. I found it difficult to keep it tightened against tripod mounting plate. In addition it deforms the rubber on the tripod mount plate. If at least the bottom of this post was roughened and not polished, the contact surface would hold better. The Oberwerk 22x100 has a black metal base plate which screws onto the bottom of the mount post. This makes a 1.5" diameter bottom contact area with the tripod mounting plate. It spreads out the area of connection to the tripod plate, eliminates the punching deformation of the rubber on the quick release shoe and it never once got loose. You could get away with mounting the 20x80 Standards on a $150 tripod setup. The 20x80s mount with a standard binocular L bracket. The Bogen 3211/3130 combination works fine. They are easily within the limits of light or basic parallelogram mounts. I've had my 20x80s mounted on a Virgo Skymount parallelogram and a UA Unimount Light Basic. All three of these setups handled this 20x80 binocular and a Fujinon 16x70 with ease. The BT100 mounts to a custom tripod mount. Any consideration to use a different mount for these would require expenditure in the range of $500 for a suitable mount. Plan on a range of $300-$350 for a suitable tripod/head mount for the 22x100s or 25x100s. If you are looking for a parallelogram mount, its probably best to direct your search to something similar to the heavy-duty models such as the UA Unimount ($429) mounted to a heavy-duty tripod or a surveyors tripod (~$150).

Inter-Pupilary Distance & Eye Relief The Celestron 25x100 has an inter-pupilary distance (IPD) range from 56.5mm to 74mm. Thats quite a wide range. Not too many binoculars get down smaller than 57mm. Eye relief is measured at 15mm, but there is a 6mm recess to the lens, so effective usable eye relief is only 9mm. Thats very short. When used with my glasses on, I have to look around a little to see the very edges of the field. This deeply recessed eye lens is very similar to that found on the Fujinon 16x70, one that also requires I look around to see the entire field of view inside. With these two binoculars, looking straight into the eyepiece while wearing glasses cuts off about 10-15% of the field of view, but tilting the head slightly allows seeing to either edge. The Oberwerk 22x100 has an IPD that goes down to 60mm+/- to 70+. Eye relief measures 12mm and with the rubber eyecups removed 11mm is usable. These binoculars have a shaped eyecup that snaps on over the eyepieces. It cannot be used with glasses.
Ive cut some cheap pliable rubber eye guards and put them over the eyepiece housing so I can use these with my glasses. I can see the entire field of view this way. Since the snap-on eye guards have a plug that acts as the eye lens cap, without those snap-ons in place I have no eye lens caps. The Oberwerk 20x80 Standard has 17mm eye relief, a very comfortable binocular to use with eyecups up or down. The IPD has a range from 59mm to 72mm. The entire field is visible with or without glasses. The BT100 eye relief is dependant on the eyepieces you select to use for your viewing. It uses any 1.25 format eyepieces. The WA eyepieces supplied have enough eye relief to use glasses but in the low powered eyepieces field cutoff is slight. The BT100 has nonrotating helical focusers that adjust the IPD from 58mm to 80mm, and can reach 58mm even with fairly wide-bodied eyepieces.
Collimation and Exit Pupils This pair of Celestron 25x100s is out of collimation by 1 arc minute. That doesnt seem like much but at 25x, some may find it annoying. Most of the time I dont see it, but I had a difficult time focusing on Jupiter and I thought this might be part of the problem. There are probably prism screws under the leatherette vinyl covering, but I did not cut holes in the cover too look for the screws. The Oberwerk 22x100 arrived slightly out of collimation. The error was only 2 arcminutes. Again, Ill say this is a small amount, but I am highly sensitive to this. The prism adjustment screws are readily accessible on the outside of the 22x100 body. This was an easy adjustment and took only about a minute to merge the images. The exit pupils in this Oberwerk 22x100 are not round, they are cats eyes. That tells me there is something askew in the optical alignment. However, I was not about to undertake any attempt to set this right. Attempting to reset exit pupils to a fully round appearance will knock the entire optical path out of conditional alignment. I know because Ive tried this before. It then becomes an even greater task to get everything back in alignment and get the exit pupils as close to round as possible. Considering that the exit pupils are not round, there is light loss. The percent loss is not an easy calculation as it would be necessary to take into consideration the percent illumination within the portion of the exit pupil that is not seen. Suffice it to say, since the outer edges of the exit pupil are not fully illuminated, then the percent light loss may be less than the percent of the area of the pupil lost. The Oberwerk Standards are nearly perfectly collimated and never moved throughout use. The BT100 is slightly out of collimation, more noticeable at high magnifications but not at 25x to 36x.

The Nikon SE, Fujinon FMT-SX, Oberwerk BT100 and Oberwerk Standard reflect less than the Celestron 25x100 and the Oberwerk 22x100. These two have objective coatings probably closer to the Oberwerk 15x70/03. Keep in mind the Celestron appear to have single coated prisms and eye lens. All these binoculars reflect a slightly different color back to you, varying from a purplishblue to blue/green/purple to yellowish-green to just green and plain blue. If coatings are applied to the proper thickness, color is not an indication of the quality, only a result of the chemical composition of the coatings. However multicoatings that are applied too thick can appear green. Blue reflections probably indicate single coated MgF surfaces. White reflections indicate a lack of coating. The more light you can see reflected in the coatings, the less light gets through the lens. What you want is coatings that reflect the least amount of light.
Field of View In the 22x100 to 25x100 range, binocs actually measure 2.8 down to less than 2.5. None that I know of reach 3.0. I have not found the need to use any sort of finder on any binocular with a filed of view down to about 2. Viewing thru the binocs seems easier to me than viewing thru a finder. FOV is not as much an issue for me when I jump to higher powered binocular viewing in the 20x to 25x range. Sure the fov gets smaller, but the image scale gets a lot bigger and objects are easier to see. Since I don't seem to have much problem finding things, maybe that's what makes it a non-issue for me. The Celestron Skymaster 25x100 binocular is advertised as 3.0. It measures 2.45. The Oberwerk 22x100 is advertised as 2.8. It actually measures Tfov at 2.7. They are still stamped on the prism housing as 3.5, an artifact I presume left over from the fact that these binoculars use the older 20x80 Deluxe housing. Both are still larger Tfov than the Pentax 20x60 (2.2). The BT100 is an f6.2 binocular. It comes standard with two sets of wide WA eyepieces, a 24.5mm = 2.5 Tfov at 25x and a 10mm = 1 Tfov at 62x. Both eyepieces have an apparent fov of 62. I use various plossl eyepieces that give a somewhat narrower field of view. 26mm TV plossls gives 24x with a 2.2 Tfov. A recent addition of a pair of Meade 13.8mm SWAs with about 65 Afov, gives 45x with a 1.44 field of view. The Oberwerk 20x80 Standard is advertised as 3.5. It measures 3.2. The Fujinon FMT-SX 16x70 is advertised as 4.0 and measures 4.05 The Oberwerk 15x70 is advertised as 4.3. It measures 4.3 Handheld, I find my favorite Tfov is about 5-6. Mounted, my favorites for field of view

are Oberwerk 15x70s at 4.3 and Fujinon 16x70s at 4.0.
Sharpness Across the Field of View We know what the edge sharpness test seems to show. Not surprising, but some of the best binoculars show significantly better sharpness towards the outer edges of the field of view. The Fujinon FMT-SX 16x70, the Nikon SE 12x50, Pentax PCF WP 16x60 and the Oberwerk Standard 20x80 display excellent sharpness across the field. The Oberwerk BT100 at f6.2, with 20mm TV plossls at 31x is sharp right up to the very edges of the field stop. You can put a 7" double right up to the edge of the field stop and still observe the two components cleanly separated. A binocular with a sharper image still sees fainter stars further out in the view. One with a less sharp image spreads the star images out over such a large area that it has already lost the faint stars from view by the time the sharpness has become only fair and has lost many more than just the faint stars at the point where sharpness is poor. In the Celestron 25x100 sharpness of image is still good at 70% out from center, but then it drops off pretty quickly beyond 70%. By 80% out it gets rather poor. The Celestron has a problem with one side of the binoc that won't allow it to reach pinpoint focus as well as the other side. The Celestron 25x100, when observing a 14" double, was very clear at 50%, still good at 60% but poor at 70%, all the way around. Doubles of 21 and 22 could still be seen at 70-75% out from center. A 29 double was the limit at 80% out. In my sample of the Oberwerk 22x100, an unusual result is found. The best image sharpness is not centered in the lens. In order to see it, sharpness needs to be tested across the lens at various hour lines on a clock. Then it becomes obvious. Along one hour line, sharpness is OK to 70%, poor by 80%. But on the opposite side, it's sharp to only 40-50% and poor by 60%. Testing other hour lines shows the sharp central point is off-axis. The other side of the binocular is mirror image. That tells me the optical axis of these binocular barrels is out of alignment in these 22x100s. The images are merged, but the optical axis of the two barrels does not center the optimum light in the middle of the exit pupil. In the Oberwerk 22x100 the 14 double, on one side was good to 60-70% and poor at 80, but on the opposite side of the same barrel was good to only 40% out and poor by 5060% out. The 21 double was seen out to 70%. Best with a 22 double was 70-75% out, less on the poor side. The 29 double could be seen at 80% out. In the Pentax 16x60, the same 22 double can be seen as two components within 10% of the edge.

