Re: [ATM] Ain't got no shperometer

[ArtfulBodger <artfulbodger@earthlink.net>]

Guy Brandenburg wrote:

> ... then it is very, very difficult to 
> avoid getting something very, very close to a sphere.

Usually, yeah.  But here's the thing: I'm trying fix a problem.  I 
skrooditup, and I did it thusly: I'm grinding two mirrors, both 8", 
one f/4.5 and the other f/5.5.  Pyrex blanks, tile tools.  So, 
obviously, the 4.5 tool and mirror are more curved than the 5.5, 
right?  Well, one fine day I took the 5.5 mirror and started grinding 
it on the 4.5 tool.  I was using 240 grit, I think.  It felt funny 
and after ten minutes I figured out that, big duh, I was using the 
wrong tool.

Man, it's tough being an idiot.

All the pressure was on the center of the mirror, and I ended up with 
a big hole in the middle.  Ten minutes was enough to do the damage.

It wasn't catastrophic.  It took a good while to get back to the 
right sagitta, but then I still had a very aspherical mirror -- flat 
outer zones, crater in the middle.  This was obvious by spherometer, 
by Sharpie, by looking at the bubbles as I ground ... but not by 
feel; the push felt smooth and even when grinding.

So anyway, I put plenty of time into spherizing the thing, while 
keeping the depth right.  1/3 COC, alternating MOT and TOT.  Now it 
seems good.  I think I've corrected the problem.

But I want to be sure.

 > Grind more, worry less.

Well, yeah ... but you can see why I'm worried, right?

Pete

-- 
Artful Bodger
http://www.artfulbodger.net
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