From: "Russell Jocoy" 
To: vorblesnak{at}yahoo.com, atm{at}shore.net
Reply-To: "Russell Jocoy" 


("atm{at}shore.net")

    David,     I think it sounds like your mirror support in your scope is
causing your astig. problems.
   You have a large thin mirror that needs mutiple supports that "bubble
wrap" can't handle.
I would suggest going to PLOP and getting a layout of what you need.  Thin
mirrors can be
tuff to figure.
                          Russ J.
("atm{at}shore.net")













>From: David Davis 
>Reply-To: David Davis 
>To: atm{at}shore.net
>Subject: ATM A big astigmatism
>Date: Sat, 19 Jul 2003 16:11:49 -0700 (PDT)
>
>
>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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