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RnSdriver
journeyman
Reged: 08/14/09
Posts: 5
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Here is some links to a chip in a low cost mirror blank I picked up not too long ago. After getting the blank I found out that a guy at my church use to teach mirror grinding classes before he moved here. When I told him of the chip in my blank he advised me to grind the chipped surface because the chip would cause a aberration in the non chipped side. The blank is 7 1/2 x 1 1/16.
I was wondering if any one had seen something like that show up in a finished mirror before. I'm planning on grinding the mirror to something close to F/6.
http://www.cloudynights.com/photopost/showphoto.php?photo=19791
http://www.cloudynights.com/photopost/showphoto.php?photo=19790
By the way this is going to be my first mirror so I will be happy to get it any thing better than 1/4 
Terry
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neo
professor emeritus
Reged: 02/12/08
Posts: 610
Loc: Iasi, Romania
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Hi Terry!
I believe there is a misunderstanding about that "aberration". Maybe he refered to grind that chip in order to prevent chipping the surface even farther.
You can use the othe side of the blank to grind your mirror.
Just grind a bit the chipped side untill you get rid of those sharp edges so you can be sure that the chip doesnt get wider.
Alex
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Russian 15x50 binos
Home made 8" f/5 Newton on eq mount
Home made 70mm f/6 (Rodenstock Rotelar lens) Apo refractor
www.astronomy.ro
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3121
Loc: Northeast
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" had seen something like that show up in a finished mirror before. "
Yup, got a flat on my bench with a chip out of the back at this moment. Nothing within 2" of that chip on the other side is any good. Chip is a clam, about the size of a quarter.
Mark
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So many projects, so little time!
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3121
Loc: Northeast
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Oh, the flat is about 1" thick, and ~12" diameter.
If the chip ocurred after figuring, you can refigure it with no issues if there's a defect. Otherwise, it's kinda useless to suggest whether there is any effect without testing. That's the first priority.
M.
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So many projects, so little time!
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RnSdriver
journeyman
Reged: 08/14/09
Posts: 5
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Well this started out as someones attempt at making a flat, so I have a ways to go before I get to figuring or testing. I'm guessing I should make sure the chipped side is flat and get started hogging the other side and see how it turns out.
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3121
Loc: Northeast
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There you go! Have fun.
M.
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So many projects, so little time!
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