[ATM] re: Working with thin mirrors

["Ellen Mackenzie" <mackenzieconsulting@sympatico.ca>]

Kudos to Bob May, in the time I have been following the list this is the
first time a passing mention is made of "walking the barrel" this is
probably the most important item a newbie must remember is to rotate the
mirror and walk around the tool to keep the wearing down even and spherical.
I usually rotate the mirror (MOT) about 20 degrees and three or four Ws and
re-position myself again - - I have often thought of an arrangement like a
potter's wheel to eliminate the walking, but the grit charge never lasts
long enough to wear you out.
    One item that I never see mentioned is sealing wax, the kind Lawyers use
to "seal" a document. It is a great material for gluing glass to handles on
the backs of mirrors - -  keeps the warmth of your hands away from the glass
- - - sorry for the guys with the twenty inchers though because to remove it
you put it in the freezer overnight and a smart sideways smack with a piece
of two by four and it falls off. We use it a lot for edging small lenses, it
takes a lot of punishment at room temperature. I have used it to center and
edge six inch refractors at low (10) rpm. The glass must be well cleaned. It
is getting hard to find at any but the old established stationers
    Maksutov correctors are very hard to polish unless you use an extremely
hard lap. Spherical mirrors faster than F/ 5 also polish slower on the edges
than in the center, we usually don't notice it because we aren't trying to
make a spherical mirror but when you need a few for a Shlieren setup, making
a perfectly polished sphere 12 inches in diameter at F / 2 isn't as easy as
it would seem.. The pressure varies as the cosine of the angle. It is where
you start to get artistic about the shape and number of channels.
    BTW I have always tempered my Burgundy pitch with about 10 percent bee's
wax the opticians of my time did it and so it goes -  if it was too hard I
would add a few drops of artists grade turpentine (the real stuff) from
Grumbaker (one teaspoon per quart).
	Before we lit up the world at night, Gregorians were superb for fine
detail, but nobody would think of running a scope at F/ 20 though they still
do. Remember it is still the aperture divided by five mm that gives the best
viewing for those that still look.


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