(sì lo so che Don Pensack non capisce ninete!... ).
CITAZIONE
Well, after 35 years of observing at the same site, with scopes of 90mm through 32", I can give you some idea what I see and have seen when viewing planets and double stars.
1) Small refractors usually beat small reflectors. I don't believe this is primarily due to the reflector vs. refractor debates but most often because the small reflectors are of poor optical quality (and made to a very cheap price) and poorly collimated, while small refractors are usually of high optical quality (where I observe, likely to be triplet apos), and good optics beat poor optics.
2) Reflectors of 10" and larger nearly always show more details on planets and resolve closer double stars, even in poor seeing. I say nearly always because the average 10" reflector is an inexpensive instrument while the average 7" refractor is likely to be an AstroPhysics where I observe, and there the optical quality will usually win.
3) Even at this late a date, most reflector owners are totally unaware of the thermal problems in their own scopes. I measure temperature differentials, mirror-to-air, of 15 degrees or more on a lot of scopes, and you simply can't expect the "local seeing" (i.e. in the scope) to allow the image to be good.
4) properly cooled and collimated, I ALWAYS see more planetary details in the big scopes, whatever the seeing. I've seen details on Jupiter in some optimized 12.5" scopes that a nearby 9" refractor could not see. And one image of Jupiter in a 28" was the closest thing I've see to a Hubble picture in a visual instrument. The really big difference between large scopes and small scopes is the color in the image. You would need an artist's palette to begin to depict the huge range of colors visible with larger apertures. Jupiter has corals, ochers, whites, grays, greens, blues, yellows, beiges, sages, salmons, blacks, reds, pinks, and a ton of other pale tints that are simply not seen in small apertures.
5) Sirius B is easy to see in a good 4" apo (and possibly smaller), but hard in most 10-12" reflectors. The reasons are multiple: thermal issues, optical quality, light scatter, poor collimation, dirty optics, etc.
But, cooled, collimated, and cleaned, a 10" reflector sees it just fine, even at f/4.5.
So, in retrospect, the issues I see with reflectors aren't due to secondary obstruction, or even seeing problems in the atmosphere. The bigger aperture always does better at "cutting through seeing" than the small aperture. The main problems are poor attention to thermal issues, collimation, cleanliness and light scatter suppression, and something that is often the "luck of the draw"--optical quality.
It makes sense, really. The typical 4' apo is what, $4000? What does the image look like in a 10" $4000 newtonian? Pretty good, I'd say, handily beating the 6" apo. (I've seen one or two over 35 years)
But is it fair to compare a normal $500 10" reflector with a $10,000 6" apo? Probably not. That so many of those 10" scopes give the 6" apo a run for the money says that aperture is a big thing, but it only gets you so far.
In the real world, then, where things are as they are, if my choice were between a 5" reflector or an inexpensive 4" achromatic refractor, I'd pick the reflector to avoid chromatic aberration (my old 4" f/15 Unitron was a great scope, but who builds f/15 refractors any more?). And I would read up on optimizing that reflector by cooling it, collimating it, cleaning it, and optimizing the suppression of light scatter.
But if the choice were between a 12.5" f/5 newtonian and a 4" apo (comparing scopes at roughly the same prices), is there really a contest? I own both, but the images in the 12.5" are simply better, in all seeing conditions, than the 4" apo.
The 4" apo gives "pleasing" views of the planets, and stupendous, almost-pictorial, views of the Moon. But the 12.5" on the same targets makes it seem like the 4" apo is viewing the universe through "fuzzy glasses". Even when the seeing is really bad. True, in the 4", the image is dancing around, but staying in focus, while in the 12.5", the image is more stationary but going in and out of focus more. But the 12.5" still sees more, even at the same magnification. The resolution difference is too big a hurdle for the 4" to cross.
