[Wavytone]

Mirrors from AASC... If they thought the buyer knew chalk from cheese, you got a good one. If they could tell you had no idea what you were buying, you got a poor one.

The way to find out is to do an optical test, either a star test or, if you have access to the equipment, perform a quantitive bench test (interferogram, Foucault or Ronchi etc...). An f/7 8" mirror absolutely must be parabolic (I had one many years ago). The wavefront error for a spherical mirror is given by the equation

w = 22.55 * D/F^3

where
w = peak to valley wavefront error
D = diameter in inches
F = f-ratio

w <= 0.25 is diffraction limited.

For a 4.5" (114mm) f/8 spherical mirror, w = 0.2 which is
diffraction limited (1/5 wave). For a 130mm f/6.9, w = 0.35,
more than 1/3 wave, which is not diffraction limited.

A 6" f/8 if left spherical would have a w = 0.26
which is right on the border of diffraction limited.

The classic Edmund "Palomar Jr." used a 4.25" f/10.6
spherical mirror, where w = 0.08 or about 1/12 wave
which is why they gave excellent images for a low
cost.

At the other extreme, The Bushnell clone of the Astroscan
uses a 4.5" f/4.4 spherical primary. This would have w = 1.19
which is.....really really bad - no matter what eyepiece you use, the image is awful.

The limits for small scopes where a spherical mirror will be OK are:

4" f/7
6" f/8
8" f/9
10" f/10
12" f/10.5
16" f/11.5

A 6" f/8 is fine as a small scope and entirely practical, but a 12" f/10.5 is quite a monster !