RE: ATM astigmic mirror.

["CSC" <optic@gwi.net>]

Jerry,

	Any small pinhole will do.  If it's really small, like 1/200" it is
probably below the size of the airy disc, that tiny dot at the center of the
image, and acts like a true star.  But for astigmatic check it doesn't have
to be star-like.  What will be most important is if the inside/outside focus
images show the diffraction rings, and how they look.

	As someone pointed out, there are some star test sites out there, one
called Aberrator is pretty neat.  The diffraction ring structure tells about
correction errors and TDE.  Your sphere should be about equal.  I think
bench star testing a sphere is instructive, as you get a feel for what a
paraboloid star test looks like in a finished good scope.

	First, re: astigmatism, what you need to know is if the out of focus images
show a non-round shape and if that shape is because of the test setup or is
in the glass.  Marking and rotating the mirror eliminates test setup errors.
If you get a weird shape on one side of focus, and the opposite shape on the
other side, you can conclude that there's some odd areas on the mirror.

	I've been through this, and have had a bear of a time to polish out
weirdness.  Something about the pitch flowing and conforming. Reluctantly, I
have simply bit the bullet and gone back to 12 micron for an hour, repolish
correctly and been much happier.

	When I used pinholes, I made several different sizes close together, and
used the brighter ones for alignment, then used the finer ones to check
image quality.  Alignment is sometimes not so simple, but once you get a
method  figured out, it's a fascinating study.

Yer on the right road,

Colin



-----Original Message-----
From: Jerry B. Hillman [mailto:JBHillman@ev1.net]
Sent: Tuesday, August 20, 2002 10:47 PM
To: CSC
Subject: Re: ATM astigmic mirror.


Hi Colin,
I did it, sort of. I made a pinpoint light source and managed to catch it in
the eyepiece.  It didn't look like I thought it would since my pinpoint
wasn't perfectly round, but I can remedy that tomorrow at work. I work for a
printing company so I can create a piece of black film with a 1/200" clear
dot in the middle.  Will this work or do I need to keep trying with the ball
bearing system.
Thanks, Jerry