Quote:
Originally Posted by astro744
You will notice more coma in an f5 or lower ratio Newtonian than in an f6 system although it is still present in all. Most users can live with the coma at f6 but most users use a Paracorr at f4.5 to f5. f5 is the fence where some users fall one way and others the other with regards to whether the coma is objectionable or not. An f5 system too requires better corrected eyepieces than an f6 system although the difference is more noticeable from say f5 to f8.
With regards to better corrected eyepieces even Tele Vue (highly corrected and tested to f4) will show coma as it is a function of the primary mirror not the eyepiece. What they wont show is astigmatism which poorly corrected eyepieces are masters of. In fact seeing coma alone with no astigmatism is almost pleasing (until you put in a Paracorr and stars are pinpoint to the edge!).
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Thanks. Interesting stuff about coma. Would F/4.7 be acceptable? Providing the mirrors are quality (which I gather they are), and the scope is well collimated (which I'll have to check and make correct every time I use the scope), then is it fair to say that F/4.7 is something you can get away with, without the need for additional devices? From the reviews I've read, the 10 inch skywatcher collapsible dob is well regarded.