This topic has been discussed here before.. Still I think it would be worth to present some more information in a form of pictures, without too many words
Recently I obtained MC Rubinar F8/500 (
http://www.rugift.com/photocameras/r...8_500_lens.htm).
The lens is very light-weight and extremely portable...
I attached here crops from image taken with it (on my 400D), from central part and from one of the corners. (30sec, ISO1600). The crops are 100% size (no resampling or resizing, and no any other form of processing, just a screen copy from RAW file displayed with DPP).
It is visible that the stars are not ideally round, but there is no difference in their shape from central and corner areas of the image.. Which means flat field and no (visible) coma and/or CA.