Posted by Andrea Tasselli at 00:18 on 2010 Oct 19
Hi Tom,Refractor unless they are large (over 6") and for achromats long (very very long; for a 6" at least f/15 to f/20 really) are not worth considering for imaging purpose. These filters are , in my humble opinion, just hiding the issue by removing or diminishing some of the offenders. Ditto for b&w whereas you can’t see it but it is there anyway. The only safe way to image using achros is to use a monochromatic imager and RGB composite technique to get rid of CA (but you’d still get chromatic spherical aberration the faster the telescope the worse it is going to be). Reg’sAndrea T.