In the Fujinon 16x70, Mizar, at 14.4, can be seen at 60% out from center and a 22 double can be seen at 80% out. A faint mag 7.0-7.2 double at 16.7 can be seen at 50% out. Another faint double Struve485 in NGC1502 7.0-7.1 at 18 is seen out to 65-70%. The BD components of the Trapezium, 19.2 are still seen at 70% out from center. A 36 double can be seen out within 10% of the edge. The Nikon 12x50 SE can see 14.2 out to 50% and can see 22 at 70% out handheld. Faint mag7 Struve2690 at 16.7 is seen out to 45%. Faint mag 7 Struve485 at 18 is out to 60-65%. Alberio at 34 is still seen at 80% out. In the Oberwerk 20x80 Standards, the 14 double is resolved perfectly at 60% out from center and can be seen out to nearly 70%. The 22 double can be seen to 80%. In the BT100 with a 26mm TV plossl at 24x, the 14 double can be put right on the edge of the field stop and still be seen as two. At 25x with the standard 24.5mm wide-angle eyepiece supplied with the BT100, a 7.4 double can be seen out to 60-70% before it becomes distorted. This method of checking doubles with various binoculars at points along the radius allows us to determine the loss of sharpness by referring to the apparent resolution at the points measured. For this, the magnification comes into play. The separation of the double multiplied by the magnification gives apparent separation as seen at that magnification thru that binocular. It should be easily understood even a fairly close double will get easier with higher magnification. However, a considerable degree of sharpness is still required to see close doubles split. Therefore, various doubles are used to test sharpness and a lower apparent separation # is better. Based on the above, the sharpness as measured by apparent resolution (in arcseconds) of various binoculars is given. When comparing one binocular to another, at any given % out from center, lower is better. Oberwerk BT100-26TV, 170 at 50-60%, 330 at 98% out from center. Oberwerk BT100-24.5 WA, 190 at 60-70% out from center. Oberwerk BT100-20pl and 17pl, faint mag 8 double, 205 on-axis, 240 at 50%. Celestron 25x100, 350 at 60-65%, 530 at 70%, 550 at 75% and 725 at 80% from center. Oberwerk 22x100A, 310 at 60-65%, 460 at 70%, 480 at 75%, 640 at 80% from center. Oberwerk 22x100B, 310 at 40-50%, 480 at 50-55% and 660 at 60% out from center. Oberwerk 20x80, 280 at 70% and 440 at 80% out from center. Fujinon 16x70, 230 at 60%, 310 at 70% and 350 at 80% out from center. Fujinon 16x70, faint mag 7 doubles, 270 at 50%, 290 at 70%. Oberwerk 15x70, 210 at 50% and 330 at 60% out from center. Oberwerk 15x70, faint mag 7 double, 270 at 40-45% out from center. Pentax 16x60, 350 at 80-90% out from center.

Nikon 12x50 SE, 170 at 50%, braced 260 at 70% Nikon 12x50 SE, faint mag 7 doubles, 200 at 45% and 220 at 60-65% out from center.
List of Stars for Testing Sharpness and Resolution You can try observing some of these doubles with your binoculars to find out how yours compare to some of those listed here with on-axis resolution and field sharpness. 95 Her, 5.0-5.1/6.3, 5 ssw of 100Her, split with BT100 at 25x Stf 232 Tri, 7.9-7.8/6.6, 2h15m +31n, near 6 Tri, split at 31x in a BT, faint, difficult. Stf 953 Mon, 7.2-7.7/7.1, 6h40m +9n, 1 s of o.c. 2264, elong. with 20x80 and 16xMon, beta, 4.7-5.2/7.3, C=6.1/10, BC=2.8, seen as A-BC = 4.7-4.8/7.3 Stf 1121 Pup, 7.0-7.5/7.4, 7h38m 24s, easily separated at 25x, in o.c. M47, 5 Ari, Mesartim, 4.8-4.8/7.8" PA 000, split at 22x100, better at 31x in BT 41 Ori, theta 1, Trapezium ABCD, A-B= 8.7, A-C=12.9, C-D=13.3, B-D=19.Del, gamma 4.5-5.5/9.6", elongated pointed in 12x50 SE, split in 16x70 Stf 1245 Cnc, 6.0-7.2/10.3, 8h25m +7d30m, just north of Hydras head Stf 954 Mon, 7.1-9.6/12.8, 6h40m +9.5n, 1/2 n of Stf 953, split in 25xBoo, kappa, 4.6-6.6/13.4, 2.0 mag difference this close makes it difficult Stf 2470, Lyra 6.6-8.6/13.4", 19h10m +34.6n, seen in 16xHerc, 5.9-6.0/14.2", nearly even mag 6 double, easily resolved even in 12x50. 79 UMa, Mizar, 2.3-4.0/14.4", difficult due to bright primary Stf 2474, Lyra 6.7-8.8/16.2", 19h10m +34.5n, cleanly separated with 15x70 Stf 2690 Del, 7.0-7.2/16.7", 20h35m +11.5n, fainter but easy, clearly separated in 12x50. Stf 485 Cam, 7.0-7.1/18, 4h05m +61n, easy, in o.c. 1502, also faint but easy in 12x50. 61Oph, 6.0-6.5/21 easily resolved at 15x 63 Ser, Alya, 4.5-5.0/22", easily resolved, seen as two 70% out in 12x50 SE. 7 Her, kappa, 5.3-6.5/28.4, near triangle of Serpens. Stf 855, Ori, 6.0-7.0/29 and C=8.9/118, 6h15m +2.5n, about 4 sw of 8 Mon. 48 Cnc, iota, 4.2-6.6/31, beautiful colored double 6 Cyg, Alberio 3.1-5.1/34", seen at 80% out from center in 12x50,color shows well. 16 Cyg, 6.0-6.1/39, easy even double within 1 of 6826, Blinking Planetary. 24 Dra, nu, 4.9-4.9/62", easily visibly separated.
Double Star Observations On-Axis Resolution How close a double you can split with a binocular is a good measure of that binocular resolution. Of course, binoculars wont allow reaching the resolution limits of the aperture due to the low magnification, so its as much a measure of your own acuity, the ability of your eyes to separate close objects. But, if the same observer performs the same type of observation thru all the instruments then you have a relative measurement. It may vary slightly for someone else with a different acuity, but they would get similar relative measurements of resolution with the same group of instruments.
Some binoculars focus to a finer pinpoint than others. And obviously, magnification provides a greater ability to split doubles. Generally, binoculars with higher magnification allow seeing closer doubles, but sometimes the binocular with a finer image will exceed the limits of one with a higher magnification. What this also means is you get to see a lot more resolution in dense clusters because binoculars with the ability to see that closer double will separate more stars in a cluster. This allows you to define that word resolved. With the Oberwerk 22x100, stars like Mizar resolve easily. Regular difficult double star targets become regularly viewed objects. Doubles like Gamma Delphinus at 9.6" are resolved. Challenges are doubles in the 7" and 8" range. Mesartim at 7.8", although not always split, became a favorite test. In Celestron 25x100s, 7.1" and 7.4" doubles are clearly separated. Mesartim at 7.8" is a successful target. Gamma Delphinus at 9.6" becomes an easy target. The Trapezium, with closest components at 8.7, provides none of the difficulty associated with 20x80s. I clearly separated the double Struve 953 in Mon at 7.2-7.7/7.1, just below the Christmas Tree cluster. I split this one "easily" in the BT100. I've seen it elongated but not split with the 20x80 Standards. Doubles, when resolved in the 16x70s, were more sharply defined than any other binocular, smaller or larger. But the higher powered 22x and 25x binoculars, even though they do not resolve to as fine a point, are capable of so much closer separations, they get to see closer doubles, which also means they are seeing more stars resolved in dense clusters. I woke up early one morning and Aries was high in the southern sky, so I gave Mesartim a try. These Celestron 25x100 do not reach a perfect pinpoint focus on one side and that hampered the view somewhat. I was easily able to see the two components, but they were not clearly separated. The position angle was evident and there were two distinct bright components, but I could not see any solid space between them. I would say they were deeply notched. The apparent separation of Mesartim with the Celestron is 7.8x25= 195 arcseconds. They should have been able to split to this apparent separation easily. The Fujinon 16x70 shows clean splits down to apparent separations under 160 arcseconds. I did see a fainter mag 7 double at 7.1 split with the 25x100s for an apparent separation of 178 arcseconds. So, why was I able to see 7.1 split, but not see 7.8 split in the 25x100s. Maybe because Mesartim, 7.8 has components both of magnitude 4.8, pretty bright. Struve 953, the 7.1 double has components of mags 7.2 and 7.7, quite a bit fainter. The fainter double causes much less brilliance in the image and the finer points are much easier to see. Once stars get much fainter than that it begins to get more difficult due to lack of light. I tried Mesartim again, this time with the Oberwerk 22x100. This time I saw a clean split, difficult, but clean. I saw two nice little points with the tiniest of black space between them. So, for me the best I could do with the 22x100s was 7.8 for an apparent

