Re: [ATM] Astigmatism on thin blanks

[Starmutt@aol.com]

I  am an old timer that rarely contributes to the list but in this case I 
will. I  have made a few dozen mirrors and if you are careful and do your 
homework I find  the thin issue to be a non-issue. I have been lucky or just careful 
and have  never had the issue with astigmatism. Thin has many advantages that 
should not  be so easily dismissed due to fear or misgiving. Saving an hour a 
night in  observing time is not a small issue. The main issue to me was speed 
of the  mirror. If you want a super figure on a thin mirror it may be a 
problem  especially if you are new to the task. I would fall back to a thicker 
mirror  depending on the need for a fast mirror. I would also indicate my 
preference for  good old Pyrex if making a thin mirror. The time needed to figure the 
mirror if  made from plate glass in a thin scope is not a small affair. Sure 
you can still  use the scope but individual need is a factor. For casual 
observing both thin  and plate glass may be acceptable. I tend to go for Pyrex when 
making a thin  mirror and definitely fine annealing. Overall the addition of 
cost to have the  glass annealed is a wise and almost necessary move. That said 
I have several  fine 13" plate glass mirrors that produce a fine image and 
lots of hours of  enjoyment. Overall my favorite is still a thin 22" fine 
annealed Pyrex mirror at  f 5.25. Just a note, though I have been a silent reader of 
the list I find it's  insights without equal. I have enjoyed the pro's and the 
newbies and read every  word. The archives are prizes to be mined and most of 
what people ask are well  covered somewhere in the past listings. Read a lot 
and reread them again. Thanks  to all. 



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