Re: Checking Optics of 10" Dob in Day Time?

From: Scott (srychnov_at_chem.ps.uci.edu)
Date: 02/10/05 

Date: Thu, 10 Feb 2005 09:44:55 -0800

In article <420b0071.1763263@news.planetkc.com>, Dan Chaffee
<dchaffee@blitz-it.net> wrote:

> Actually, Carl does measure astigmatism with the simple foucault test
> and the couder mask. My experience is that large and medium scale
> roughness is VERY easy to recognize and I had no problem seeing this
> in my ealiest mirror making days. Micro-ripple is tougher to confirm
> and requiring some experience, but not as damaging as large and medium
> scale roughness.
>
Dan,

Astigmatism is very hard to measure in a Foucault test unless it is
really bad. I participated in a mirror testing program with the ATM
list. One of the mirror was astigmatic by about 1/4 wave. Of the 15
people who tested the mirror by Foucault, Ronchi, and other simple
bench tests, NOT ONE OF THE TESTERS IDENTIFIED ASTIGMATISM! However,
there were about four interferometry tests, and all four tests easily
identified the astigmatism.

I know that Carl tests for astigmatism. It is easier to identify
BEFORE PARABOLIZING with a "ball bearing" bench test. I suspect that
that is how Carl does it, but I don't know.

Back to Wayne. You can do a star test either at night or in the day
using a remote star or artificial star (see Suiter's book.) Any more
sophisticated bench test will require you to remove the mirror from the
telescope. You could track down a mirror maker to help you set up a
bench test like the Foucualt test. If you wanted to I could help you
do it, but we are not very close in space.

Scott

Relevant Pages

  • Re: Checking Optics of 10" Dob in Day Time?
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