[ATM] Sun Test During Rough Grinding

[Francis J. O'Reilly]

Joe,

Most of us wait at least until we have the curve out to the edge of the 
mirror before testing.

Testing with the sun will not tell you anything about the figure. And at 
rough grinding, it is premature.

Francis
----- Original Message ----- 
From: "Joe ROWLEY" <jrowley2 at msn.com>
To: <atm at atmlist.net>
Sent: Saturday, April 03, 2010 5:15 PM
Subject: [ATM] Sun Test During Rough Grinding


>
> I'm nearly finished with rough grinding on my very first mirror (10" f/7) 
> and all is looking good. My FL is correct, the sagitta is almost down to 
> my target depth and the ground surface is gradually working out toward the 
> edge.
>
> My question came up while doing a sun test this afternoon to make sure my 
> focal length was still where I want it to be. When I reflect the image of 
> the sun onto the fence in the backyard (about 2 p.m. so the sun was pretty 
> high in the sky) it is oblique vertically when I am outside of the focal 
> length, then rounds out to a spot as I step forward and then goes oblique 
> horizontally when I am inside the focal length.
>
> At first I was worried that it meant I had some really bad astigmatism, 
> but the obliqueness stays where it is as I rotate the mirror in my hands. 
> Question 1: Am I right in not worrying about it and assuming that this is 
> just a trick of the crude reflection and the angles at work? Question 2: 
> If so, (just out of curiosity because you wouldn't be into ATM if you 
> didn't want to learn about almost everything you see) what causes this 
> affect?
>
> As always, thanks for all your help and sharing your experience.
>
> Joe Rowley
> Boise, ID
>
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