darker sky background because the extended object will appear darker by the same amount the background darkened. OK, how do you improve your chances of seeing faint extended objects? Well if you keep magnification constant and increase aperture, exit pupil will get larger and very likely a bright sky would generate a more washed out image. But, if you keep exit pupil constant and increase both magnification and aperture, you will increase the amount of light gathered and you will increase your potential for contrast detection. This is the old tried and true Aperture Rules formula, except that you pay attention to what size exit pupil works under your bright skies. Even though both have an equally bright 4mm exit pupil, a 25x100 will see faint extended objects easier than a 20x80. So it depends on what you are looking for. If you are observing for clusters, you can improve apparent contrast against a bright sky by simply increasing magnification and decreasing the exit pupil. If you are searching for faint extended objects, the only way to improve contrast detection is to use more aperture. Field of View TFOV - AFOV I have not found the need to use any sort of finder on any binocular with a field of view down to about 2. Viewing thru the binoculars 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. The Tfov 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. Oberwerk 25x100 IF is specified as 2.5. It measures 2.35. Afov = 58. M44 easily fits in the fov. Celestron 25x100 Skymaster is specified as 3.0. It measures 2.45. Afov = 61. Garrett Optical 20x80 Gemini is specified as 3.15. It measures 2.9. Afov = 58. Orions Belt can be seen. The BT100 straight-thru binocular is f/6.2. It comes standard with two sets of 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, but a much more highly corrected field of view (my personal preference) compared to typical wide-field eyepieces. Among my eyepiece pairs are: 26mm TV plossls give 24x with a 2.2 Tfov. Afov = 52. 20mm TV plossls give 31x with a 1.6 Tfov. Afov = 50. 14mm TV Radians give 44x with a 1.35 Tfov. Afov = 60. All of these Tfovs have been measured in the field to within a few percent of expected. Unlike some other interchangeable eyepiece binoculars, the Oberwerk BT100 does not have any internal field stop between the eyepiece holder and the prism. Therefore, the eyepiece field stop, nominally the Afov of the eyepiece, determines the field of view in
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

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. I used a Cheshire eyepiece to correct most of the alignment. The condition is now much better. Collimation The Celestron 25x100 was 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. I made no attempt to realign this binocular. The Celestron does not have readily accessible adjustment screws. They are very likely right under the coating, but that means cutting or poking thru it to get at the screws. I decided not to. The G.O. 20x80 Gemini has the same prism housing as the robust Oberwerk Mariner line. The Oberwerk Mariner 10x60, the Oberwerk 25x100 IF and the Garrett Gemini 20x80 have the exact same prism housing construction. If you were to remove the barrels and cover the nameplate you would not be able to tell one from the other. My G.O. 20x80 arrived with point source images well collimated to within 30 arcseconds. Field of view overlap is nearly matched. To get an idea of the comparative difference in collimation error as viewed by your eye, the 25x Celestron at nearly 1 arcminute error has an apparent error of 25 arcminutes. The 20x Garrettt at only _ arcminute has an apparent error of only 10 arcminutes. The BT100 at 40 arcseconds error before correction had an apparent error of 17 arcminutes at 25x, but it had an apparent error of 41 arcminutes at 62x. After correction, I estimate the apparent error is now 6-8 arcminutes at 25x and 15-20 arcminutes at 62x. At 44x with a 14mm Radian, that would be an apparent error of 11-15 arcminutes and it is not noticed. Misalignment of the Field of View The Oberwerk 25x100 arrived with point collimation dead on, but there was a difference in barrel field of view overlap. Imagine looking thru the eyepieces of the binocular and what you see is two circles but they dont overlap perfectly. Best way I could describe it is this. If I were to fix the binocular field of view on a yard stick at some distance away, in one barrel I would see from the 5" mark to the 30" mark. In the other barrel I would see from the 7" mark to the 32" mark. This exaggerated image here should help. The field of view in the Oberwerk 25x100 is about 2.35 or about 140 arcminutes. In the binocular I received, the overlap was off by about 8 - 10 arcminutes, or by about 7% of the field. However, stars were merged almost dead on. You would say these binoculars were collimated on point sources. So what was the problem?

