I've been intending to try this experiment for some time now. Clear skies under a bright Moon provided me with some "garbage time" to actually go ahead with it. I made a 90mm off-axis mask for my 10" f/4.5 Dob.
I used "black on black" foam-core board from Wal-Mart. While I was at it, I also made a ring baffle for the mirror end of my scope, to prevent stray light (streetlights, headlights, reflected moonglow) from entering the tube from that direction.
Now, I had my own prejudices going in. I've always felt that off-axis masks are a waste of time and aperture, and that a 10" Dob that is optically decent, thermally stabilized and well-collimated should beat a 90mm unobstructed aperture on all objects under all conditions. I thought of several potential exceptions: 1) Really bad seeing conditions, under which resolution is so limited that both apertures might reveal the same planetary/lunar detail, and the smaller one might appear to give a steadier image. 2) Using a cheap wide-field eyepiece on a bright open cluster, the image at 2.5mm exit pupil in a 90mm f/12.7 scope might be more pleasing than the image at 7.0mm exit pupil in a 10" f/4.5 scope. 3) The larger diffraction pattern, dimmer image and lack of obstructions in the 90mm might provide a more pleasing image of bright double stars, resulting in the appearance of better "separation".
Once again, the most transparent night of the month happened under a nearly full Moon. Naked eye limiting magnitude was 5.0-ish. My naked-eye assessment of the seeing showed a lot of twinkling, so I thought it was a pretty good night for the test. My first target was Saturn. In the 10" @ 160x, it showed the typical view in mediocre seeing. The Cassini Division was visible except where the rings crossed the disk. The Crepe ring was visible where it crossed the disk but only fleeting at the ansae. The South Equatorial Belt was visible as a darker band crossing the disk, and the South Polar Region was also noticeable. Five moons were easily visible. On went the mask. The image got a lot dimmer, but most of the detail was still visible. The exception was that the Crepe ring was essentially impossible to pick out at the ansae, and Tethys and Dione were difficult and sometimes invisible. The Cassini Division appeared thinner and less-defined. Large seeing tremors were still visible. The SEB and SPR appeared a bit darker against a dimmer disk. Which view was better? The full-aperture view. How much better? Probably a question for aestheticians rather than one worthy of quantification. If this were a 90mm refractor, would I be happy with the off-axis mask view? Yes. I would return to Saturn later.
Double stars were a natural for this test. Given the seeing, I didn't try to split any close pairs. Instead, I went to three that I usually look at this time of the year. First was Rigel. At 160x, the companion was easy with or without the mask. With the mask, Rigel's image appeared dimmer and steadier, and the fairly dim companion was well-separated. Without the mask, Rigel was dazzlingly bright and of course the diffraction spikes were present. The companion was much brighter as well. I preferred the full-aperture view, but both were more than equal to the task.
Zeta Orionis was next. This belt star has components of 1.9 and 4.0 separated by a little over 2 arcseconds. This was actually fairly difficult with the mask on; I had to concentrate a bit to pick out the secondary. At full aperture, it was immediately and indisputably double. Seeing at this point was probably at its worst for the night. When I turned over to M42, my first goal was the Trapezium. I thought I glimpsed the E star occasionally with the mask on. Without the mask, the E star was visible much of the time, and I had an occasional glimpse of F. Later, when the seeing improved, F would be visible most of the time at full aperture. Even E remained fleeting at 90mm, though.
OK, how did the nebula look? At 160x, even under the moonlight, lots of subtle detail was visible in both apertures. The visible nebulosity grew noticeably and got a lot brighter when I removed the mask, but I was impressed with what 90mm could show. I switched to low power, and the 32mm GSO wide-field Kellner. Despite its wide field, it's often hard for me to choke down the view through this eyepiece in the 10" f/4.5. Before getting rid of it, I wanted to see how the edges looked in the 90mm at f/12.7. While stars at the edges looked a bit better, they were still seagulls. Maybe Bonaparte's Gulls rather than Glaucous-winged Gulls, but gulls nonetheless. At full aperture, one complication is astigmatism in my eyes. I notice this at about 5mm exit pupil and up. Also, the full-aperture view should show more coma. But even working at f/12.7, which should be easier on the eyepiece, the Kellner still isn't much to crow about.
I wanted to see how open clusters looked at 36x in the Kellner. First, I pulled over to the Double Cluster in Perseus. There really wasn't any contest. In the 10", more stars were visible and they showed more of their true colors. With the mask in place, the cluster looked dull and dim. I've enjoyed the view of this object at similar magnifications in my 60mm refractor (albeit under better sky conditions). Compared to the 10", the 90mm wasn't doing much. Admittedly, this was at 2.5mm exit pupil; maybe a 50mm eyepiece would have given a more pleasingly concentrated view, but decreasing magnification won't show fainter stars. The Pleiades, with its bright stars, made for a closer match. M35, closer to the Moon, looked pitiful at both apertures, but bumping the 10" up to 76x (15mm Antares Elite) made for a very attractive view of this cluster in spite of the narrower field.
At the same magnification, the smaller aperture gives a much darker sky background. Along with the steadier star images, this definitely contributes to the adjective "refractor-like". I turned to Comet Machholz. At 36x90, it was a fuzzy round ball with a bright core. It was attractively placed in a field with 3rd-magnitude Gamma Persei. Taking off the mask was like opening a floodgate with light pouring in. The sky background was much, much brighter and less aesthetically pleasing. The stars were much brighter as well. The comet didn't seem to gain as much on first glance, but with a second look it was clear that the coma size had nearly doubled and there was a tail. Increasing the magnification to 76x darkened the sky background and made the tail easier to see. Going to 76x90, part of the tail was visible with difficulty.
