From: "Bob May" 
To: "atmlist" 
Reply-To: "Bob May" 


The shape may be from the initial grinding which you did and didn't turn
the mirror enough and the tool enough as you went. I teach a stroke where
the mirror is basically turning by about 5-15 deg. per stroke pair by the
allowance of one hand to do the pushing, letting the wrist flex a bit and
then the other hand doing the pulling with the same results.  Every few
strokes, you want to change the angle of the path across the tool as the
old method of walking around a barrel, more like a slow shuffle, would keep
the angles on the tool constantly changing. It may also be that the tool
surface wasn't flat when you started - the pyrex blanks that are sold have
a lot of interesting shapes on them and are definitely not flat anywhere
and thus as you get close to the complete grinding of the surface, there
often ends up being odd shapes of the final flat section.
Keep grinding and you may want to drop down to 120 grit to get the big pits
out of the surface now.  If the hole shrinks by more than 50% (all of it is
really best but 75% shrinkage is good) then the next grit can be used to
shrink it the rest of the way and that will guarantee that the surface is
well ground.
The hole is more from the extremely fast shaping of the surface not getting
all the way into the center of the tool and you may be having the same
thing on the outer edges of the mirror if the glass was flat. Bob May
http://nav.to/bobmay
bobmay{at}nethere.com
NEW! http://bobmay.astronomy.net

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