This is what I think. There are four prism tilt screws on a binocular, one for each prism. I think the images in this binocular originally were not merged and the wrong screws, the eye prism screws, were adjusted to merge point sources. If the objective prism was tilted from the light path, but the eye prism was adjusted to merge the out-of-line images, then afterwards that barrel would show a different field of view than the other barrel. What I did by trial and error was throw the merged images out of line little by little by turning the eye prism screw and then realigned the field little by little using the objective prism screws. Once I got the field to match in both barrels, I needed just a slight readjustment of the eye prism screws to make a fine adjustment to merge the images. Just to raise a few questions: How many of you have ever used a little jewelers screw driver to adjust the prism screws on your binoculars? Did you access the eye prism screws or the objective prism screws? How do you know if you adjusted the prism that was tilted in the first place? Ive seen one set of instructions for collimating that dont even show the access holes for the objective prisms. So Im guessing that it is most likely the screws accessed to adjust prisms most often are the eye prism screws. Just one more question for thought. What do you think is the likelihood that it is always the eye prism that needs adjustment and the objective prisms dont? Im happy now with my Oberwerk 25x100. I now have near perfectly matched field of view AND merged point source images. A Floating Prism Shelf The Oberwerk BT100 prism housings hold what are described as massive prisms. There are three separate cover plates over openings into the prism housings. These tiny coverplates must be removed to access and make adjustments to the prisms if necessary. This is not like the common binocular with prism tilt screws. Collimation is quite different, very precise, rugged and well designed. The massive prisms are mounted to plates that float on three points, each point finely adjustable. Inside the prism housing are three little wheels. Whereas on a closed binocular you find little screws outside the prism housing, these wheels perform the same function. The wheels look like little tinker toy sprockets, if you can remember what those are. As you rotate a wheel about the axle, it raises or lowers that side of the prism plate. I believe it is best to adjust these exactly like you would perform collimation on a three screw SCT secondary mirror. You never make all of the adjustment with one screw. If you need to raise one screw you also equally lower the two screws on the opposite side of the prism plate. So adjustments need to be made in baby steps. I measured point source image separation between the objectives originally at approximately 40 arcseconds separation. How could I measure it that fine? I observed a 62 arcsecond double star Nu Draco to test collimation on point sources. I could easily watch as both components tried to merge. I could see the error in mis-alignment was about 2/3rds of the space between the components of this double star. Normally

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.

CHROMATIC ABERRATION

Chromatic Aberration (CA) is a function of the inherent nature of a lens to focus different wavelengths of light, blue, yellow and red wavelengths, all at slightly different points along the focal length axis of the lens. By using a doublet lens, with each glass having different properties, the focus point for some of the wavelengths can be brought closer together. By using a triplet lens, the focus points of the wavelengths can be brought even closer together. CA can never be eliminated entirely. Lateral Color Error Keep in mind each color (wavelength) focuses to a different focal length. That means also that each color results in a very slightly different magnification. Since image scale varies with wavelength, objects off-axis show color fringes because variance in magnification causes color images off-axis to not coincide. In essence, lateral color is a form of distortion. This is why much more color is seen off-axis than on-axis. Longitudinal Chromatic Aberration To get a feel for the degree of the focus differential between the yellow focal length and the blue/red focal length, in a 20x80 binocular of approx 350mm focal length, if CA is well corrected to 1/2000 f, the difference in focus distance is about 0.18mm. My sample 20x80 triplet has individual focus eyepieces that have 5.5mm of travel with a rotation of 270. Therefore a 0.18mm of focus travel is a turn of 10. This aberration is seen as color fringe completely around a bright object when viewed on-axis. In a well corrected doublet, the size of the blue/red CA blur can be 3x the size of the Airy disk. That is, for a 80mm lens with an Airy disk radius of 138/80 = 1.72 arcseconds, the blue/red blur may have a radius of 1.72 x 3 = 5.1 arcseconds. For faster systems (binoculars), this blur size will increase. For larger apertures, the blur size will decrease. Focusing to a point between the yellow wavelength focal point and the blue/red focal point will gradually reduce the blue/red blur. As focus approaches the blue/red focal point, the yellow/green blur will increase in size. The smallest blur image will occur when focused midway between the two, but neither wavelength will be clearly focused. 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 bright objects. Only when precisely focused on the object is CA minimized. This helps explain why binoculars when used for terrestrial viewing may show considerably more false color. Only the object at the precise focus distance will have CA minimized. Every other object in the field of view, closer than or further away than the precise focus point, may show greater CA. This may explain why a particular model terrestrial binocular with a low CA seems to have a greater depth of field. When used for astronomy, focus is on infinity and there is no depth of field to compare. This is also why we see CA on bright objects and why it can be so difficult in binoculars to suppress CA and focus on especially large bright objects. Is this cause for alarm?

