[ATM] Re: Astigmatism in a Newt

[Daniel Reinders <danreind@shaw.ca>]

Hi All,

I recently put together a 6" F8 newtonian, mostly using parts from a
christmas trash telescope.  One thing I ditched was the mirror, which
suffered from severe spherical abberation.  I ordered a 6F8 mirror (meant to
be the higher quality 1/6 wave or better) from a local web supplier.  After
collimating visually I got a fantastic sharp view of Saturn, leading me to
believe all was well - I hadn't star tested it though and I regret that now.
In the mean time I devised a wire spider support, and messed around in
general.  Now things aren't as sharp, and when I try to star test I detect a
pretty major astigmatism.  Strangely enough things continue to seem at their
best when I just collimate visually, when I've tried to star collimate (to
better center the astigmatic circles) things just get worse and worse!  Is
it because I'm trying to fix an astigmatic image?

Now I'm starting to question my recollection of the image of Saturn - maybe
it wasn't as good as I remember it and it was just such a contrast from the
previous trash mirror?  Which leaves me in a dilemma.  How do I know if I
received a bad mirror or not?  Could the astigmatism be coming from the
secondary, and if so how do I tell?  Are there any collimation failures that
could explain the astigmatism - mirror not centered, secondary not centered
in the tube (possible with the wire spider) etc?

Any advice would be greatly appreciated.

Dan Reinders


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