separation of 7.8x22 = 172 arcseconds. With the 25x100s I got 7.1 for an apparent separation of 7.1x25 = 178 arcseconds. Eventually, I did split Mesartim with the 25x100s. I tried Mesartim with the 20x80 Oberwerk Standards, but these also have a problem bringing things to a pinpoint focus. With the 20x80s, I could not focus out the flared rays of the two stars and this caused too much interference in the image. I was just able to see the elongation of the pair. It would be pushing it, but I think a better sample of the 20x80s might be able to do it. With Oberwerk BT100s, along with Mesartim at 7.8, other successfully split doubles were Struve 953 Mon 7.2-7.7/7.1 at 24x = 170 arcseconds, Struve 232 Tri 8.0-8.0/6.6 at 31x for 204 arcseconds, and 95 Herc 5.0-5.1/6.3 at 25x = 158 arcseconds. I have no eyepiece combinations for the BT between 31x and 25x. With the Fujinon 16x70, on numerous occasions I have seen Gamma Delphinus 9.6 split at 16x for an apparent separation of 154 arcseconds. To date, I believe this is the best resolution I have recorded with any binocular. With the Nikon 12x50 SE, the best Ive tried was 100 Herc at 14.2 x 12 = 170 arcseconds. Im expecting much better out of this binocular and Ive picked some targets from the list above for additional testing. I tried Struve2470 in Lyra, but with a difference of 2.0 magnitudes between components, this is a very difficult double. Best chances would be a more even double at about 13, perhaps the A-C components of the Trapezium.
Limiting Magnitude Limiting magnitude gives you an indication of how deep you will see into the many faint stars within open clusters. The deeper limiting magnitude will see more faint stars. Assuming equal quality lenses and coatings: Every 10% increase in magnification provides about 0.1 magnitude gain. Every 10% difference in the area of the aperture provides only 0.03 magnitude gain. In mag 4.7 skies the Fujinon 16x70 can see mag 10.7 stars. In mag 5.0 skies the Oberwerk Standard 20x80 can see mag 10.9 stars. A selection of observations, all at mag 5.4, with the number of stars visible in M45 15x70/03 Oberwerk 128 stars to mag 10.84 16x70 Fujinon FMT-SX 133 stars to 10.84 20x80 Oberwerk Standard 184 stars to mag 10.96 25x100 Oberwerk BTstars to mag 11.18 w/24.5 WA Ober 36x100 Oberwerk BTstars to mag 11.68 w/17 Sirius plossl
Under my best skies ranging from mag 5.4 to 5.8; 12x50 Nikon SE see stars to a limit of mag 10.8, 16x70 Fujinon FMT-SX see stars to a limit of mag 11.0, 20x80 Oberwerk Standard see stars to a limit of mag 11.2, 22x100 Oberwerk is capable of seeing stars as faint as 11.8, 25x100 Celestron could reach nearly to stars at mag 12.0. Both the 100mm binoculars lost about a half magnitude in the viewable area beyond 50% out from center. In most binoculars, the loss grows to about a full magnitude or more as you move out closer towards the edge in the field of view. The following observations are taken on a field of stars in M44 west area. First observation was viewed entirely within the center 50% of the field of view. Second observation is exact same field, but viewed entirely using the outer 50% of the field of view. Note the drop-off in the number of stars seen. This illustrates the loss of magnitude and hence faint stars in the outer area of the field of view. 15x70 Oberwerk - 20 stars seen but only 18 when viewed in outer 50% 16x70 Fujinon - 23 stars seen, but only 21 in outer 50% 25x100 Celestron - 28 stars seen, but only 23 in outer 50% 25x100 Oberwerk BT100 - 32 stars seen, but only 28 in outer 50% (w/24.5WA Ober ep.) Two recent observations show a similar result and show the magnitude loss exists to some degree even in two of the finest binoculars. On the same field and on the same night in mag 5.4 skies; Fujinon 16x70 sees mag 10.87 out to 50-60%, 10.64 at 80% out and 10.37 at 90% out. Nikon SE 12x50 sees 10.64 out to 50-60% out, 10.37 at 80% out and 10.2 at 90% out. The loss of edge magnitude cannot be offset. The better the instrument, the more you will see towards the edges. Loss of sharpness will contribute to additional loss of limiting magnitude at the edges, but these are two different things going on. Loss of magnitude can be noticed more easily over the sharp view area. Illumination of the exit pupil gives an indication of magnitude loss in the outer edges of the field of view.

Illumination of the Exit Pupil The illumination of the exit pupil may give an indication of the light drop-off in the outer portions of the field of view. Along with the loss of sharpness across the field identified above, this is responsible for the loss of magnitude observed in the outer field of view. The illumination is recorded by moving a laser light across a scaled flat glass surface placed over the objective end. The exit pupil, enlarged and projected on a white card, will be seen to vary in width according to the position of the laser light on the objective.
As the light is moved from the center towards the edge of the objective, measurements are recorded for the width of the exit pupil. Significant readings are the position of the laser on the objective at the point at which the exit pupil first drops below 100% full size, the position at which the exit pupil has only 50% the full diameter and the size of the exit pupil when the laser it at the extreme edge of the objective. For comparative purposes, I have listed the 70% position readings here. I chose 70% out from center of objective because that represents at which one half the area of the objective lies within the line and one half the area outside the line. Celestron Skymaster 25x100 Light entering at 70% out on objective radius fully illuminates only the central 40% diameter of the exit pupil. The central 20% of the exit pupil receives no light at all from the outer edge of the objective. Oberwerk Giant 22x100 Light is off-center in both objectives. Light from the center of the objective does not produce the maximum exit pupil. Light at 70% out on the objective radius, on one side of center, illuminates 80% of the exit pupil. On the opposite side of center, no point in the exit pupil is fully illuminated and the central 20% diameter of the exit pupil gets no light at all. On average, the central 40% of the exit pupil receives no light at all from the outer edge of the objective. As a comparison Fujinon FMT-SX 16x70 Light entering at 70% out on the radius of the objective lens illuminates the central 52% of the diameter of the exit pupil. The central 20% of the exit pupil receives no light at all from the outer edge of the objective. Oberwerk BT100 with stock 24.5mm eyepiece at 25x Light entering at 70% out on the radius of the objective lens illuminates the central 40% of the diameter of the exit pupil. The central 10% of the exit pupil receives no light at all from the outer edge of the objective. Oberwerk BT100 with 26mm TV plossl eyepieces at 24x Light entering at 70% out on the radius of the objective lens illuminates the central 64% of the diameter of the exit pupil. Light entering at 100% out, at the very edge of the field stop illuminates the central 10% of the diameter of the exit pupil.
What Can Be Seen The combination of magnification and aperture is so overwhelming in the 100mm binoculars, even the 16x70 Fujinon is no match for a 22x100 or 25x100 binocular. Fainter galaxies seem to be easy pickings for these 100mm binoculars. While M66 is seen bright, and M65 is difficult in the 16x70s, both are readily seen in the 25x100s. In addition, NGC 3628, the companion to M65/M66, not seen by me in any smaller binocular, was visible several times in the 25x100. M105 and its companion NGC 3384 make a nice pair. M96 is seen, but M95 remains difficult. Keep in mind, you will not see as much handheld as you will with mounted binoculars. It is very difficult even with the best 70mm binoculars to see stars beyond mag 10 when handheld. Seeing stars at mag 10 requires a completely quieted binocular on a stable mount, viewing without touching the eyepieces. A binocular used for scanning, with slight minor shake eliminates most stars over mag 9.0 from view. Absolute steadiness and some persistence is required to see mag 10.2 and 10.3. This translates to how much can be seen in star fields, open clusters and dense clusters. Deeper magnitude means more stars seen. A steady binocular means more resolution in clusters. M101 was found instantly in the 25x100, it was seen equally well in the 22x100, but on the same night it was only suspected in the 16x70s. M57 is an obvious torus at 25x. In the area just south of M11, oc M26 is seen easy in the 16x70 Fujinon, but nearby gc 6712 and oc 6664 are not seen at all. In 20x80 Oberwerk Standards, 6712 is seen and in 6664 only 3 or 4 stars can be glimpsed. In both the 22x100s and 25x100s, 6712 is seen readily and 8-10 stars can be counted directly in 6664. On M17, the nebula was by far the best in the 25x100s. In the 20x80s, M17 was seen but not with the same size and brightness. In all smaller 16x70 binoculars, M17 appeared smaller with much less prominence. Some few objects are so huge that they will not fit into the small field of view of these high powered binoculars. The Hyades is too large. Even the Pleiades is just a little too large to be viewed in context, but the amazing depth provided by the 22x100 and 25x100 binocs allows seeing about 200 stars in this cluster. Comet NEAT was near the same fov as M44, just about 3 below it. In the Oberwerk 22x100, it was just out of the fov with M44. In Celestron 25x100s, the field was too narrow to capture both at once, but NEAT looked better. The 22x and 25x gave a much better view than the Fujinon 16x70 because the magnification allowed seeing distinction between the core and the halo. At 25x, even the core had size to it. There was a distinct core with a fairly large halo, maybe 20 arcmin. In the 25x100, it appeared to me there was a slight stubby fan-tail pointing out to the left/upper left.

