Birding Catadioptric Scopes? published November 1993, updated 2007

Questar Birder
Celestron C5
It's All Done With Mirrors

Most of us, at one time or another, have seen birders in the field with one of those funny-looking mirror spotting scopes. Short, fat, with eyepieces at strange angles, they stand out in a group of refractors like a Snipe in a flock of Yellowlegs. If you have shopped around at all, you know that some of those mirror scopes cost upwards of $2000 on the street, two to three times the price of the best of the refractors. So what's the deal? Are these mirror scope people just eccentrics, or do they know something the rest of us don't?
Most of the mirror scopes you will see in the field are actually compound telescopes, catadioptrics (Cats for short), which use a combination of lenses and mirrors to form the image. (All telescopes use some lenses anyway, at least in the eyepieces which magnify the image once it is formed. See Beyond the Basics for more on the technical details of mirror scopes.) Every optical decision is a compromise. You must take the bad with the good, and choosing a Cat scope over a standard refractor is no different.
The Disadvantages of Cats
On the down side, Cats can be more delicate in the field. Some require periodic adjustment of the position of one or both mirrors to keep them in top form. The reflective metal coating on the mirrors can (and does) tarnish over time, degrading the performance of the scope sometimes to the point where it is necessary to have the mirrors recoated, and that can be quite expensive. Cats, like all mirror scopes, have a central obstruction ‹ the small secondary mirror which directs the image to the eye is out in front of the main mirror where it blocks some of the light entering the scope. In bright light, this obstruction can cause an annoying dark shadow to float in the view, especially for eyeglass wearers who can't get their eyes close enough to the eyepiece, and it does cause some degrading of the image, loss of contrast, definition, and brightness when compared to a refractor with an objective of exactly the same diameter as the mirror (for more on this be sure to read Beyond the Basics). Then there is the Cat focus effect ‹ something which is hard to describe but obvious to anyone who has taken a thoughtful look through a Cat. Moving the focus can, especially at first, make objects in the view pop in and out like one of those trick perspective drawings. Also, out­of­focus images in the Cat are made up of tiny fuzzy donuts instead of tiny fuzzy points (again it is the central obstruction at work) and some people find that disconcerting. Finally, because of their longer focal lengths, all Cats have narrower fields of view at their lowest power than your average refractor spotting scope.

The Cat Advantage
So why would anyone what to own and use a Cat? Consider the up side. Because the Cat folds the optical path back on itself, optical designers can fit a large aperture, long focal length, instrument into a very small package. Large aperture and long focal length become important when you need to use high powers to reach those birds out at the limits of conjecture. Even the best prismatic refractor spotting scopes aren't much use at anything over 60-80X. The average birding refractor is fairly useless even at 60X, yielding an image that is often slightly blurry, quite muddy, and very dim. High quality Cats in the 90-100mm range have no problem reaching those powers, with an image that is still sharp, contrasty, and bright, and can often go quite a way beyond. (In fact, the lowest power available on the stock Birder model Questar is 54X, and the C5 with its supplied eyepiece yields 42X. These Cats, then, begin where many prismatic refractors leave off!) Then too, 90-100mm Cats are less than a foot long and weigh between 4 and 5 pounds. The equivalent refractor would be 4 feet long and weigh somewhere in the neighborhood of 12-15 pounds. Even at lower powers and closer distances, the image produced by a high quality Cat is very impressive. Many birders see increased definition, better color fidelity, and more brightness than they have ever seen in a refractor scope (see Beyond the Basics for more on this also). Very few standard spotting scopes have the close focus ability of Cats. You simply have to see what a bird 15 feet away at the feeder looks like through a Questar to believe it. The B&L;8080 focuses to 6 feet, close enough to count breast feathers on a hummingbird. If you are into photography, Cats are much easier to adapt for photography than most refractors, and, in good light, can produce excellent photographs. Finally, all of the Cats are much more useful for a secondary interest in astronomy than any of the refractor scopes (with the possible exception of the Tele Vue Pronto or Celestron S80).