Certainly not! It is inherent in all doublet lens systems. It shows up more in shorter focal length fast systems such as binoculars. But you should understand the implications of CA in an optical system. CA has an affect on focus, light grasp and contrast. Focus - CA makes it more difficult to reach precise focus. CA is the result of the various wavelengths in the light not reaching focus at the same precise focal length. CA has other significant effects on the image. As distance off-axis is increased, this becomes more noticeable. Light Grasp - As CA increases, the light grasp of the instrument is reduced. CA affect on light grasp is a result on not being able to focus all the light into the Airy image. The CA light escaping to the fringes does not contribute to the overall image and this in turn affects the contrast in extended images. In a more highly corrected system, there is greater light in the image. This might explain why some observers say their apochromatic refractor reaches deeper than an achromat of equal diameter. Even though some observers might not see such a great improvement in the image from the reduction of false color, more light is being delivered to the image and the improvement is real. Contrast CA has an affect on contrast for two reasons. CA reduces the amount of light focused into the image, so overall contrast is reduced due to light loss. Also, in extended object images, any degree of false color CA will cause a slight color blurring at the fringes of dark detail and bright background. This will reduce the contrast needed to see such detail. In the Oberwerk 25x100 IF no CA is seen when observing Saturn. Jupiter did show a bit of red CA, not at all objectionable, and it did not interfere at all with the detection of Io. The moon has a thin yellow band. When eyes were moved off axis the band would change to blue and purple and become thicker. Staying on axis really minimized the CA and it was no more objectionable than some other binoculars. In the Garrett 20x80 triplets I saw no CA at all on a crescent moon. On a 3/4 moon I saw virtually the same band of CA in the Garrett 20x80 as I saw in the Oberwerk 25x100. With the moon placed right of center the outer edge was yellow green. When placed left of center the outer edge was reddish purple. Neither binocular showed CA when viewing on axis. BOTH showed considerably LESS CA than the Fujinon 16x70. In the Celestron 25x100, Saturn produced no CA at all. No CA was seen in Venus onaxis, 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. The f/6.2 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 16x70 shows blue CA around Jupiter when it is placed about 60% out from center.

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

resolved with a dense background glow. Using the BT100 with a pair of 12.5mm UO orthos at 50x, I could not count all the stars seen in M37. Globular cluster M71, open cluster H20 Globular cluster M71 lies near the center of the little constellation Sagitta. M71 is faint even in a 15x70. This particular globular cluster may be one of the youngest globulars. Some even say it is either a very loose young globular or a very old distant open cluster. I see it as a difficult object to resolve in any case and GO 20x80s showed it as just a faint irregular glow. M71 was seen fainter in a 15x70 and was barely seen in the 10x60. It was fairly large in the Oberwerk 25x100, but it is not resolved. A 5 scope at 220x resolves only 10-15 stars in M71. The open cluster Harvard 20 is less than half a degree sw of M71. A view of M71 with the Oberwerk 25x100s also shows nearby open cluster H20. I have only suspected seeing Harvard 20 in the Celestron 25x100s. It was seen in the BT100 at 37x like a faint glow around a short string of stars. M11, M26, NGC6664 and NGC 6712 In the area just south of oc M11, oc M26 is seen easily in the 16x70 Fujinon, but neither the very faint oc NGC 6664 or the difficult small globular cluster NGC 6712 could be seen in the 16x70s. With 20x80 Oberwerk Standards, 6712 is seen as smaller and much fainter than M26 and in 6664, although very faint, 3 or 4 stars can be glimpsed. In both the Oberwerk 22x100s and Celestron 25x100s, 6712 is seen readily and 8-10 stars can be counted directly in 6664. In the GO 20x80, M11 appeared partially resolved. In the Oberwerk 25x100 I saw 10-20 stars resolved in M11. M26 was fairly bright. In oc 6664 I could count 9 stars and NGC 6712 was found quickly. In the BT100 at 31x, M11 was pretty well resolved, especially with averted vision. OC 6664 shows 12 stars. M12, M14, NGC7006 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 at 36x did in fact provide some resolution in the outer edges of M12. M14 was observed with Obwerwerk 25x100 and a 15x70. It appeared twice the size and brightness when viewed in the 25x100. NGC 7006 in Delphinus is one of the most difficult globular clusters I have observed. It was seen only if the Oberwerk 22x100 and the Oberwerk 25x100 and only after a great deal of persistence trying to see it.

