Re: ATM 6" mirror

[The Silent Observer <silent1@ix.netcom.com>]

Michael Coslo wrote:
> 
>         Here are a few things to try:
> 
>         Check the manner in which you mount the primary and secondary
> mirrors. If you use clips, are they binding on either of the mirrors?
> Especially check the secondary, lots of bad mirrors are actually
> poorly mounted secondaries.
> 

Another source of astigmatism at the secondary (which, I might add, you
can probably replace for around $30 if you find it's bad) is if it's not
flat.  I'm told it's fairly common for diagonals to have some sphericity
(it's much harder to get a real flat than a perfect parabola), and if
so, the off axis reflection from a spherical surface will introduce
astigmatism.  If, as suggested below, you can rotate the primary and
don't find the astigmatic axis changes, and assuming you don't find a
pinch or strain in the mounting of the diagonal, I'd be very inclined to
suspect the secondary isn't flat.

>         Try rotating the primary if you can. If the astigmatism
> changes directions when you rotate it, it is the primary, if not,
> look elsewhere.
> 
>         I'd be willing to bet a cup of coffee that your problem is in
> the mounting system, and that is good news, because you can do
> something about it.
> 



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Opinions expressed are my own -- take them for what they're worth
and don't expect them to be perfect.