Cost is, of course, a consideration in any optical purchase. Of the scopes in this test, the 90mm Questar field model lists for over $3000, and sells for around $2000. The Questar is obviously only for those who are willing (and able) to pay for premium performance. The Celestron C90 with special coatings lists for around $1000 and can be had for about $500, making it very competitive with the best 60-80mm spotting scopes. (You can save $150 or so if you buy the scope without the special coatings, but I would not recommend it.) Even the Celestron C5 (with an aperture of 5 inches!) can be purchased for about what you would pay for a ED 60 or 80mm refractor. Bausch and Lomb's little 80mm Cat spotting scope/telephoto lens is very competitive at a street price of under $300. (Meade also makes a 100mm Cat spotting scope, the 2045, but I was not able to get one for testing.) With the exception of the Questar then, on cost alone, most of the Cats should be considered quite good buys.
I want to reiterate here some advice I gave in the last spotting scope issue (February 1993, Refractor Spotting Scopes). Before you even consider a scope, spend your money on the best binoculars you can buy. Most of us will get far more use and satisfaction from high quality binoculars than we ever will from a scope. Then too, having now tested most of the spotting scopes available to birders, I have to say this: there are very few scopes that can provide a 20 power view with the same visual impact as the 8 power view through excellent birding binoculars. Most of us buy a scope for its reach. We want those birds way out there! 20 power is nice, but it is really something like 45-60 power that we are after. Very few scopes indeed can provide anything like the same apparent image quality at 60 power as you are used to seeing in your binoculars at eight. To get beyond 60X, reaching for those far wee specks, requires a scope of truly exceptional design and quality. There are far fewer scopes on the market than there are binoculars in the first place, and with scopes, because of the cravats above, you come to the short list much quicker.
It is a testimony to the utility of the Catadioptric design that three scopes, from a field of only four Cats, would make my overall spotting scope short list. In saying that, I freely admit that I am willing to sacrifice some durability and field worthiness for image quality. It is the view of the bird that I am after. I want to be able to see. If I have to exercise some extra care to do it, well, that's okay. In general, even the least expensive of these Cats provide optical performance at least equal to, and often far superior to, anything else in their price range ‹ and that is saying quite a bit.resolution
In evaluating Cats, I have used the BVD Reference Standards for refractor spotting scopes for comparison. That's the real question, isn't it ‹ how well do Cats compare to the best of the standard scopes? The answer, in general, is that Cats do very well: with the exception of the B&L;8080 (which has its own strong points), all of the Cats bettered the optical performance (resolution, contrast, color fidelity, and bright­ ness) of the Nikon Fieldscope ED, the Reference Standard for 60mm scopes. When you move up to the 70-80mm refractor class, the question becomes more complex. Certainly the Questar, the Celestron C90 and the Celestron C5 offer slightly better raw optical performance (again: resolution, contrast, color fidelity, and brightness) than any of premium 77-80mm prismatic refractors (i.e., the Kowa 77mm Fluorite, the Bausch and Lomb Elite 77 ED, the Optolyth 80mm Fluorite, or the Swarovski AT80). It is in comparing the Cats to the Tele Vue Pronto (the current reference standard in the 70 80mm refractor class) that things get complicated. Under identical test conditions, all three of the top Cats, the Questar, the Celestron C90, and the Celestron C5, yield higher resolution on the test chart than the Pronto. The C5, with its huge aperture, provides truly exceptional, one could almost say "amazing," resolution, nearly 85 times that of the naked eye. When it comes to brightness and contrast though, only the C5 can equal the Pronto at equivalent powers. Don't mistake me here. Both of the 90mm Cats provide very bright, very contrasty views ‹ better than any standard 80mm spotting scope...the kind that would satisfy even the most exacting is just that the Pronto is slightly brighter at any given power. All three of the top Cats provide better color fidelity, deeper color detail, and more intense color, than the Pronto (see Beyond the Basics).
