From: David Davis 
To: atm{at}shore.net
Reply-To: David Davis 


For some reason  my first two messages got et by the net.  I suspect Yahoo.
 It's OK though as the first was a paniced rambling and the second was
defeat.  In a nut shell ...

I was given a mirror project that the owner felt had astigmatism.  He had
long since gone on to other things and sensing a kindred soul, or a pigeon,
he gave it to me.  It is 17.375 inches in diameter by 3/4 inch thick.  Has
about a F5.8 curve, (101.75 inches focal length).

I know nothing about mirrors and packed the thing away for two years until
this last March when I took it to Bellingham to the Optics Workshop. There,
two of the keynote people, Mel Bartels and Karl Zambuto, looked at it for
me, gave me some lessons in looking at mirrors and checked it for
astigmatism.  Mr Bartels said he did not see any overt signs of
astigmatism. I still did not know what to do with it to correct what they
showed me till John Dobson took the time to look at it taught me some
tricks to work on the errors.

I have been working on it and finally put it in a assembly and tried to
star test. I have big time astigmatism.  I have tweaked it, checked the
mount, checked the secondary.  I think the cell is adaquate. Mr. Dobson
looked at it and suggested I put a piece of small bubble wrap on the wood
and then set the mirror in.  I have done that.

What I have found is,  the astigmatism seems to stay in the same place when
I rotate the mirror.  I took the mirror and put it back on the test rack
and checked it all the way around, rotating it about sixty degrees and
checking again, to see if the ronchi lines jumped at all.  The lines stayed
right where they were in and out of focus.  I cobbled together a pin hole
tester and used a 11.5mm eyepiece to do the pin hole test.  The only thing
I can say for sure is the out of focus image on both sides was round,  (
and I have more work to do on the figure, with a tip of the hat to Mr.
Dobson on reading star tests.)  The problem was I was sliding the whole
gizmo back and forth by hand, and of course never in a straight line.  I
will work on the thing and try again.

I am looking for ideas here, if anyone wants to toss a few into the ring. 
One thing I wondered was, does it make a difference if the optical focus is
different than the mechanical centerline?  I set the mirror up in the rack
and using a right triangle and some string figured where the centerline of
the glass circle, (or imagine a glass cone 17.375 inches on the bottom and
203.5 inches tall) lay.  I then used a laser pointer and eyeballed where
the reflective focal point was. After about two hours of repeated testing
they appear to be about 7/8's of an inch apart.  OR stated more
technically, if you are at the ROC looking at the mirror, down the
mechanical centerline,  the optical focus is about 7/8's of an inch to your
right.

The last thing I did was to use the sun to collimate the system,  shooting
the reflection into a target on the focuser.  Now I will wait till dark and
try another star test.  Don't be afraid to email me if you have an answer
or idea.

David Davis
vorblesnak{at}yahoo.com
vorblesnak{at}peak.org

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