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Old 22-11-2013, 04:31 PM
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Shiraz (Ray)
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Shiraz is offline
Join Date: Apr 2010
Location: ardrossan south australia
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Originally Posted by gregbradley View Post

I have wondered about this diffraction limited theory and limited by the atmosphere. It does not match my experience unless most scopes do not come close to being limited by the seeing.

Use an Astrophysics APO refractor or a top Tak or other tp brand APO and you will see much greater sharpness than say a lesser figured but still close to 1/4 wave ED80 or similar.

Longer focal lengths hit the wall of seeing much earlier than shorter focal length scopes down to FSQ type scopes not really seeing affected at all.

Rick from Planewave was telling me that they can get a mirror and measure it on a table and get 1/4 wave. But when the mirror is installed in a mirror cell that 1/4 wave can disappear and so a lot of attention was placed on the mirror cell. So these mirrors that are tested are no doubt tested on a bench before installation. If they were tested after installation they may reveal much lower ratings.

Nevertheless optics like Orion Optics UK mirrors which offer a 1/1th wave upgrade would on the surface seem a waste of money as 1/4 wave is considered diffraction limited. But as you can plainly see from Mike's and John's images the 1/10th wave mirror definitely adds to sharpness. Same with Rolf's Newt and David Fitz's mirror. They are really performing.

So perhaps getting the highest rated mirror/lens is the way to go to allow for the anomalies introduced by mounting the lens/mirror.

Marj Christensen posted once she has seen Roland rub a lens with his finger to get that last tiny bit of imperfection out. We must be talking some almost unmeasurable imperfection that would respond to that.
But you can definitely see it.

Perhaps its because APOs generally are short to modest focal length scopes and so aren't always hitting the limit of the seeing so much as the longer focal length scopes of about 1500mm plus.

This seeing limited argument does not bar getting the highest optics you can possibly get in my opinion.

With your indulgence Allan

the angular size of the resolution spot from a scope will decrease with increasing aperture, but the seeing spot remains the same size. Small scopes have spot sizes roughly similar in size to the seeing spot, so the scope quality does have a major impact - the scope imperfections are not swamped by the seeing and a better scope is obvious. This also means that small short scopes are not affected so much by seeing because the spot size (or resolution) is mainly determined by the scope and not the seeing in many conditions. Sampling also comes into it when imaging, but that is another issue.

Above about 6-8 inches in Australian conditions, the seeing dominates and the scope quality becomes less important for DSO imaging - many of the best big scopes have huge secondary obstructions that really spread out their diffraction spots, but they still work perfectly because the resulting spot, while large compared to that of unobstructed optics, is still smaller than the seeing. Some people buy high spec mirrors and then put in an MPCC to fix the coma, which probably turns their 1/10 wave Newtonian into worse than a 1/2 wave system - others use focal reducers or field flatteners that do who-knows-what to the spot diagrams of their high end scopes - but none of this matters because the resulting scope is still better than the seeing. By all means get high quality optics - you will not go wrong that way. But for DSO imaging with apertures above about 6-8 inches, it probably does not matter much. What matters more is the mechanical quality of a good scope (eg the mirror cell/focuser/thermal) and that can have a profound effect on performance - nothing is worse than a scope that drifts out of focus and alignment with temperature and attitude - on a second rate mount.

The best illustration I know of showing how fundamental seeing can be is the before and after images of the 1987 SN taken by the AAT. Top quality big scopes do not work better than smaller scopes in poor to average seeing - they just work better in very good seeing.

Last edited by Shiraz; 22-11-2013 at 04:59 PM.
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