But, of course, as important as resolution, brightness, color fidelity and contrast are in scope performance, they aren't the whole story. Both the Fieldscope (or any of the high quality prismatic refractors) and the Pronto have much wider fields of view at lower powers than any Cat...indeed, none of the cats will reach down to anything like the 20X that most refractor makers call low power. Then too, the view through the refractors at lower power is "easier" than the view through the Cats. There is no central obstruction shadow to get around, the perspective is somehow slightly more natural, and the straight through or 45° correctly orientated view of the prismatic refractors is easier for most people to deal with. At moderate distances and low to medium powers (20-45X), it is doubtful that any birder really needs the resolution of a C5. The Nikon Fieldscope (or any high quality 60mm ED refractor, the Kowa 614 comes to mind) will show you just about all there is to see, and the Pronto will show you things you didn't know were there. And, once again, for year­in­year-out durability, there is nothing to equal a well made refractor. All of the Cats feel somewhat delicate compared to the Fieldscope, and downright fragile compared to the Pronto.
In the end your decision comes down to something like this: Do you prefer a compact, somewhat delicate scope that offers great medium power views, and the best possible views at high powers, up to and beyond 150X (a Cat)...or do you prefer a rugged scope that gives great low power, wide­field views, and good (to very good) views up to about 60X (an ED or Fluorite 60-80mm refractor)? It really depends on how you bird. Only you can answer that question (and unfortunately, where scope use comes in, you probably can't answer it until you have already bought and used at least one high­quality scope).
In addition to the Cats in this issue, I have also included one alternative mirror design. The Zuka Scope is a Newtonian with a unique hand holdable design. (In a separate section, you will also find reviews of the new Bausch and Lomb 77mm Elite with ED glass and the redesigned Celestron S80+ refractor spotting scopes.)

The Questar Birder
The Questar is undoubtedly a scope to reckon with. The Birder is the standard Questar field model with the addition of a lower power, wider field eyepiece and a 10X finder scope. The Questar design is quite unique. The finder scope is mounted underneath the main scope, between the scope and the tripod. The eyepiece is mounted at the top of the scope, at a right angle to the view. To use the finder you simply look through the main eyepiece and flip a small lever/knob to the correct position for an instant 10X view. When you have centered the object of interest, you flip the lever once more, fiddle with the main focus knob as necessary, and you have the 54X view. Not enough power? Flip yet another lever/knob and a barlow lens slides into place (you can see it slide in), fiddle the focus once more, and you have 87X. (A barlow is a specially designed lens that increases the magnification of any eyepiece it is used with. Barlows, in general, have the advantage of increasing magnification without reducing eye relief...your eye remains a comfortable distance from the glass...though putting in the barlow does decrease the image brightness and the field of view, just as a higher power eyepiece would. The Questar barlow has the additional advantage of being custom designed to give optimum performance with the Questar eyepieces.) Once you get used to the funny little machine shop lever/knobs on the back of the Questar, the system is very practical in the field, making it easy to find birds, and to select the appropriate power for a good view. (Questar makes a 30mm eyepiece that gives you 43 and 68X, for those who prefer lower powers and wider fields.) I found the truly tiny focus knob (screw head would describe it more accurately) to be somewhat awkward with bare hands, and I can't imagine using it comfortably with gloves, though the focus is both easy and precise. The Questar (like many Cats) focuses by moving the main mirror. In some scopes this can cause the image to jump around (and in a worst case can degrade the image due to mirror flex). Questar avoids these problems by mounting their mirror at the center hole and sliding it in and out on a heavy spindle. The image is very stable during focus.
Questar (and Questar owners) often make the claim that the Questar will exceed its "theoretical" resolution limits under good seeing conditions. (See the discussion of resolution near the resolution test chart for more on this.) I was able to confirm that claim using the USAF chart at 36 feet and an eyepiece that yielded 130X. The Questar easily resolved the lowest block on the chart, which corresponds to a resolution of 1.31 arc seconds, exactly the theoretical limit for a 90mm scope. In addition, the lines were sharp and well defined, indicating that optical abberations are exceptionally well controlled. To go beyond 36 feet I have to go outside where atmospheric effects come into play, but even so, at 66 feet I measured the Questar's resolution as 1.01 arc seconds, about a third of a second better than theory. Combined with the exceptional color fidelity and color detail provided by the Cat design, the Questar's resolution provided truly breathtaking medium and high power views in the field. The Questar is the kind of scope that keeps you at the eyepiece long after you have made the identification. It is a scope for those who love the beauty of birds. You can get lost in the fine details of feather placement and color...and seeing every blink of the eye, every twitch of muscle as though the bird were perched on your finger can give you a sense of the living bird like nothing you can get through binoculars.
