Re: [ATM] Which diameter gets worked for a given stroke direction?

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

Astigmatism occurs when the mirror and grinding/polishing work isn't done
right.
The admonition constantly is to "walk the barrel" and turn the top item as
you go.  In addition, you should regularly reset the lower item so that it
doesn't gain astigmatism from being poorly supported.  It is the random
stroking of the mirror and lap that makes the final finish of the mirror a
true figure of revolution.  Working the mirror more than 3 strokes in any
particular direction will make the mirror astigmatic to a supprisingly large
amount!  Keep the mirror and lap moving in what angular direction that you
are stroking and you will gain the true figure of revolution that you
desire.  Also note that turning the lap with the mirror and the stroke
direction (basically keeping the mirror aligned with the lap but stroking
the top in a different direction) will not alleviate the probability of
getting a saddle shaped surface which will show as astigmatism.
Bob May
bobmay@nethere.com
http://nav.to/bobmay
http://bobmay.astronomy.net

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