Refractor contrast vs aperture advantage

I built a 6-inch Newtonian with a tiny secondary mirror on a curved spider. The increase in contrast over the old larger secondary was quite noticable. My 6-inch Newt outperformed 8-inch Schmidt Cas scopes at a star party (for contrast and detail), using Mars as the sample target. My question is, when does aperture become an advantage? Will I get better planetary images with a 9.25 or 11-inch Schmidt Cas (with 33% obstructions) than I do with my 6-inch Newt (with 12% obstruction)? By "better" I mean enhanced contrast and more planetary detail as well as sharpness. Anyone have any experienced with reduced secondary obstructions? Thanks!

Larry, What is the f ratio of your telescope? Your diagonal is a 3/4"(12%)?

Larry Brown said:

I built a 6-inch Newtonian with a tiny secondary mirror on a curved spider. The increase in contrast over the old larger secondary was quite noticable. My 6-inch Newt outperformed 8-inch Schmidt Cas scopes at a star party (for contrast and detail), using Mars as the sample target. My question is, when does aperture become an advantage? Will I get better planetary images with a 9.25 or 11-inch Schmidt Cas (with 33% obstructions) than I do with my 6-inch Newt (with 12% obstruction)? By "better" I mean enhanced contrast and more planetary detail as well as sharpness. Anyone have any experienced with reduced secondary obstructions? Thanks!

Yes you will. The old saw, "Aperture Always Wins," still applies. A good C8 will deliver more on the planets.

My observing experience has shown that the "conditions of use" are a big factor in what you see at the eyepiece. What I mean is that "aperture rules" only when allowed to rule under the right conditions. Number one is that the big aperture must be good aperture (well corrected). I have also found that the larger the central obstruction is in a telescope, the more sensitive a telescope is to the seeing conditions. Despite all the number crunching concerning an obstructed telescope's contrast factor etc. etc., I have noticed an obstructed telescope's seeing sensitivity more than that of the contrast factor through many obstructed instruments.

Temperature plays a big part in telescopes as well. If you don't control the "weather" in the telescope it will not perform as good as the aperture should allow. Collimation is another factor in performance. Many SCT's are severely crippled in planetary performance by many who don't know how to properly keep one collimated.

I could mention more factors but I hope you get the idea. A well corrected 8" SCT should beat a 6" with a minimum obstruction on planetary work but this is not always the case :S There are a lot of things that have to be right with the SCT.

I'm currently using a 10" F/8 Newt with a 15% obstruction.

Question: the telescope you compared your well designed instrument to, did it have good overall correction, was it well collimated, cooled off?

Keith

I don't claim to know everything but I do have a lot of experience in designing, building and using telescopes. Almost 40 years. There are some of us that have had a lot of hours behind the eyepiece and are not trying to be know-it-all's when we share our real life telescope experiences. At one time I was quite content with my Sears 60mm refractor when I was stationed in Alaska. That was a long time ago.

By your response it seems as though you have never seen Jupiter or Saturn through a good 8" to 12" telescope on a night of good seeing. A crisp view Saturn or Jupiter at 400x is not to be forgotten. Though a 4" will give a good "view" it can never match the details seen through a large high resolution telescope.

I'm not aganist smaller telescopes. I have a 90mm refractor that I use quite often. I use it on those so-so nights of average seeing. Very rarely is the seeing bad enough that I have to put the scope away. I can't say the same for my 10" reflector.

Despite how good it is, the 90mm refractor can never approach the level of definition that my 10" can provide on those rare nights of good seeing. Some of us are willing to pay the "price" (either in dollars, comfort or patience) for such views.

Keith

Unless you never have anything else to do in your life, most people do not have the time to wait two hours, especially when work awaits you early the next morning. If I can see 4 or 5 bands clearly with my old 102, why wait 2 or 3 hours to try to see one more band, which will not be as clear.

My experience:

It is pretty simple to plan ahead far enough to set the scope out an hour before I plan to observe and start the fan running.

I have a good 4 inch apo refractor and I do use it very frequently. But if I want contrasty, detailed views of Jupiter, Saturn, Mars, there is no doubt in my mind that I will go with a Newtonian of 10 inches or more. Viewing Jupiter, it's not just the number of bands, it's the details and contrast within the bands, this is what aperture buys you. A 4 inch might make 200x on a good night but the image is dim and near the limit of the eye's resolution but at 200x, a 10 inch is just getting started.

It's true that for a given given aperture, planetary contrast is greater for a telescope without a central obstruction, this is a result of diffraction. But it is also true that the major diffraction effect is the result of the "outer obstruction", that is the aperture of the scope... that's why a 10 inch Newtonian will easily outperform a 4 inch refractor on any night when the seeing is sufficiently stable to make viewing the planets worthwhile.

Jon

Here in "Snowkane", I mean, Spokane, we get over 110 nights of frost. So I have a spot made-out in our not 100% insulated garage. When I know the good night is coming, the to-be-working scopes get to camp in the cold a few nights ahead. No waiting, just a very frigid telescope waiting to be hugged (and stuck to if you like to lick 'em!)
Yes, waiting extra hours for a cool down is a pain. But if you plan ahead an inch....

All the best