Chromatic Aberration Chromatic Aberration (CA) occurs because all wavelengths of light come to focus at a slightly different focal length. In faster focal ratio instruments it can be more pronounced. To get a feel for the magnitude of the focus differential between the centered yellow focal length and the blue and red wavelengths focal length, in a 22x100 binocular, approx 460mm focal length, if corrected to 1/1000 F, the difference in focal length between the blue and red is 0.46mm. The Oberwerk 22x100 moves the eyepiece assembly 1mm in or out with a 60 turn of the center focus dial. So the 0.46mm difference in the red/blue and yellow focal lengths is about a 30 turn of the focus dial. If corrected to 1/2000 F, the turn would be 15 of the focus dial. In a good corrected doublet, the size of the blue/red CA blur can be 3x the size of the Airy disk. That is, for a 100mm lens with an Airy disk radius of 138/100 = 1.38 arcseconds, the blue blur or red blur may have a radius of 1.38 x 3 = 4.1 arcseconds. For faster (shorter focal length) binoculars, this blur size will increase. Since it can be considered that any point of light on an extended object (like Jupiter) forms an Airy disk, you may now see why it is at the edge borders of bright objects that we see the colored band of CA. In the Oberwerk 22x100, Jupiter on-axis has very little CA, off-axis has moderate blue to one side. CA on the moon was a thin band and it was easy to move my eye placement to make it go away.
In the Celestron 25x100, Saturn produced no CA at all. None Seen in Venus on-axis, but blue and sometimes a purple or green was seen off-axis. Very little CA is seen on Jupiter out to about 50-60% from center. Beyond that, Jupiter produces blue CA on the inside edge and yellow/green on the outer edge. Ca on the moon was green towards the outer edge. With Jupiter at the very edge of the FOV in the 22x100s, CA is a pronounced red/ purple on the side towards the center of the lens. At the edge of the 25x100s, Jupiter produces a blue CA on the side towards center. Simply moving your head from side-to-side and looking into the exit pupil at a different angle can produce a different color CA around these bright objects. As a comparison, the CA in the long focal length 20x80 Standards is very well suppressed. I saw no CA on axis and almost none when viewing Jupiter at 80% out from center of field. The f6.2 Oberwerk BT100 produced a CA free view of 5/8 phase Venus and a slight thin blue CA ring entirely around Jupiter, but only when Jupiter was about 70-80% out from center of view. Fujinon 16x70s show blue CA around Jupiter when it is placed about 60% out from center.
Optical and mechanical deficiencies I had some problems with the Celestron 25x100, that the left eyepiece would not achieve a pinpoint star focus, overall the view was pretty good, but the finest point star image thru the left eye was noticeably enlarged. This indicates some problem. Had these been a purchase, I would have sent them back as defective. I thought the exterior construction around the prism housings was a bit chintzy, not what I would have expected in a $300 binocular. The center mounting post was simply too short. I could not mount these binoculars on any tripod without the bottom of the barrels rubbing against the top of the tripod mount plate. If I were a person with a very narrow inter-pupilary distance, I would not have been able to achieve anything less than about 60mm with the currently supplied mounting post. There is no way to take that post off the binocular and replace it. The Oberwerk 22x100 uses the body from the older model 20x80 Deluxe. Instead of dew shields screwed onto the front end, the extension is now part of the binocular body and the objective lens is moved out about 3 further. The 22x100, just like the old discontinued 20x80 Deluxe has a right diopter adjustment that seems set incorrectly at the factory. I use my corrective glasses with all my binoculars. Therefore, with many binoculars, the right eye diopter is set on or near zero. With the 22x100, it is almost at the very end of the minus diopter range. Like the older 20x80 deluxe, I could not use these binoculars without my glasses if I wanted to. The illumination of the exit pupil test on the Oberwerk 22x100s shows an unusual result for this sample. This off-center illumination is, in my opinion, a substantial reason to

consider this sample unacceptable. However, at this time I know of possibly only a handful of people beside myself who would ever perform such a test to find this anomaly. The off center illumination in the 22x100 coincides exactly with the non-symmetrical sharpness test. The side of the lens that shows the least sharp image is the same area that shows the least illuminated exit pupil. These 20x80 Oberwerk Standards have a pronounced flare in the image of bright objects. It produces radial points around bright objects. A mag 1 star appears with 40-50 spikes flared around the point image. Stars of 5th or 6th magnitude are not bright enough to show the flare in the image. So while this caused a great deal of interference in the image of bright objects, it seemed to contribute no interference on faint objects. This BT100 has a bit of astigmatism in one barrel. It is more pronounced with some eyepieces than with others. It is very difficult to focus the image in this barrel to get as fine a pinpoint to match the nice image in the other barrel. The out-of-focus diffraction image in the affected barrel is decidedly oblong horizontally one side of focus and vertically the other side of focus. A small cross appears at the point of image focus. It appears to be less noticeable at lower magnification, I assume simply because the image is smaller.
What comes after 25x100? The next step is not necessarily a larger aperture beyond 100mm. There are even larger fixed power binoculars but another option is a variable power binocular. You would be surprised how much more can be seen with just a little more magnification. Interchangeable eyepieces provide that variable magnification. There are a variety of binoculars on the market in the 80mm to 120mm range that fit this bill. The one I am using is the Oberwerk BT100. The BT100 is a different animal altogether than any other 5# to 12# binocular in the 80mm to 100mm range. The BT100, at 26# (12kg), is heavier than my largest telescope, a 4 foot long 6" refractor. The BT100 cannot be picked up while mounted on its tripod. You must first set out the tripod then attach the BT100s. You would not move it while mounted. You must first take the binocs off the tripod. The views are simply stunning. It does come standard with two sets of WA eyepieces, a 24.5mm = 2.5Tfov at 25x and a 10mm = 1Tfov at 62x. Both eyepieces have an apparent fov of 62. My binocular shows a bit of difficulty with the 62x eyepieces. The supplied WA eyepieces are fine, especially the 25x pair, for wide field easy observing. For detailed work I have been using various plossl eyepieces that give a somewhat narrower field of view. My favorite eyepieces in this instrument are: 26mm TV plossls for a 4.2mm exit pupil that give 24x with a 2.2 Tfov; 20mm TV plossls for a 3.2mm exit pupil that give 31x with a 1.67 Tfov;

17mm Orion Sirius plossls with a 2.7mm exit pupil puts up a great 36x with a 1.44 Tfov. A recent addition of a pair of Meade 13.8mm SWAs will give me an exit pupil of 2.2mm at 45x with a 1.44 field of view. Also I have used a pair of 12.5mm UO orthos that gave a 2mm exit pupil at 50x, but with only a 53 arcminute field of view. However, the depth of magnitude seen in clusters was absolutely stunning. Oberwerk 22x100s and Celestron 25x100s make M12, a globular cluster, look like it is on the verge of resolution in the outer edges, where the Fujinon 16x70s could not resolve M12 at all. The Oberwerk BT100, using a 17mm Orion Sirius plossl at 36x did in fact provide some resolution in the outer edges of M12. One of the best high-powered views ever in the BT100 was with a 12.5mm ortho at 50x when I resolved some 30 to 40 stars in the very dense open cluster M11. The best low power views Ive had were with the 26mm TV plossls that allowed seeing the Merope and Rosette nebulae. These fixed power 100mm binoculars provide about a 2.5 field of view at low power. The BT provides 2.5 at low power 24x to 25x and would provide a 1 field of view at about 62x. Highly corrected wide field eyepieces can be used to get a wider field of view at a substantial cost above initial investment. Most people are using fixed binoculars and prefer the wider field of view. 25x100 will not allow you to resolve globulars, but 25x100 with a 2.5 fov and a 4mm exit pupil will show you many faint nebulae, galaxies and clusters. More power is not always the answer to see an object. These binoculars provide exit pupils ranging from 5mm (30mm ep in BT100 = 20x and 5mm e.p.) to 4mm at fixed low power and a BT can be used at the same low powers or at higher power, generally with exit pupils about 1mm or larger.
Are These 100mm Binoculars Worth It? Keep in mind a binocular allows you to see with both eyes. A 100mm binocular has two 100mm apertures with a combined area of aperture equal to a 141mm telescope. Binocular summation allows brain to process information from the eyes giving an end result that is greater than what would be seen with one eye in one of those apertures. Depending on the type of objects observed, that summation ranges from 1.2x for light gathering to 1.4x for contrast beyond what is seen in a single 100mm aperture. I really do agree whole-heartedly that a larger glass won't completely compensate for a mediocre design. However, I've put three different pair of 100mm binoculars up against the 16x70 Fujinons and Oberwerk 20x80s. The fact is, even with each of the 100mm binoculars with some optical deficiencies, the Fujinons could not see as much as any of the 100mm binoculars. 22x100, 25x100 and variable power BT100s see so much more

