Re: ATM Astigmatism

["Bill T." <twentiethwave@hotmail.com>]

I've been star testing a 20", 1.5" thick mirror these last several nights.

A Ronchi null test against Arcturus easily shows the difference between when 
the mirror is supported by it's full 18 point cell, as opposed to when it is 
supported only by the ends of the three collimation bolts.  This result is 
with the scope pointed ~20 degrees from vertical.

Whatever the PLOP-predicted deformation difference between an 18 point and 3 
point support might be, that amount of distortion is detectable with a 
Ronchi null star test.  I estimate that maybe as little as 1/8 that amount 
might be noticeable on steady night.

Bill T.

P.S...  In regards to my earlier question about spin figuring, thanks for 
the overwhelming reponse.  I have been spinning a spherical mirror on a 78 
RPM platter in the hot sun for over a week now, but the glass has not yet 
flowed into a parabola.  Am I doing this right?  Is clockwise the correct 
direction for the Northern Hemisphere?  Is it possible to achieve a parabola 
in a single Summer?






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