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Old 22-11-2013, 11:42 PM
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gregbradley
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Join Date: Feb 2006
Location: Sydney
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I agree with Peter. He has expressed better than me but he certainly is pointing out the phenomena I have observed which was my point.

His RHA image says it all. You can't get those tiny stars with other gear. The other scope I have seen tiny stars like that is the infamous ASA 12. When it works its a tight machine.

There is apart from seeing and distortions the factor of scatter.

If your argument were true then there would be no point in getting those Ion Milled RCOS's that essentially had perfect figure. Yet have a look at those Rob Gendler images taken on a 14.5 inch RCOS with Ion Milled optics in WA on his astropic site.

They have very tight stars, show detail in commonly imaged southern objects you just don't see on images from this site except Martin's from Sierra Nevada.

So there is obviously another factor or factors that come into play than simply seeing limited, 1/4 wave limited etc. Scatter is one, sharp focus as Peter points out is another. Having had many scopes some definitely snap to focus better than others. Some have sharp focus all across the field and others super sharp in the centre and not so much in the corners etc. Addded to that would be deformations in the performance of the mirror/lens with different temperatures (Paul Haese and Bird can tell you about the importance of cooled optics) as well as deformations caused by stress from a lack of proper mirror support in the scope (as Rick Hedrick from Planewave was telling me, your 1/10th wave mirror once mounted may be way less than you think). So mechanical issues as above become very important factors in getting that sublime optical system which is quite a rare instrument. I guess that is why the high end stuff often seems overengineered like Tak scopes, AP scopes, etc.

Greg.
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