[ATM] astigmatism in star test

["Jerry B. Hillman" <JBHillman@ev1.net>]

Hi all,
>From my own experience, astigmatism is very easy to introduce into a good
optic.  I have tried everything I could think off to support my mirror
without inducing astigmatism.  Both inside the box and outside of the box
thinking.  Some of my attempts were very different.  Several different sling
designs using different materials, several different two point edge support
sytems, using pads, wire, ball bearings, etc. I even wrapped the entire cell
in duct tape.  This actually worked for awhile, till the duct tape started
shrinking.
My latest attempt is a sling made from the same metal strapping tape that my
spider is made from. This tape is the same width as my mirror is thick,
attached to the cell 180 degrees apart at the halfway point of the mirror.
The first star testing session yielded the fact that there is no visible
astigmatism in my primary mirror, or the rest of the scope for that matter.
Additional star testing will be done when the skies clear, but so far I know
that Ronchi testing on a star yields straight lines edge to edge regardless
of how many lines I have visible, all the way down to one.  The star test on
Arcturus shows a focused star image that expands evenly either side of
focus, with the inside of focus image being somewhat sharper than the
outside of focus image.  The black hole in the center of the out-of-focus
image shows up sooner in the inside of focus image than the outside of focus
image.
This testing was done under less than ideal conditions, so, as I said, When
the skies clear.
BTW, my mirror was undercorrected.  Now, not so much.
A note on bench Ronchi testing.  If you don't know exactly how many lines
per inch, and Ronwin20 wants black lines per inch, don't bother even trying.
The results will be meaningless and worse yet, misleading. This will mean
more to those of you who make your own ronchi grids.
Clear skies, Jerry




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