[ATM] Help my 17.5 inch coulter mirror is astigmatic

["Carl Vehmeyer" <carlveh@wxs.nl>]

Hello everybody,

 

7 years ago I bought a old coulter 17.5 inch mirror to build my first big
telescope. Unfortunately I wasn't able to test the mirror at that time and
could only startest the mirror after I had finished the telescope a year
ago. It turned out to be undercorrected and had severe astigmatism. The
mirror looked somewhat like this mirror
http://www.astro-foren.de/showthread.php?p=34674 . With help from some
fellow amateurs I tried to correct the problem by refiguring the mirror. We
started out with grit 400 and worked our way up. All seemed to go well, but
after polishing and figuring I ended up with well corrected mirror which
still has astigmatism. We're sort of stuck, any help or advise is welcome.
Does anybody have experience with astigmatism of this kind, Coulter mirrors
or with refiguring.

 

Thankful for any advise,

 

Carl Vehmeyer

The Netherlands

 

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