Re: ATM Question about grinding

["Bob May" <bobmay@nethere.com>]

The Sharpie test is useful for finding out if the surface is spherical
(provided you rotate often enough) but it won't tell you if the surface is
fully ground out.  The "Red-out" test will tell you if the surface is all
ground to the same amount but won't necessarily tell you if the surface is
spherical.  I will say that if the surface is fully ground out the
probability is very strong that the surface is also very close to being
spherical and the Sharpie test is done with the last (hopefully!) wet to
insure that the surface is indeed spherical.
Each test in its order.
Bob May
http://nav.to/bobmay
bobmay@nethere.com
NEW! http://bobmay.astronomy.net