It has to be asked. Are the view, the practical field design, and the obvious high level of workmanship of the Questar worth $2000? Of course, I can only give you my opinion. If money is an issue, and you are willing to put up with a little inconvenience for an excellent view of birds, you can equal the optical performance of the Questar at a considerably lower price. Either the Celestron C5 or the Tele Vue Pronto will give you overall optical performance at the Questar level for less than a third of the price. Both are physically bigger and heavier scopes. Neither has the convenience of the flip in finder or barlow, and both (for different reasons) are harder to handle in the field, but the performance is there. Even the Celestron C90 with special coatings will give you about 85­90% of the Questar's overall performance (my estimate) in a similarly sized package, at a fraction of the cost . Still, if I had $2000 I wouldn't miss, I would not hesitate to buy a Questar. There is something about the overall intelligence and the simple elegance of the design that puts it, at least for me, in the "priceless" category. Besides providing a level of field performance that you could not better at any price, the scope would be a continual joy to own and use. Only you can say if it is worth $2000 to you.

The Celestron C5
The current C5 is a reincarnation of one of Celestron's early Schmidt Cassegrain offerings. The original was the only scope in its size class, and was very popular with astronomers and astrophographers who needed a portable scope to take out to dark­sky sites. When it was taken off the market, it left an obvious gap. The current scope is a slightly improved model, and in its astronomical incarnation, should appeal to the same kind of astronomers as the original. In addition, it is being offered in a spotting scope configuration, without the clock drive and fork mount necessary for many astronomical uses. As such, it has much to offer to the long distance birder.
The C5 is actually not much bigger than the Questar Birder. The tube is about 6 inches in diameter and 11 inches long. It stands no higher off the tripod than the Questar with it's finder scope. It weighs about 6 pounds to the Questar's 4. Into this small package, Celestron packs the resolution and brightness of a 5 inch mirror. Both in the field and on the resolution chart, the effect of the 5 inch aperture is obvious. You can see detail that simply is not there in a smaller scope. Then too, when you need high powers, the combination of resolution and brightness make for exceptional views. In the field, at 150X (the highest power I could get with the eyepieces I had at hand) I did not feel that I was reaching the limits of the scope's useful reach. Because the C5 takes standard 1.25 inch astronomical eyepieces, you should be able to find a set of eyepieces to suit your exact needs. Celestron offers everything from low power wide field eyepieces (in the Ultima line) to high power eyepieces with exceptionally long eye relief (in the LV line). Other suppliers offer eyepieces in almost any configuration imaginable.
The C5, as a spotting scope, is supplied with Celestron's 45° (right side up, right way around) diagonal, a 5X24 finder scope, and a 30mm Ultima eyepiece. The eyepiece, which provides 41X and a reasonably wide field of view, is one of the best on the market, very popular among demanding astronomers, and certainly up to the needs of the most exacting birders.
The diagonal, on the other hand, is somewhat of a disappointment. While the correctly orientated view is easier for most people to deal with, no roof prism erecting system is going to give you the same level of performance that you will get with a simple right angle prism or mirror. Actually, on the C5, the 45° degrades the image less than I would have expected. You lose about a tenth of a second of resolution, a good deal of image contrast, and some color fidelity, but, with the long focal length of the C5, it is actually usable. (The effect is much more pronounced on shorter focal length instruments. The 45° noticeable degrades the image on the C90, and turns any high power view on the S80+ to mud.) Still, if I were going to carry the C5 out into the field it would be because I wanted absolutely the highest resolution views I could get, and the 45° diagonal is just not up to that job. Substitute the standard Celestron multicoated 90° star diagonal and you will see everything the C5 can show you.
The finder scope has cross-hairs for centering the object of interest, mounts on the top left of the scope where it is sometimes awkward to get your eye to it, and is just adequate for quickly locating birds.
