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Old 23-11-2013, 02:52 PM
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Shiraz (Ray)
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Join Date: Apr 2010
Location: ardrossan south australia
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Originally Posted by gregbradley View Post
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.

hi again Greg.

Tight stars do not of themselves indicate high resolution. Sure the optics must be good enough for seeing limited performance, but the star tightness is then mainly a function of the sampling. I think that Peter gets about 1.63 arcsecond pixels when he uses a 16803 with the short focal length RH, which is undersampling in most conditions. A star that would cover say 4 pixels in Peter's system (at 1.63 arcsec) will cover about 25 pixels in your CDK/16803 system (sampled at your system scale of about 0.64 arc sec). With the 694 on your CDK, the same star will cover about 100 pixels - so Peter's stars will look tiny compared to yours, as they should - not because of any optics magic, but because of sampling.

One can certainly get tighter looking stars (and a huge increase in sensitivity) by undersampling, but at the cost of not recording the full detail in extended objects. Undersampling is certainly a valid strategy though if you want to image large faint targets - eg as Bert does.

agree that mechanical quality appears to be a common characteristic of larger high end astrographs - and I would guess that it is very often the difference between an optical system being seeing limited or being a poor performer.

Regards Ray

Last edited by Shiraz; 23-11-2013 at 09:24 PM.
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