Quote:
Originally Posted by naskies
Given the 1625 mm focal length, the field curvature's not as intense as with say a short focal length refractor. I have a KAF-8300 chip CCD too - the curvature is noticeable in the corners if you look for it, but it's mild enough that you could probably get away no field flattener.
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Actually, it
is as "intense" as in a short FL refractor. That is why many people find that refractor field flatteners work well with their RCs.
<in a Cassegrain radius of field curvature is proportional to difference in radii of primary and secondary mirrors. Flat field systems have R of primary similar to R of secondary; but that necessitates huge obstructions so it is not done often; and definitely not in GSO RC case.>
Using the data measured by Wolfgang Rohr
http://www.astro-foren.de/showthread...7440#post47440
I find radius of curvature of the field to be around -320mm, which is pretty much equivalent to a refractor of approx 800 to 900mm focal length.
Main problem BTW with those RCs is vignetting. You have about 5-10mm (at best) of unvignetted and then it drops of fairly rapidly as cone gets cut off by baffles. So for most of your frame you are operating more at f/10+ after you factor in the obstruction. Large chips will be even worse.