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Old 22-11-2013, 02:43 PM
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If you really want to see how good Newts and SCTs are Allan, have a look at what the planetary imagers do - Newts and SCTs are the main two types of scope used and both can get down to the diffraction limit in good seeing. For DSO imaging though, the atmosphere is so messy that it really doesn't matter how good the scope is (within reason) - the atmosphere will determine what you get, not the scope. Arguing over spot sizes completely misses the elephant in the room - the seeing.[/QUOTE]

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.

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