MOUNTING

Mounting Considerations for 10# 100mm Binoculars (and 8# 20x80s) These 25x100s are much larger than all 70mm binoculars and even still quite a bit larger than many of the 20x80s. These are big, real big. A 10# binocular requires a substantial mount, not just any tripod but a heavy-duty tripod with a heavy-duty head. Most light or basic parallelogram mounts groan under the weight. Many lightweight tripods are woefully inadequate to carry the load.

Settling times for parallelogram mounts and tripod/head combos UA New Unimount Light on Bogen 3246 with 7.5# 20x80 damps in 6 sec max, 3-4 sec for bumps. UA New Unimount Light on Light Surveyor with 10# 25x100 damps in 8 sec, 5-6 sec for bumps with 7.5# 20x80 damps in 5-6 sec max, 3-4 sec for bumps. UA New Unimount Light on Medium Surveyor with 10# 25x100 damps in about 5-6 sec max, 3-5 seconds for bumps. with 7.5# 20x80 damps in 5-6 sec max, 3-4 sec for bumps. UA Microstar deluxe on Bogen 3246 with 7.5# 20x80 damps in 2-3 sec, bumps die out quickly. Scopestuff 2D Cradle on Bogen 501 head on Bogen 3246 tripod with 10# 25x100 damps in 8-10 sec, 5-6 sec for bumps BT100 on BT100 custom wood surveyor with 26# Oberwerk BT100 damps in 1-2 sec Orion Paragon HD-F2 w/ Paragon fluid pan head: with 7.75# 20x80s on fully extended center shaft 73" high = 5-6 seconds. Bogen 3011 w/3130 fluid pan head: with 7.5# 20x80s on fully extended center shaft 75" high = 5-6 seconds. Bogen 3246 w/501 fluid pan head: with 10# 25x100 fully extended center shaft 75" high damps in 2-3 sec with 8# 20x80s fully extended center shaft 75" high = 1-2 seconds.
Integral Vertical Mount Post The Oberwerk 25x100 vertical mount post is too short. Especially when using a substantial head like the Bogen 501, that means the binocular barrels rest on top of the tripod head and there is limited range to slide the vertical post for balancing. When pointed up, it didn't seem to pose any problem. While the slide rod is probably 10" long, once that vertical post is attached to the top of the tripod, you can only use about 5" of that full slide rod length. I think a taller vertical post, similar to the post on the older 20x80 Deluxe, is a necessity for these giant binoculars.
I added a 1 extension post obtained from ScopeStuff. It solves the problem very nicely. With the 1 post added, this binocular mounted on a Bogen 501 head clears the head platform and allows full use of the slide bar and a full range of IPD adjustment regardless of where the binocular is balanced on the slide bar.

