"donutbandit" wrote in message
...
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.
Unfortunately, you can't tell from this figure.
'1/4 wave', is one of these phrases that is often used, and doesn't really
mean very much. Firstly, you need to know if this is RMS, or peak. But, even
with this extra data, there is a problem. It is possible (for instance), to
have a mirror, which has a small 'bump' in the middle, making it '1/4 wave',
yet the majority of the surface, is fantastic. Conversely, another mirror
could meet the same criterion, and be covered in imperfections. If you
actually have numeric values for the RMS, and peak errors, you can get a
very good idea. The other number that can help to 'quantify' the overall
quality, is the Strehl ratio.
It is possible for some 1/4 wave mirrors to be the basis of a fine
instrument. I have actually seen one scope, that on test, would have
probably only 'scraped home' as 1/2 wave, yet is superb (it had the central
hump described above).

Best Wishes