Quote:
Originally Posted by Shiraz
On the same cloudy weekend...
The atmosphere will limit you to maybe 1.5 arcsec FWHM in excellent seeing on long exposure imaging (if you are lucky). There is not much point in getting optics that will do a whole lot better, since you will not see the difference. Average commercial 1/4 wave reflecting optics of 8 inches and above will do way better than 1.5 arcsec FWHM if properly mounted, so they are more than good enough - you will not get any better results with 1/12 optics than you would with 1/4 wave optics. Even if you have top notch optics, you will compromise that when you put in the necessary coma corrector. A good example is the MPCC coma corrector, used effectively by a lot of Newtonian imagers. It is likely that it turns diffraction limited optics into 2/3 wave optics, but even that is quite good enough in most seeing. If you have truly exceptional seeing, a better CC may just possibly be warranted, but 1/4 main optics will still be more than good enough.
Situation is different if you are observing visually or at high framerates and magnification - that is where better optics may pay off, since you can get fleeting bursts of good seeing that allow viewing or imaging near the diffraction limit. But even then you may only see some advantage if the mirror is cooled close to ambient to keep boundary layer effects at bay.
Then there is the difference in mirror mounting systems. If you buy an expensive scope, you should expect astigmatism, pinching etc to be sorted out, but a cheap OTA may have both in abundance - not because the optics are no good, but because the mounting systems are not up to scratch. Poor alignment stability may also make it more difficult to set up a coma corrector in a Newtonian, so scope mechanical quality definitely does matter.
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So why were some of the now ( no longer produced ) RCOS scopes ion milled to 1/100th wave accuracy?