than the 16x70 Fujinons that in some cases it's like seeing it small in the 16x70s and then finally adding enough magnification to really see it. 22x100 or 25x100 will not allow you to resolve globulars, but 25x100 with a 2.5 fov and a 4mm exit pupil will show you many faint nebulae, galaxies and clusters. 25x100s show nearby companion NGC galaxy to M95/M96 and to M65/M66, where the 16x70s do not (in my mag 5.4 skies. I suspect if skies were mag 6, the 16x70s would show the NGCs). 22x100s and 25x100s made M12 look like it was on the verge of resolution in the outer edges, where the Fujinons could not. The BT100 at 36x did in fact provide some resolution in the outer edges of M12. Finding M9 and the fainter NGC globular (in the same fov to the northeast) was easy in the 22x100 and 25x100s, not so easy in the 16x70s, but it was still seen. The number of fainter stars seen in open clusters with both of the 100mm binoculars is greater than the number seen with the 16x70s or 20x80s. Using BT100s with 36x eyepieces, the number of stars seen and the faintest magnitude reached increases dramatically even over both of the other 100mm binoculars. Of course everything needs to be taken into context. I have no doubt the Fujinon 16x70 reaches the finest pinpoint sharpness and resolution. I haven't found a binocular to beat it. Maybe the 12x50 Nikon SE is in the same category. None of these fixed power 80mm or 100mm binoculars bring stars to as crisply defined focus as the Fujinons. The contrast in the Fujinons may be closely equaled, but not beaten. But, when it comes to how much can be seen, in just about every side-by-side viewing session, the larger binoculars saw more, and easily, not just barely. The 16x70s have a finer view, but the 22x100s and 25x100s will see more. If there are no serious optical deficiencies present that take away too much from the observation, then even if you didn't add aperture for most objects adding magnification allows you to see more. Will I ever get rid of my 16x70 Fujinons in exchange for a 22x100 or 25x100. Not a chance. I haven't found another binocular that equals the quality of the Fujinons. But there is room on my equipment shelf for a 100mm binocular that sees more than my Fujinon. A good sample of either one of these would make a nice choice. Click to discuss this article in the forums

But there are exceptions. Certain types of objects demand superior contrast and the bright image from a larger exit pupil. My fixed power binocular of choice for observing the North America and Veil nebula would be the Fujinon 16x70. These fixed power 100mm binoculars provide about a 2.5 field of view at low power. The BT provides 2.5 at low power 24x to 25x and would provide a 1 field of view at about 62x. Highly corrected wide field eyepieces can be used to get a wider field of view at a substantial cost above initial investment. Most people are using fixed binoculars and prefer the wider field of view. The 20x80s have just a bit wider fov at 2.9. More power is not always the answer to see an object. These featured binoculars provide exit pupils ranging from 4mm at fixed low power to high powered BT100 combinations with 4mm to about 2mm exit pupil. In most cases, for observing more stars you will benefit by more power or higher magnification. But the BT100, even though capable of higher powers, with 26mm eyepieces at 23.8x gives a 4.2mm exit pupil in a fine instrument with high contrast, a better suited power for viewing faint extended objects. Binocular Summation A binocular allows you to see with both eyes. Binocular summation allows our brain to process a signal from two eyes giving our brain more information. The end result is we see more than what would be seen with one eye from one aperture. Some studies show, depending on the type of objects observed, that summation can range up to +20% for light gathering to +40% for contrast beyond what is seen in a single 100mm aperture. Percent increases are based on what could be seen in equivalent area of aperture. So for a 100mm objective, rather than (nominal) 10,000 sq mm, in a binocular the effective area for information gathering is 12,000 sq mm or 14,000 sq mm or the equivalent of what would be delivered from as much as a 118mm aperture. The Celestron 25x100 is smaller than the Oberwerk 25x100. Shown here for comparison next to a Celestron 15x70 which are the same size as the Oberwerk 15x70, you may be able to see some difference.
Oberwerk 25x100 IF These are really big. They are just over 17 1/4" long, 9 1/4" wide and 4 1/2" high. The spec says they weigh 10 pounds. They are waterproof. These come in a real nice hard storage case.
This Oberwerk 25x100 was the instrument used for my recent deep sky object tour, Galaxy Hunting with Big Binoculars a report found in the CN Binocular forum through Best Of Observing. Faintest stars seen with the Oberwerk 25x100 are about mag 12.0 to 12.5. On-axis resolution is very good. Sharpness across the field is good to 85-90%. Contrast and light transmission of this binocular make it a great choice for deep sky viewing. The Obie 25x100 IF measures approximately f/4.85. The center vertical mounting post was simply too short. The Oberwerk 25x100 coatings are similar to the BT100 coatings. This binocular sells for $425.

Celestron 25x100 This binocular weighs in at 9.75#. At 16 long, it is slightly smaller in overall dimension than the Oberwerk 25x100. It is waterproof.
The Celestron 25x100 is an individual focus binocular. Overall the view was good, but it had some problems that the left eyepiece would not achieve a pinpoint star focus and point collimation was just about at the limit acceptable. This lack of pinpoint focus may be indicating an eye lens not seated properly. Had these been a purchase, I would have sent them back. The binocular is covered with a gluedown pebble finish. There is no readily accessible external prism adjustment screws. I thought the exterior construction around the prism housings was a bit chintzy, not what I would have expected in a $300 binocular. They come in a nylon suitcase. Center vertical mounting post was too short.
Faintest stars seen with the Celestron 25x100 are about mag 12.0. On-axis resolution is slightly less good. Sharpness across the field is good to 70%, but poor beyond 80%. Contrast and light transmission of this binocular make it a great choice for deep sky viewing. The Celestron 25x100 is approx. f/4.4. No undesirably noticeable pincushion or barrel distortion. There are considerable differences in advertised eye relief, fov & coatings with reality. This binocular did sell for $349. Currently $329 and it can be found for as little as $248.
Garrett Optical Triplet 20x80 Gemini These binoculars are about 12 3/4" long, 8 3/4" wide and 4" high. Since the binocular body is smaller than the 100s, the short vertical mount post does not cause as much problem. They are waterproof. GO 20x80 weight is about 7.5#. That allows greater flexibility in mounting options. It seems the difference between 7.5# and 10# is a threshold that once crossed requires much more substantial support. My sample of the GO 20x80 had a significant issue with off-center sharpness across the field of view. I do not expect this would be present in every sample. If the sharpness were centered, the sharp area would be about on par with the Celestron 25x100. Faintest stars seen with the GO 20x80 are about mag 11.3 to 11.5. On-axis resolution is very good. The contrast and light transmission of this binocular make it a great choice for deep sky viewing. The Garrett 20x80 Gemini is approx. f/4.6.
This GO 20x80 was the instrument used for my recent deep sky open clusters tour A Prolific Night of Viewing with the Garrett Optical 20x80, a report found in the Binocular forum through Best Of Observing. These come in a real nice hard storage case. This binocular sells for $279.
Oberwerk BT100 Binocular Telescope The BT100 is a different animal altogether than any other 5# to 12# binocular in the 80mm to 100mm range. The BT100, at 26# (12kg), is heavier than my largest telescope, a 1200mm f8 6" refractor. The BT100 dwarfs even the Oberwerk 25x100. This binocular comes in a unitized housing that measures approximately 24 x 12 x 6. The integral mount adds to the overall dimension. You must first set out the custom tripod then attach the BT100s. You would not move it mounted as it is very much top-heavy. My BT100 is a straight thru viewing binocular. The custom tripod requires the need to sit in a chair under the binocs to view at any altitude. Actually, Ive found for most altitudes this can be very comfortable.

the BT100. That would not be the case for instance in the Miyauchi Saturn which do have an internal field stop which limits fov. Some other binoculars for comparison are: Oberwerk 15x70 is specified as 4.3. It measures 4.3 Fujinon 16x70 FMT-SX is specified as 4.0 and measures 4.05 Oberwerk 20x80 Standard is specified as 3.5. It measures 3.2. FOV Object Size Some few objects are so huge that they will not fit into the small field of view of these high powered binoculars. The Hyades is too large. You cant see all three belt stars of Orion at once in a 25x100, but you can in a 20x80. Even the Pleiades is just a little too large to be viewed in context with a 25x100, but the amazing depth provided by the 25x100 binocs allows seeing about 200 stars in this cluster. Using the BT100 with a pair of 12.5mm UO orthos (50x), the Christmas Tree cluster nearly fills the entire fov (it's only about 0.9). Inter-Pupilary Distance & Eye Relief The Oberwerk 25x100 has an inter-pupilary distance (IPD) range from 61mm to 71mm. That is really quite a narrow range. Although it should accommodate most users, this closest limit of 61mm means this binocular may not work for any users with an IPD that measures only in the 50s. That would be almost all young children, many of whom I have found have IPD of 55mm to 58mm. Oberwerk 25x100 eye relief is substantial and easily allowed me to view the entire field from edge to edge while viewing with my glasses and the eyecups folded down. I measured 18mm, with a 3mm recess to the lens leaving 15mm usable eye relief. The Celestron 25x100 has an inter-pupilary distance (IPD) range from 56.5mm to 74mm. Thats quite a wide range. Not too many binoculars get down smaller than 57mm. Celestron 25x100 eye relief is measured at 15mm, but there is a 6mm recess to the lens, so effective usable eye relief is only 9mm. Thats very short. When used with my glasses on, I have to look around a little to see the very edges of the field. This deeply recessed eye lens is very similar to that found on the Fujinon 16x70, one that also requires I look around to see the entire field of view inside. With these two binoculars, looking straight into the eyepiece while wearing glasses cuts off about 10% of the field of view. That could be considered substantial for an eyeglass wearer. Considering the Tfov is only 2.45, the net field of view due to short eye relief is only about 2.2. Tilting the head slightly allows seeing out to either edge of the field. The Garrett Optical 20x80 Gemini has an inter-pupilary distance (IPD) range from 61mm to 71mm, exactly the same as the Oberwerk 25x100. Thats not surprising since the GO 20x80 Gemini has the same prism housing as the Oberwerk. The Oberwerk Mariner 10x60, the Oberwerk 25x100 IF and the Garrett 20x80 Gemini have the exact same prism housing construction. Remove the barrels and cover the nameplate and you would not be

