RE: [ATM] Re: Astigmatism in a Newt

["Thomas" <tomjan@ozemail.com.au>]

A quick and dirty way is to rotate the primary, if this is the problem
the axis of astig will move, if it is the secondary the astig will
remain in it's old orientaion.

Clear skies,
Thomas Janstrom
http://www.tjanstrom.com
http://www.norsewines.com.au
"Your nobody untill you've been ignored by your seventh cranio-facial
nerve."


-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf
Of Daniel Reinders
Sent: Monday, 17 May 2004 3:57 PM
To: ATM
Subject: [ATM] Re: Astigmatism in a Newt


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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