Old parabolic mirrors from AASC

[Foxinsox (Graeme Fox)]

I have owned for an old F7 8" Newtonian Reflector from Amateur Astronomers' Supply Company In Crows for many years. I also own a 12" Meade Dobsonian trust newtonian that I bought from Bintel a few years ago.

Is the new mass produced 12" mirror in my Dob the same parabolic shape of my ancient 8", hand-made Pyrex mirror in my AASC Newtonian ? Or is it spherical ?

[barx1963 (Malcolm)]

Without knowing much about your 8", they should both be parabolic. At f5 the 12" would be horrible if it was spherical, and my old 12" had the same make mirror and it was pretty good.
As I understand it, and the experts on optics can correct me if I am wrong, on slower scopes a spherical mirror is close enough not to cause major issues, but I think you need to get to f8 or slower to get that.

Malcolm

[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 !