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Old 23-11-2013, 11:50 PM
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Originally Posted by Shiraz View Post
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
I see. I can see the sampling being part of that. But if I imaged that same scene with my high quality APOs of around the same focal length I don't think I would get stars that small. So the accuracy of the optics and that optical system is having some effect there.

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