Focus is controlled by a large knurled knob at the lower right of the back plate. It takes a number of turns to move the mirror (and the focus point) any distance at all, but, in compensation, the focus is extremely smooth and precise.
In the field, at medium powers (40-60X), the C5 provides the kinds of views you have only dreamed about. It is one of the few scopes on the market that actually gives the impression of giving you a better view, brighter, richer, more detailed, at those powers than you get through your binoculars at 8X, and, as I said above, there seems to be no upper limit to the amount of detail you can see at higher powers. The C5 resoloves finer detail than my test chart shows at 36 feet with the lines very sharply defined. There is little or no sign of the line blurring that results from abberations in the optical design. Outside, the C5 still resolves the finest lines on my chart at 66 feet, and might go a bit beyond. That translates to a resolution of .71 arc seconds, a fifth of a second better than theory, and more than twice the resolution of the best refractor that I have tested. You just can't get much better resolution than that! If you regularly need extreme high power to reach across a lake or tidal flat, I would have no hesitation in recommending the C5 as your best choice. Even if you are only going to use the scope at moderate powers, you really can not buy a better view at any price.
As a bonus, if you are interested in astronomy, the C5 is undoubtedly the best choice you could make for a dual purpose scope. It has enough light grasp to make it really useful for deep sky objects, and a long enough focal length to yield excellent planetary detail. Put it on a sturdy tripod and take it out under the stars. Even without the clock drive and fork mount you won't be disappointed, and you could always add the astronomical accessories later. (Or better yet, if you can afford an extra $200-$300, buy the astronomical package to begin with. The C5 has a quick mount system that allows the optical tube to dismount from the fork in a matter of seconds for use on a standard tripod, and you will get the much higher quality 90° diagonal as part of the standard package.) The C5 can give you the best of both worlds: stars, galaxies, nebulas, and planets by night...birds by day. That is saying quite a bit for a scope that costs less on the street (even with the astronomical package thrown in) than most of the premium 77-80mm ED or Fluorite spotting scopes. Such a deal!

The Celestron C90
The C90 has had a following among birders for quite a number of years now. I tested the model with special coatings. It comes equipped with a "hybrid" 45° diagonal (it adapts the .975 inch back of the C90 to use standard 1.25 inch eyepieces), the same high quality 30mm Ultima eyepiece as the C5, and the same just adequate finder scope. If you have not looked though a C90 lately, the addition of special antireflective coatings and the change to the better eyepiece have made an incredible difference in the image you see. If you add the necessary adapters to use a standard 1.25 inch star diagonal, the C90 provides exceptional performance in a very small package.
This is a field scope, not an astronomical scope adapted for field use. Focus is controlled, camera lens style, by a large rubberized ring that rotates the whole front of the scope. While this arrangement is quite a bit faster than the knob at the back of the other Cats, it is not nearly as precise. You sometimes have difficulty hitting the exact focus point. The scope is also available in a completely armored model with a heavy rubber casing for field durability. It is quite compact, less than 9 inches in length and only 4 inches in diameter, and weighs in at about 3 and a quarter pounds.
Optically, the special coatings model is among the best spotting scopes I have tested. Although not up to the level of either the Questar or the C5, and not quite up to the level of the Tele Vue Pronto among the refractors, the C90 provides bright, contrasty, high resolution views (tested resolution was 1.66 arc seconds, or about a third of a second off theoretical limits) at medium powers (the supplied eyepiece provides 33X), and will easily reach 100X with an eyepiece like the long eye-relief 10mm LV.
The C90 will cost you somewhere in the neighborhood of $500, and for that money you simply can not buy a better medium power view of the bird. Both the Kowa TS614 and the Nikon FieldScope IIED come close to (or even, under some conditions, equal) the C90's optical quality (slightly lower resolution, but higher brightness and contrast), and offer lower powers and wider fields, but both cost more on the street. All of the 77 80mm ED or Fluorite scopes are considerably more expensive, and would not give you a significantly better view than the C90. Among the refractors, only the Tele Vue Pronto would exceed the C90s performance and then you would have to be willing to pay somewhat more and carry a much heavier and bulkier scope out into the field (and a correspondingly larger and heavier tripod to go with it). All in all, the C90 with special coatings makes a very attractive scope for birding, and, quite possibly, is the best buy of any spotting scope currently on the market.