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

Differences Noted in the Coatings. Both the Celestron Skymaster 25x100 and the Oberwerk Giant 22x100 have objective coatings that appear purple/green with very little reflection off the objective lens. Looking at reflected light inside the Oberwerk 22x100 shows a green reflection off the prism face towards the objective. The same prism face in the Celestron 25x100 shows a light blue reflection. The coatings on the Oberwerk eyepieces appear greenish. The eye lens and prism coatings on the Celestron 25x100 appear like they might be single coated MgF. The 20x80 Oberwerk Standard has a fine coating on the objective that produces almost no reflection with full daylight. The coatings on this 20x80 binocular seem to reflect so little light, they appear better than the Oberwerk 15x70/'03 and maybe should be considered nearer to the Fujinon 16x70. The coatings on the Oberwerk BT100, judged by the amount of reflections coming off of them and the difficulty of seeing any detail in those reflections, appear to be one of the finest coatings I have seen. One way to tell, comparatively, how much light gets through the coatings is to look for how much light is being reflected back off the coatings. You really can't tell much by looking at just one binocular. You need to see a range of binoculars to see different amounts of reflection. Heres how I compare the coatings on binoculars. Standing with my face in full sunlight and looking into the binoculars objectives as a mirror, I look for my reflection and try to see how much detail I can pick out in light reflected off my face. I look for reflection of my sunlit face and the outline and color of my dark hair. I look for clear facial features, eyes, nose or ears. Then standing in the shade but still with bright outdoor light on my face, again I use the binocular objective like a mirror, and I look for all the same facial features reflected in the coatings. Now I must tell you I hold the Fujinon FMT-SX in the highest regard. The 10x70 and the 16x70 are the same binocular with a different eyepiece. For a long time, I have felt they have the best coatings of any binocular I own. But, when I first picked up a pair of Nikon 12x50 SE I was startled when I looked at the coatings. At first glance, you can't see any light reflected off the coatings. Keep in mind the Fujinon FMT-SX and the Nikon SE are the best of all the binoculars I own. Probably right in with those is the Oberwerk BT100. Without a doubt the Nikon 12x50SE reflects LESS light off the objective lens than the Fujinon FMT-SX. It's hard to see any reflected light coming off the Nikon 12x50 SE. Its very difficult to see the entire outline of my head. No facial light, no features visible, just a faint dark outline of my head.

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.

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.
In the 22x100s, NGC 6934 in Delphinus was an obvious globular cluster. It was just seen in 16x70s. The 22x100 caught glimpses of another globular in Delphinus, NGC 7006. NGC 7006 was not seen in any binocular smaller than 22x100. When you view faint clusters in 25x100s you see stars that just were not there in any smaller binocular. Star counts on clusters like IC4665 in Ophiuchus show 51* in the 25x100, 50* in the 22x100, but only 40* in the 16x70. The BT100 at 36x sees 60 stars in the same field. On another night, nearby IC4756 was observed in the BT100 with 20Tvplossl at 31x. Surrounded by about 8 bright stars, sprinkled with faint stars counted over 100 stars at 31x100, with moonlight in the sky. In 20x80s, this cluster looks somewhat sparse because the faintest stars are not even visible. 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. This is a comparison of observations made on an area with controlled boundaries around M45, The Pleiades. All observations were not made on the same night, but these are all in mag 5.4 skies and under the same controlled conditions. 16x60 Pentax PCF V WP see 141 stars to a limit of mag 10.84 15x70/03 Oberwerk see 139 stars to a limit of mag 10.84 16x70 Fujinon FMT-SX see 156 stars to a limit of mag 10.94 20x80 Oberwerk Standard see 184 stars to a limit of mag 11.18 Best views yet with the BT100 were with the Televue 26mm plossl 2.0fov and with Televue 20mm plossl 1.5fov eyepieces. View is quite a bit narrower than the stock WA 2.5fov 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. Magnifications I use the most often in the BT100 are 24x, 31x and 36x. Each step up increases the number of stars that can be seen and none of the fixed power 100mm binoculars can equal the performance of the BT100. The BT100 can see stars to mag 12.0 easier and more directly than any of the other 100mm binoculars. The best low power views I had in the BT100 were with the 26mm at 24x. The only instrument I've ever own/used that was able to see a portion of the Merope nebula is the Oberwerk BT100 binocular telescope with a pair of 26mm TV plossls at 24x with a 4.2mm exit pupil and a 2.2 fov. Using the stock 25x WA eyepiece, I saw slight definition to the Rosette nebula.

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;

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