Re: [ATM] Detecting Astig in Ronchi images

["Jeff Baldwin" <bald@centurytel.net>]

I have a hard time testing a mirror for astigmatism with ronchi bars.  I 
have had much better success putting a focuser and an eyepiece next to the 
ROC while I shine a small mag light at a ball bearing at the other side of 
hte ROC.  You focus on the "star" seen in the ball bearing.  I use tiny ball 
bearings for this.  If you use the highest power eyepiece (and maybe a 
barlow) you'll ever use in the telescope while you are at the ROC, then it 
is twice the magnification of the spot since the ROC is twice the f.l.  If 
you can't see it here, you won't see it in your telescope.  You may be way 
ahead of me in that you are going to scan the images and do a lot of math to 
determine your astigmatism, but it seems that directly viewing the "false 
star" at the ROC is going to tell you instantly how your surface of 
revolution is (or isn't).
Bald

Jeff and Glenda Baldwin
Beaver WA
http://home.centurytel.net/bald

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