able to tell one from the other. The same comments made about the Oberwerk 25x100 must be made about this binocular. Although it should accommodate most users, this closest limit of 61mm means this binocular may not be the right choice for any users with an IPD that measures less than 61mm. GO 20x80 eye relief is measured at 18mm. With the eyecups folded down, recess depth to the eye lens is 5mm, so the effective usable eye relief is about 13.5mm. I could not quite see the field stop all the way around all at once with my glasses on, but I was only losing a very small fraction. With the eyecups fully extended, and without my glasses, I needed to scrunch in tight to see the entire field of view. The BT100 has an inter-pupilary distance (IPD) range from 58mm to 80mm. Its possible some very wide eyepieces might hit together when used for a narrow IPD. However the supplied WA eyepieces are pretty large, 50mm in diameter, and these do not come in contact even at the narrowest setting. For my IPD setting of 62mm, the supplied eyepieces have a 12mm space between them at the narrowest point. The BT100 eye relief is dependant on the eyepieces you select to use for your viewing. It uses any 1.25 format eyepieces. The 62x WA eyepieces supplied have enough eye relief to use with glasses but in the low powered 25x eyepieces there is very slight field cutoff. The Meade 26mm SP LP has a deeply recessed lens, so there is a bit of the field of view lost when wearing eye glasses. The TV 26mm plossl eyelens is right at the surface and more prone to eyelash dirt but with glasses has no loss of field. Baffles and Blackening Control of stray light is important to keep contrast. There will be stray light inside any instrument. If it reflects off of any shiny surface, it will potentially cause ghosting and reduce contrast. All of these binoculars have the internal surface of the objective barrels sprayed with a dull black of dark gray material to subdue reflections from stray light. In the Oberwerk and the Garrett it can easily be seen that the surface coating is not particularly evenly applied. It is not dark black. But at least the entire surface is covered such that there are no shiny surfaces exposed. In the BT100, the entire interior surface is well coated with a roughened black coating. Also, there are four baffles between the aperture and the prism. It appears the first baffle may also be acting as an aperture stop, reducing aperture by just a few mm. I did not see any baffles in any of these other binoculars. Lens Caps I must be old school. I like lens caps that slip on over the objective barrels. The Oberwerk 25x100 and Garrett 20x80 objective lens caps are rubber and they stick into the barrel. They are a real pain to get to stay in. Not to mention, when the lens caps are cold, they shrink and they no longer fit. They fall off as if they were sized for a binocular a few mm smaller. This is a deficient design. However, I do like the one piece hinged

eyelens caps. They fit very well whether the eye cups are up or down and you are far less likely to lose an eye cap since it is a pretty big piece. The BT100 objective lens cap is a solid one-piece hard plastic insert. It fits the entire body housing. It easier to get in/out than the soft rubber inserts mentioned above. The BT100 stock eyepieces do not come with any caps. I purchased field lens and eye lens caps from Orion for about $3 each. Thats 8 caps, so about $25 to put caps on eyepieces that in my opinion should be furnished with caps. When the eyepieces are not in the BT100, the eye end openings have a screw on metal cap. Hinges The BT100 is a solid body binocular. Only the prism housing moves to adjust IPD. It is fairly easy to grasp the entire prism housing and adjust the IPD. It will not move without a fair amount of force so there is no chance of knocking it out of your IPD setting. The Oberwerk, Celestron and Garrett are all substantial sized binoculars with a fairly robust hinge and a front objective support hinge. It takes a fair amount of force to change the IPD setting. These are easily adjust in the field if being used by several individuals all of varying IPD setting. Ive heard some people complain that the supporting rod where it attaches to the prism housing is not rigidly stationary. I would only be concerned it this attachment were loose to the point of causing a lack of support. This can be tightened at the support bar shaft sleeve near the prism housing. Optical and Mechanical Deficiencies I had some problems with the Celestron 25x100, that the left eyepiece would not achieve a pinpoint star focus, overall the view was pretty good, but the finest point star image thru the left eye was a noticeably enlarged round blob, not a fine point. This indicates some problem. Had these been a purchase, I would have sent them back as defective. On both the Oberwerk 25x100 and the Celestron 25x100 the center mounting post was simply too short. I could not mount either of these binoculars on any tripod without the bottom of the barrels rubbing against the top of the tripod mount plate. If I were a person with a very narrow inter-pupilary distance, I would not have been able to achieve anything less than about 60mm with the currently supplied mounting post. There is no way to take that post off the binocular and replace it. The Celestron 25x100 showed some slight ghost images on bright objects. This would indicate there are some internal reflections. This is consistent with surfaces that are not FMC. The Garrett Optical 20x80 has an exposed prism edge resulting in minor prism light cutoff. Also the GO 20x80 shows an exit pupil size inconsistent with 20x80. It may be higher magnification or there may be an internal aperture stop. This BT100 had a bit of astigmatism in one barrel. It was more pronounced with some eyepieces than with others. It was very difficult to focus the image in this barrel to get as

COATINGS

Antireflection Coatings An uncoated lens surface can reflect 4% of the light hitting it. MgF single coating can reduce that reflection to 1.5%. Properly applied multi-coatings can reduce that reflectance even further to 0.5% or 0.25% per lens surface. Furthermore, improperly applied multicoatings may do no better than or not even as good as good single coatings. A binocular may have as many as 14 lens surfaces. Even a properly multi-coated binocular can transmit at best only about 93% to 96% of incoming light. The same binocular with 14 surfaces if MgF single coated may transmit only 79% of the light.
Reflected Light Shows Light Not Passing Thru the Lens Not all anti-reflection coatings are created equal. An indicator of the quality of antireflection coatings may be reflections you can see when looking into them. Any reflection you see in the coatings is light reflected off the lens and not passing thru the lens. Step outside under full daylight and let the sunlight shine on your face. Use the objective lens like you would a mirror and look for your reflection in the coating. The desired result is the least amount of light reflected off the coated surfaces. That means the most light is getting through. It is difficult to see any reflections at all in Fujinon or Nikon SE coatings. Look at your reflection in a binocular with lesser quality coatings and you will be able to see your full reflection in color with detail in your face. Look into the objectives of an Oberwerk BT100 binocular telescope and you will barely see the outline of your head. You will not see any detail in your face. The BT100 has a nice even coating with a green/purple tinge. The Oberwerk BT100 reflects less light than either the Oberwerk 20x80 standards or the Oberwerk 15x70. The Fujinon 16x70 reflects just a bit less than all of the above.
The Oberwerk 25x100 has fully multi coated optics on all surfaces. The coatings reflect very little light. No internal reflections were noticed.

The Garrett Optical 20x80 Gemini has fully multi coated optics on all surfaces. The coatings reflect little light.
The Celestron Skymaster 25x100 has objective coatings that appear purple/green with little reflection off the objective lens. However, the objective prism face and the eye lens in the Celestron 25x100 shows a light blue reflection. The eye lens and prism coatings on the Celestron 25x100 appear like they might be single coated MgF. Internal glass surfaces that are not multi-coated reflect more light back off the surface and do not permit that light to get through the lens. Not only does that reduce the amount of light that gets thru the lens, but also the light reflected internally causes contrast losses in the image. So, lower quality coatings reduce light in the exit pupil and reduce contrast. These two things can contribute to a less bright image in the exit pupil, even when compared to a binocular with equal exit pupil.

