ATM Astigmatism detection?

["Bob Walker" <tac@i1.net>]

I know, the star test is the final arbiter, but it's been relentlessly
hazy/cloudy for days.  See if this seems a valid test for astigmatism:  Do a
standard Foucaualt/Couder test, rotate the mirror 90 degrees, do it again.
See if the measurements are more different than your normal error envelope.
Since the definition of astigmatism is different focal lengths for different
mirror orientations for a given zone, this tells the tale ... BUT
abberations have different effects on image quality depending on where they
are on the mirror.  So, use Sixtests to calculate the Strehl ratio for each
mirror orientation.  If they are not significantly different, your
astigmatism isn't significant ... BUT Sixtests normally allows the radius of
curvature to float, finding the best fit to a paraboloid -- analogous to
refocusing the telescope.  Obviously, you can't do that in real life; the
focuser can be at only one position at a time.  So, what I did was make a
note of the ROC specified for the best-fit paraboloid for both mirror
orientations.  I then selected an intermediate value and entered it as the
ROC for the Target, and set it as Fixed.  I set the Target b= -1.00 and set
it as Fixed.
Flame Suit = ON
It seems to me that if both axes produce an acceptable Strehl ratio under
these conditions, then whatever astigmatism exists is acceptable.  I brace
myself for the onslaught pointing out flaws in my reasoning.
In the case of my 16" f/5.4, the best-fit Strehls 90 degrees apart are .874
and .896.  Selecting the intermediate ROC value as I described, the Strehls
are .820 and .824 respectively.  From this I conclude I have a pretty good
chunk of glass.  Right? Wrong? Maybe?
Flame Suit = OFF
Rouge Mode = ON
I'm determined to get my Strehl into the .9's, so I shall proceed with
figuring (step # 52 next) with rouge, lest the Cerox allow me to make
mistakes faster than I can correct them!

 *  Best regards, Bob
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