RE: [ATM] Am I there yet?

["Jerry" <wa4guu@verizon.net>]

	For a "planetary" mirror I would not stop now. It is at best a
marginally good mirror. Since it is a thin mirror I'm not so sure how good
one should expect. It is a bit rough and a little astigmatic. But the
roughness seems to be areas such as at 7 to 8 oclock from about 1 inch to 2
inches in from the edge that look like a few square inches of dug out area
that don't extend all the way around in the zone. There are a number of
other such areas distributed somewhat in a zonal pattern but astigmatic.
Also look close to the edge just outside that depression I mentioned at 7
oclock and compare at the half inch wide edge zone at the 7 and the 11
oclock position and see the difference.

	The astigmatism is most easily seen if you picture a close to, but
not quite equilateral triangle with the base  higher on the right side than
the left. The top of the triangle points to about 11 or 11:30. Do you see
that? It is real astigmatism probably from poor support or not rotating the
mirror in its support frequently enough as you work if you are working TOT.
If you are working MOT then you are turning your mirror in increments that
are too regular and too large an angle (in this case 120 degrees) and
working across the same diameters too long. I'm sure it must be TOT and
either three support points and infrequent change in mirror and support
orientation. It may be you need a better support system than what you have
for mirror that thin. 

	I'm guessing the low areas are probably too much weight or tool not
pressed well or both. If you are into figuring with 3 to 5 minute or less
sessions then maybe something is changing drastically in the short time from
beginning of the session to the end. So the work doesn't get done evenly all
the way around in the zones. I'm thinking a possibility is changing the
temperature from ambient prior to work. Some people give the lap a bit of
warm water treatment and a short press just before work. Personally I don't
like that idea. The heat will go from the lap to the mirror unevenly and may
be the source of such areas as the one at 7 to 8 oclock and 1/2 to 1 and 1/2
inches from the edge. I like to have the mirror and lap press without weight
at working temperature until rather well stabilized at room temperature.
This will get more uniform contact. Put a damp towel around the the edge of
the lap and mirror to keep it from drying out while pressing. Go as slow
with the stroke as you can and still keep steady motion.

	That triangular astigmatism will require frequent reorientation of
the morror on its support. On the other hand if the lap is on the bottom and
you have three supports at 120 degrees don't bother reorienting the lap with
respect to the support. The lap was pressed in with the tool supported at
those points. You want all the work done in a session with the lap supported
just as it was supported when it was pressed.

	The edge could be better but it could be a lot worse. Notice that
you can judge differently depending on what clock position you chose to look
at.

	With the general roughness and the astigmatism you can not rely
heavily on your zone readings.

	You have probably said before but I don't remember. What thickness
is your mirror? Hope something I said helps. 

Jerry





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