RESOLUTION

On-Axis Resolution - Double Star Observations and USAF Line Pairs Charts Double stars provide a good measure of binocular resolution. Of course, binoculars wont allow reaching the resolution limits of the aperture due to the low magnification, so its as much a measure of your own acuity, the ability of your eyes to separate close objects. But, if the same observer performs the same type of observation thru all the instruments then you have a relative measurement. It may vary slightly from someone else with a different acuity, but they would get similar relative measurements of resolution with the same group of instruments. Some binoculars focus to a finer pinpoint than others. And obviously, magnification provides a greater ability to split closer doubles. Generally, binoculars with higher magnification allow seeing closer doubles, but sometimes when comparing binoculars with very close magnification, such as 8x vs 8.5x or 15x vs 16x, the binocular with a finer image may exceed the limits of one with a higher magnification. What this also means is you get to see a lot more resolution in dense clusters because binoculars with the ability to see that closer double will separate more stars in a cluster. This allows you to define that word resolved. On-axis resolution can also be measured with USAF Resolution Charts. The results you will get using line pair charts in daylight will far exceed the resolution you can see on stars at night. Ive tested and recorded resolution on stars for every binocular I own and Ive tested about two dozen of those binoculars on USAF line pair charts. Almost every one of those binoculars can resolve line pairs in daylight about 40% closer than what can be seen when splitting stars. For example, I can see about 150 arcseconds apparent separation clean splits of equal magnitude stars. With the same binoculars I can clearly see line pairs resolved down to 90 arcseconds. (150-90)/150 = 40% closer separation on line pairs. This is consistent across almost all binoculars I have tested, within a few percent. Line pair chart resolution is a good indicator of contrast transfer. The resolution is measured center to center of two thin black lines separated by an equal width white space. Lines are exactly 5x longer than they are thick. Lines are much easier to see resolved than point sources. But seeing equal spaced black lines and white spaces requires a high contrast transfer in the instrument. When you reach the point where you can no longer see lines, but can only see gray, you reached the limit of contrast transfer. When the results of these tests are provided in terms of Apparent Separation or Apparent Resolution, you can then compare one binocular to any another with much different magnification. For instance, I have several different binoculars that can see 150 to 160 arcseconds apparent separation on point sources. Using 150 arcseconds apparent for the example, that would mean I have observed a 15 double in a 10x binocular, a 12.5 double in a 12x binocular, a 10 double in a 15x binocular and a 7.5 double in a 20x binocular. Using the line pairs chart all of these binoculars can see from 87 arcseconds to

has already lost the faint stars from view by the time the sharpness has become only fair and has lost much more than just the faint stars at the point where sharpness is poor.
Sharpness as Measured by Apparent Resolution (in arcseconds) Checking doubles at various points along the radius allows us to determine the loss of sharpness by referring to the apparent resolution at the points measured. For this, the magnification comes into play. The separation of the double multiplied by the magnification gives apparent separation as seen at that magnification thru that binocular. It should be noted here, the placement of the test star is measured accurately by placing a chartable field star either in the center of the fov or right on the edge of the field stop and then measuring a large scale chart distance from the test star to the chart star. It should be easily understood even a fairly close double will get easier with higher magnification. However, a considerable degree of sharpness is still required to see close doubles split. So a higher magnification binocular must be able to split a closer double to get at same apparent resolution as a lower magnification binocular. As an example, a 12 arcsecond double in a 20x binocular would be equivalent to a 9.6 arcsecond double in a 25x binocular. Therefore, various doubles are used to test sharpness. Based on the above, the sharpness as measured by apparent resolution (in arcseconds) of various binoculars is given. When comparing one binocular to another, at any given % out from center lower is better. With the Oberwerk 25x100, I was able to observe a 7.3 arcsecond double (11 Mon) still cleanly separated at about 50% to 60% out from center for an apparent resolution of 183 arcseconds. That's really very good central area resolution performance and at the 60% out mark exceeds the capabilities of the Celestron 25x100 by nearly a factor of 2. It also exceeds the capabilities of the Oberwerk 22x100 which at very best achieved a resolution of 310 arcseconds at 60% out. The Oberwerk 25x100 is still able to resolve doubles of 18 and 19 at 75% to 80% out from center.
In the Celestron 25x100 sharpness of image is fair at 70% out from center, but then it drops off very quickly beyond 70%. By 80% out it gets rather poor. The Celestron has a problem with one barrel of the binocular that won't allow it to reach pinpoint focus as well as the other side. The Celestron 25x100, when observing a 14" double, was very clear at 50%, still good at 60% but poor at 70%, all the way around. Doubles of 21 and 22 could still be seen at 70-75% out from center. A 29 double was the limit at 80%. The Oberwerk BT100 at f/6.2, with 20mm TV plossls at 31x is sharp right up to the very edges of the field stop. You can put a 7" double right up to the edge of the field stop and still observe the two components cleanly separated. Why? Because it is using a well corrected TV 20mm plossl. With the BT100, superior sharpness is seen with the TV 26mm plossl 2.0 fov and with Televue 20mm plossl 1.5 fov eyepieces. View is quite a bit narrower than the stock WA 2.5 fov eyepieces, but you can literally put objects right out to the edge of the field stop and still see a near perfect view with almost no distortion present. In the BT100 with a 26mm TV plossl at 24x, a 14 double can be put right on the edge of the field stop and still be seen as two. At 25x with the standard 24.5mm wide-angle eyepiece supplied with the BT100, a 7.4 double can be seen only out to 60-70% before it becomes distorted. I compared Meade 26mm SP LP to Televue 26mm plossls and also to the stock Oberwerk 24.8mm WA that came with the BT100. Also I compared stock WA 25x eyepieces to Obie 25x100 IF. Both 26mm plossls are much sharper towards the edges than the stock WA eyepiece. There is very little loss of image sharpness. Stars are nice round sharp stars right up to the edge. The BT100 stock WA 25x eyepieces are equivalent to the Oberwerk 25x100 IF out to 60%. But by 80% to 90% out the 25x100IF holds up better and the stock WA eyepieces really lose sharpness of image. Off Center Sharpness When testing the Garrett Optical 20x80 Gemini, I found this particular sample has an off center lens sharpness problem. I reported this same anomaly when I reviewed the Oberwerk 22x100. Most binoculars have the sharp field of view centered and may provide a clear sharp field out to 60%-70% or occasionally even 80% of the field. This one is off center, sharper towards the 9 o'clock position and deteriorating rapidly towards the 3 o'clock position. Both barrels showed the same result. A 13" double can be seen 65% out towards the left but only 35% out towards the right. An 18" double can be seen 70% out towards the left but only 40% out towards the right. A 36" double can be seen 90% out towards the left but only 50% out towards the right. Several times during the night of deep sky viewing, I would move a faint diffuse object towards the right and it would become exceedingly more difficult to see as I moved it further out of the central sharp spot in the view. It obviously had little effect on my night of observing as I captured over 60 deep sky objects in one night of viewing and some very close double stars are separated or elongated. But best view is not centered.

LIGHT GATHERING

Brightness of the image A binocular with a larger exit pupil will have a brighter image. Well, most of the time. Often we read of a user comparing the apparent brightness of the image in the exit pupil from one model binocular to another. Assuming equal size aperture, sometimes it seems difficult to understand how binoculars with equal size exit pupil may differ in brightness or why it might be that a smaller exit pupil binocular may appear to have a brighter image. Standard calculation tells us the larger exit pupil provides a brighter image. But that is the simple calculation, and all is not always what it may simply appear to be. Assume for example two different 100mm binoculars one with a 4.5mm exit pupil and the other with a 4mm exit pupil. Everything tells us the binocular with the 4.5mm exit pupil should give a brighter image. Why then might it be possible for the 4mm exit pupil to give a brighter image than the binocular with a 4.5mm exit pupil. In addition to the potential light lost to the exit pupil based on the quality of the coatings and the number of uncoated or single coated surfaces, the following may also contribute to light lost from the exit pupil. Studies have shown that not all binoculars deliver all the light expected based on the simple calculations of aperture area and exit pupil. The brightness of the exit pupil is considerably affected by vignette in the binocular system, especially in the prisms. Even for some of the best binoculars, vignette is present. The amount present can cause considerable differences between models of the same sizes and between varying sizes of objectives producing the same size exit pupil. Total brightness cannot simply be assumed based on transmission figures or light gathering area and exit pupil. If all the light that enters the front prism hole does not exit the back prism hole, it is bouncing around inside the prisms. This does several things. It causes what is known as vignette and it reduces contrast in the image. It will also reduce the brightness of the image in the exit pupil. So it is possible to have two equal sized binoculars with equal size exit pupil and yet one binocular can have a brighter image than the other. Although present even in closely sized and quality binoculars, it may not be as apparent as it sometimes is between high quality and cheap binoculars. But it is a factor. Two measures used to help determine which binocular does a better job of delivering all the light it gathers are Limiting Magnitude or faintest point sources observable and Contrast or highest contrast images of the faintest extended objects. Neither alone truly tells the whole story of which binocular produces a brighter image, but a binocular that does both best would likely have the least amount of losses due to all of the above mentioned issues. Limiting Magnitude Limiting magnitude gives you an indication of how deep you will see into the many faint stars within open clusters. The deeper limiting magnitude will see more faint stars.