Beyond The Basics
It's All Done With Mirrors

Way back at the very beginnings of optical history, no less a name than Sir Isaac Newton (of the laws of gravity fame) demonstrated that there are two ways to bring light into focus to form an image. (If you know much about Newton you might suspect that at least part of his motivation was just that, to show that there was another way, his way, of doing the thing.) He built the first mirror telescope, employing a design that bears his name to this day. The Newtonian telescope uses a concave mirror (the primary mirror), instead of a lens, to catch and focus light, with a small flat secondary mirror mounted in front of it at an angle to direct the light off to the side where the image can be magnified and viewed (see illustration, next page). Newtonians use a parabolic primary mirror to bring all the light that enters the scope to the same focus. The parabolic mirror produces very sharp, bright images in the center of the field, but introduces increasing distortion as you move out to the edges.
There is only one Newtonian scope included in this issue. Newtonians, with optics open to dust and moisture, their long tubes and awkward eyepiece placement, are not, in general, much use in the field. The Zuka scope overcomes many of the Newtonian's limitations and actually manages to turn some of them to advantage. (See the review.)
The other scopes in the test are catadioptrics...scopes which use a combination of corrective lenses and spherical mirrors to form the image (see illustration). In theory, the use of properly corrected spherical mirrors produces more precise images over a wider field. Maksutov Cassegrains (the Questar and Celestron C90) use a deep, bowl like, corrector lens. The secondary is actually a spot of reflective metal applied directly to the inside surface of the corrector. Schmidt Cassegrains (Celestron C5 and Bausch and Lomb 8080) use a flatter, but more complexly shaped corrector plate, with a separate secondary mirror bolted to it in its own adjustable mount. The B&L;8080 is actually a modified Schmidt design, since it uses a small compound lens placed in the light path after the secondary mirror.
The Distance Factor
The major advantage of mirror optics, as noted in the introductory article, is compactness...the ability to build a large aperture, long focal length instrument into a very small package. Large aperture and long focal length equal better high power performance. Even the little Bausch and Lomb 8080 Cat, with an 80mm mirror and a focal length of 800mm, provides quite good performance at 100 power. Reaching 100X with most refractor spotting scopes, with their much shorter focal length objectives, would require an eyepiece with such a short focal length that your eye would be all but in contact with the glass, and, even then, it is doubtful you would see much useful detail in the image. Both the C5 and the Questar have focal lengths of over 1200mm and enough light gathering ability to provide fairly bright images at high powers. 100 power is no strain for either of them, and the C5, in particular, yields very good images, even in subdued light, at 150X. When distance is a major factor, there is simply nothing to match a good Cat equipped with the proper eyepieces (unless, of course, you are willing to carry a big, long focal length astronomical refractor out into the field with you).
Aberrant Behavior
In addition to usable high powers, many birders, when they first look through a high quality Cat scope, are amazed at the snap in the image. They might use words like "sharp," or "bright" to describe what they see, but what they are actually seeing is a negative quantity...the almost total lack of chromatic aberration. As noted in past discussions of ED and Fluorite lens elements, no lens can bring all the colors of light to exactly the same focus. Lenses bend light as it passes through, and each color is bent at a slightly different angle. The presence of out of focus color in the image is called chromatic aberration. In the worst case, out of focus colors can appear as pale rainbow fringes around bright objects, but the general color smearing also mutes and muddies every shade in the image. The achromatic objectives used in most refractor spotting scopes bring two primary colors to the same focus. The latest apochromatic objectives use exotic glass or non-glass elements to bring at least three colors to the same focus. In the real world, though, even the best ED and Fluorite apochromatic refractor spotting scopes that I have tested so far show some out of focus color.
Mirrors, since they do not bend light at all, are inherently free of chromatic aberration and false color. What impresses birders is the all but perfect definition and delineation of each individual shade and tint in the image. The image in a scope like the C5 or Questar, or even the C90 with Special Coatings, can be breath-taking in good light, and will glow with subtle shadings well into twilight.
