In message , donutbandit
writes
I ran into a great deal on a 6" f/4 parabolic mirror. All the stats on it
look good, except that it is a 1/4 wave mirror.

I was raised believing that no less than 1/8 wave will suffice, but how
about a low power rich field scope? The magnification would never be
greater than 40.

The fast focal ratio and geometric aberrations are more likely to hurt
image quality than whether or not the image is perfectly diffraction
limited. Few eyepieces will tolerate fast f4 light cones and your
secondary will necessarily be rather big. The latter factor cuts both
ways.

It is probably worth a try if the mirror is reasonably cheap. Don't
expect great planetary details though!

Regards,
--
Martin Brown