CONTRAST

Think of the sky as the backdrop in a portrait. Your subject must stand out from the backdrop to be seen. Consider both the object and the background to be on a gray scale. If they share an equal shade on the gray scale you cannot see the difference between object and background. If the object is a little brighter and the background a little darker, you can see the object. Contrast allows seeing faint objects against the sky background. The qualities of an instrument that help contribute to better contrast are several. One is the control of stray light by baffles and blackening. A premium lens that contributes the least light scatter in the image is another. Premium coatings will allow the most light to pass or improve the transmission and increase the potential for contrast detection. Coatings will also reduce internal reflected light and prevent contrast losses due to interference. And a lack of chromatic aberration will allow more light to be concentrated in the Airy image and further improve the object image for contrast detection. Detection of Broad Extended Objects Higher magnification reduces exit pupil and darkens the image, including the extended light of the background sky until it approaches a completely dark sky, to the point you can no longer see the field stop. Lower magnification with a larger exit pupil delivers a brighter image, not only from the object, but also from the background. Larger exit pupil can be beneficial or detrimental, depends on the sky condition. Generally, lower power makes it much more difficult to see most small subjects, but in dark sky conditions may allow you to see large broad nebulous objects. Higher power may help darken a bright sky background and may help bring out faint extended objects from the background, but also reduces the overall brightness of the image. If you increase magnification with intentions of darkening the sky background, you reduce exit pupil and you also darken the image of the extended object. If you increase the aperture too much without increasing magnification, you increase exit pupil and you will brighten the entire image and you may get a washed out image. However, if you increase aperture AND increase magnification, maintaining the same exit pupil, you may improve the contrast detection of the faint extended object because you have gathered more light. Of course this will only work when sky is dark enough to permit the lower magnitude limit of contrast detection. If you are after open clusters, globular clusters and double stars, then magnification would be of greater benefit. But, if you are after faint extended objects with low surface brightness, then aperture may have greater benefit. The Merope Nebula in the Pleiades Cluster I've observed M45 with William Optics 7x50 ED, Fujinon 10x70s, Nikon SE12x50s, Oberwerk 15x70s, Fujinon 16x70s, Orion 16x80s, Garrett 20x80s, Celestron 25x100s, Oberwerk 25x100s and many others. I've looked hundreds of times with 20 or 30 different instruments. None of them, except one, showed the Merope nebula. The first time I've ever recorded seeing the Merope nebula in M45 was with an Oberwerk BT100 under transparent mag 5.8 skies using a pair of 26mm TV plossls at 24x with an exit pupil of 4.2mm. Not only does it take excellent skies and a high quality instrument, but it takes persistence! The bright portion of the Merope nebula extends southward from the

brightness of 13.9, about the same as M33 (Sb 14.0), easier than M74 and much easier than IC 342. I've only seen M74 in binoculars once in my life and that was with the Oberwerk BT100 at 24x with 26mm TCV plossls. I recently spent a great deal of time trying to see IC342 in Camelopardalis. From the three very bright stars found at the top of Kembles Cascade, the very faint galaxy IC 342 lies about 5 directly north. Next to the M31 galaxy group, this may be the closest galaxy to the Milky Way. I have tried dozens of times to see this faint glow and even though on this night with the Garrett 20x80 Gemini I suspected something just a little different about the spot, and I do mean the exact spot, at no time have I ever been able to say for sure that I have seen IC342. I spent a great deal of time at the eyepiece in my attempt to see IC342. Sometimes I spent several minutes without moving my eyes away at all. I had hoped to try for IC342 again in the early morning hours after a full night sleep and after reaching much darker adaptation. But weather did not cooperate. I did try with the BT100 at 31x and 25x, the Oberwerk 25x100 and the Garrett 20x80. In each binocular, there was suspected a very faint diffuse area against the background sky. But it was never confirmed as a positive detection of the object. IC 342 has a Sb of mag 15.0 and only little brightening towards the center. It is a very difficult object to bring up to a level of contrast threshold detection. Maybe Ill get it one of these nights when I get my best skies of mag 5.6-5.8. M101 in UMa is elusive and is not always seen with big binoculars. Unlike many other galaxies that appear with a bright center and some extension, M101 is nothing more than a faint glow covering a wide area. Often it may be necessary to tap the binocular so it wiggles a bit and that might allow you to see it. The best of nights are required. M101 has a Sb of mag 14.6 but it has some slight brightening to the core. One night, M101 was found instantly in the Celestron 25x100, it was seen equally well in the Oberwerk 22x100, but on the same night it was only suspected in the 16x70s. On two occasions recently, when awakening at 4AM and going outside completely dark adapted, M101 was found instantly with the Oberwerk 25x100. In the GO 20x80, M101 appeared as just a slight bit brighter in difference in the gray color of the sky. M65 , M66 and the challenging companion NGC 3628 are found in Leo with M66 being the easiest, M65 difficult, and the companion NGC 3628 not seen by me in any binocular smaller than 100mm. NGC 3628 has a Sb of mag 13.5. While M66 and M65 are just seen in 15x70s, 16x70s and 20x80s, the companion NGC 3628 will not be seen. Both M65 and M66 are readily seen in the 25x100s. With the Oberwerk 25x100, 3628 was seen instantly and directly. 3628 was seen averted several times in the Celestron 25x100. M66 to the east is the brighter and larger of the two. It appears slightly oval. M65 on some occasions is not easy to see with 70mm binoculars. One morning not only was it seen easily with the Oberwerk 25x100, but also the faint third companion galaxy NGC 3628 was seen instantly. M65 to the west has a longer narrower dimension than M66, but to me M65 is half the size of M66. On the other hand, NGC 3628, even though very faint, was seen as very long and narrow. Both M65 and M66 had a bright core. 3628 has no

core at all. Its just a long faint extension. All three lie within a 1 field and make for a great view in the 25x100. M95 / M96 / M105 galaxy group is also in Leo. These always take a little bit more time to find. Of the pair M96 / M95, many times M95, to the west, is very difficult to see. Less than 1 north of M96 is M105, also pretty bright, although smaller and rounder than M96. And right next to M105 is the even smaller NGC 3384, but it has a fairly bright core. NGC 3384 has an Sb of 12.9. 3384 is seen easily in the Oberwerk, and was also seen pretty easily in the Celestron. Only once it had been identified in the 25x100s was it seen as just more than a fuzzy starlike object in a Fujinon 16x70. M95 in Leo at Sb 13.6 was not as easy and was found easier in the Oberwerk but only suspected in the Celestron. All four of these span only about 1.5, so they were all seen in the same field of view. Light Grasp, Resolution and Contrast in Open Clusters and Globular Clusters The binocular that would be best for broad diffuse extended objects may not the same binocular that would be best for most other objects. For most everything else, a higher magnification (that produces a smaller exit pupil) provides a larger image and a bit deeper limiting magnitude that allows seeing more. M38 has stars that are spaced openly. All the stars are widely spaced but pretty faint, probably the reason why so many cant seem to find this cluster with mid-sized binocs. It has many stars easily seen, well spaced and all evenly bright, but all pretty faint with the brightest star only mag 9.5. In 20x80s, M38 is really quite spread out. So, at 20x80 it looses its appearance as a cluster with star glow. In a 10x60 you will still see a more compact cluster with a glow of stars. Too much magnification can make M38 almost disappear. M38 has a very small distant cluster nearby, NGC 1907, visible in 25x100s. It was seen easily in 20x80. On one excellent morning 1907 was seen in 16x70s. A close observer may also find a nearby planetary nebula. M36 is much different. It is very compact, with stars tightly grouped. Its difficult to separate the stars with mid-sized binoculars. On different nights, a 15x70 saw 4 to 6 stars, the Fujinon 16x70 saw 8 and 11 stars. A glow is readily apparent because about 10-15 of the tightly packed stars are pretty bright. Both good resolution and good contrast are needed to separate them. The Garrett 20x80 saw 14-15 stars and could see the 11 double star in the center of M36. The 25x100 can see about 20 stars. M36 has 15 stars between mag 8.9 and 10.65. In the BT100 at 44x, M36 was fully resolved to 22 or 23 stars. M37 is the most impressive of the three. It is large and very densely populated. M37 is like so many stars crowded together, there seems almost no room for all of them. Its very difficult to resolve this cluster. The stars are mostly evenly bright, but only one is very bright. Anything that can be resolved is seen across a bright, diffusely lit background. Mid-sized binoculars have a difficult time resolving the dense cluster, so even at 20x, it still holds that impression of a broad unresolved glow. But at 20x, it looked a little grainy. 25x100s can resolve maybe 40 or 50 stars, but they are so tightly packed they cant be counted. In the BT100 at 44x, it showed probably 40-50 stars

Sidgwick, J. B., Amateur Astronomers Handbook, Dover Publications, 1971. (optical theory, aberrations and formulae) Seyfried, J. W., Choosing, Using & Repairing Binoculars, University Optics, 1995. (information on how binoculars work and especially collimation.) Skiff, B. and Luginbuhl, C., Observing Handbook and Catalogue of Deep-Sky Objects, Cambridge University Press, 1990, 1998. (object descriptions, deep sky objects only, listed by NGC# within constellation). Tirion, Wil. Rappaport, B. and Lovi, G., Uranometria 2000.0, 2 Volumes, WillmannBell, 1987, 1989. (large scale star charts, 473 charts). Tirion, Wil & Sinnott, Roger, Sky Atlas 2000.0 2nd Edition Deluxe, Cambridge University Press Sky Publishing 1981,1998. (Star Charts in color, black stars on white background plotted to magnitude 8.5 stars. 26 foldout charts are 21x16 each). Other Reports and Articles by Ed Zarenski: How to Understand Binocular Performance, CN Report Feb. 2003, http://www.cloudynights.com/howtos2/binocular-performance.htm Limiting Magnitude in Binoculars, CN Report Sept. 2003. http://www.cloudynights.com/documents/limiting.pdf Affect of Eye Pupil on Binocular Aperture, CN Report Aug. 2004. http://www.cloudynights.com/documents/binoexit.pdf 100mm Binoculars What Can You See?, CN Report Oct. 2004, Binocular Review - Oberwerk 22x100 vs, Celestron 25x100 http://www.cloudynights.com/documents/binos3.pdf