Being Obstructive
In the lead article we mentioned the effect of the central obstruction on image quality. Having the secondary mirror in the light path blocks some of the light, making for a dimmer image than you would get from a perfect lens of the same diameter. The secondary also introduces diffraction effects that reduce the overall contrast of the image. (Light is bent from its normal path as it passes the edge of the secondary, and these wandering rays enter other areas of the image, blurring distinctions between light and dark areas.) The amount of lost light is equal to the percentage of obstructed area. The Celestron C5 has a primary mirror 5 inches in diameter and a secondary of 1.9 inches. The percentage of light lost is 12%, giving it the same brightness as a refractor with an objective of 4.6 inches. The Questar and the C90 have 3.5 inch primaries and 1.125 inch obstructions, giving them the same light grasp as a 3.35 inch refractor. Scientific studies (as sighted in a very interesting article by William P. Zmek in the July 1993 issue of Sky and Telescope Magazine) show that the amount of lost contrast is exactly equal to the effective aperture of the scope, where effective aperture equals the diameter of the primary mirror minus the diameter of the secondary. Thus the C5 should show the same amount of contrast as a refractor with a diameter of 3.1 inches (80mm) and the Questar/C90 would show the same contrast as a 2.375 inch refractor (60mm). That's theory. In my experience, once you get over about 90mm, the contrast of a properly baffled and coated Cat scope in full daylight differs very little from a prismatic refractor of the same or very similar size. The Questar visually equals any 80 prismatic spotting scope in contrast and is noticeably brighter. The C90 with special coatings is close behind. I have not been able to compare either scope to a high quality astronomical refractor with an objective of exactly the same size...perhaps there you would begin to see some difference. Even there, unless the refractor was exceptionally well corrected for chromatic abberation, the extra saturation and definition in the colors through the Cat might offset any loss of contrast enough to make it very difficult to say which provided the better image. Only in a small Cat like the B&L;8080 does the brightness and contrast loss become apparent, and then it is no worse than theory. The 8080 should be as bright as a 70mm refractor and have contrast equal to that of a 45mm spotting scope, and that is pretty much what you see.
Remember, the central obstruction affects only brightness and contrast. You still get the resolving power of the full aperture...a fact which is amply born out by the resolution figures of the 8080 and all of the larger Cats.
Prism Pollution
One reason the Cats do better than expected on contrast and brightness, and hold their own on resolution, is the absence of elaborate erecting systems. There is no doubt that the prisms used in your average refractor spotting scope limit brightness, contrast, and resolution in the final image more than any other single factor. Putting a single prism or diagonal mirror of reasonable quality behind a Cat compensates somewhat for any contrast or brightness lost due to the central obstruction. As a case in point, you may have noted the comments on the Amichi 45° diagonal supplied with the Celestron scopes. Putting the 45° behind those Cats cuts resolution and contrast much more than any central obstruction could, and the effect increases as the focal length of the objective decreases. The 45 is reasonable behind the C5. It turns any high power image in the S80, with its much shorter focal length, to mud. Switching to a simple 90° diagonal turns the S80 into another scope altogether at high powers. It would be interesting to see what a high quality ED objective like the three­element, air­spaced in the Nikon 60mm Fieldscope II could do without the erecting prisms built into the scope. Perhaps someday someone will make a scope body with detachable, replaceable erecting systems. You would be able to bayonet on a 45° or straight­through back­end for low power convenience, and then, with a twist, replace it with 90° mirror diagonal for high powers. (We purists could keep the 90° on all the time, of course.) It is to the Cats' advantage (with the exception of the Questar, which has its own built­in conveniences) that they provide exactly that option.

Comparison Chart
Make /ModelApSup.E.PDim.Wt.ListAv.
B+L 80808025,40X12X42.75500330
Celestron C909033X10X53.9920550
Celestron C512542X14X85.61300750
Questar Birder8954,87X11X9426002200

Ap=aperture in mm, Sup.E.P.= power of supplied eyepiece(s), Dim.=lengthXmax. width in inches, Wt.= weight in pounds, List= approximate list price, Av.= rough average of street prices.
*plus eyepiece, $150 for 20X, $200 for zoom