The real answer to the brighter sky background in a larger aperture is to increase the magnification. Unfortunately, a higher magnification means a narrower field of view. To get an exit pupil of 2.5mm in my 10" f/4.5, I would need an 11mm eyepiece. Even an 11mm Nagler is only going to give me a true field of view of about 45'. The 32mm Kellner gives a true field of over 1.7 degrees. Now, normally I observe deep-sky objects under relatively dark skies, so an exit pupil of 4mm or so will still give a pleasingly dark background. I guess I need to see how a 20mm Nagler (ouch!) will perform in the 10". Nevertheless, I can see the pleasure that a small-aperture, short focal length refractor would give in wide-field views of star fields.
Flocking and baffling might also improve background brightness in the larger aperture. I constructed my rear baffle after demonstrating that bright moonlight reflecting off frost on my ground tarp and entering the mirror end of the scope had a noticeable effect on low-power views. When I first got this scope, I temporarily placed a piece of felt opposite the focuser, but I never felt ('scuse me) that I could tell the difference. A focuser baffle is another possibility, as is an extension of the tube further beyond the focuser. On a typical fall, winter or spring night here, frost forms on the inside of the tube after a couple of hours of viewing. I'm therefore a little dubious about putting felt or flocking paper there, and worried that perhaps they might cause problems when trying to dry out. A tube extension might act like a dew cap for the tube and delay frost formation.
Back to the sky. I took another look at M42 with 44x (26mm Celestron Plossl). At 90mm, the nebula was fairly dim in its outer regions, but showed some nice subtle detail. I could make out some faint blue-green tints. Off came the mask, and the nebula went to Technicolor. Even under the moonlight, the brightest parts were a fluorescent green and there were tinges of red in the wings. I'll take the brighter sky background.
I revisited Saturn. The seeing had improved. I worked my way up through my powers. 275x was the most consistently good power at both apertures. Yes, I said that. My Dob with a 90mm off-axis mask was giving a good image at over 75x/inch. The image at 330x90 was "usable", but definitely worse. At 330x254, the image was less stable than at 275x, but occasional steady moments favored the higher magnification. This is an example of the atmosphere, rather than the optics, setting an upper limit on useful magnification. There will be few times when even an optically perfect 10" scope will give a good image of Saturn at 800x, but on those canonical nights when "the atmosphere limits planetary observing to a maximum magnification of about 300x,"* a 90mm foam-core mask (or, a 90mm refractor) will appear to be overperforming relative to its aperture.
Now, which aperture gave the better view? The 90mm view was very clean and detailed, and relatively dim. The Cassini Division was visible except where it passed in front of the disk. The Crepe Ring was visible across the disk and fleetingly at the ansae. The same disk banding was visible, and there were some subtle brightness variations in the southern part of the Equatorial Zone. The 10" view was much brighter, and most of the details were better-defined. The disk banding appeared to have more subtle hues, but more structure was also visible. The Crepe Ring was obvious at the ansae. The Cassini Division looked wider and blacker, and was fleetingly visible in front of the disk. I strongly preferred the full-aperture view, but I'll admit the view at 90mm looked nicer than I expected. A couple of notes: 1) I expected a reduction in scattered light around the planet from the lack of central obstruction, but it didn't seem to scale down any more than overall image brightness. 2) Focusing is more forgiving at f/12.7 than at f/4.5, but it didn't really affect my enjoyment of the view. 3) If I were sketching the 90mm view, I would have categorized the best seeing periods as Antoniadi II; with the 10", I would have called them Antoniadi III.
I left the Moon for last because I didn't want to screw up my dark adaptation for the other tests. I was pretty tired by the time I got to it, so I didn't do a thorough exam. Seeing had also gone back downhill. My impressions were that the full-aperture view was a lot brighter and showed finer detail. The 90mm view looked "coarser" for lack of a better word. But, I wasn't in the mood to try to quantify it, so I called it a night.
This was a fun experiment to do on a night that didn't have much going for it. In the best of worlds, I'll try the deep-sky experiments again on a night with no Moon. I wonder if I'll find the time, though. I might find the time to try planetary/lunar/double star observations on a night when the seeing is truly suckalicious; that ought to be interesting. I doubt that I'll take the time to try the mask on a night when the seeing is nearly perfect, though. (Assuming that ever happens.)
A 90mm off-axis mask is not a 90mm APO refractor, and I'm sure some of you have been sputtering that as you've been reading. More light is lost and scattered from reflecting off two mirrors in the Newtonian optical system, versus passing through those two or three lens elements. Even if my mirrors were spotless, this might make image brightness equivalent to an 80mm refractor, and who knows about the light scattering? My 90mm off-axis Dob remains a 10" Dob with a focal length of 1144.5mm at heart, without all the finery and portability of a short-focus APO in its mount and and accessories. But, I didn't see anything to counter "aperture wins" when it comes to revealing detail, even under light-polluted skies. I also reminded myself that a lot of stuff IS visible with 90mm of quality aperture.
*Loosely quoted from "Choosing Your Eyepieces" by Al Nagler