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Re: [ATM] The 15.5 Ultrathin Project, Astigmatism discussion
Could you test from above with the mirror lying flat?
Gary Fuchs
---- Original message ----
>Date: Fri, 20 Jan 2006 09:55:26 -0800 (PST)
>From: vorblesnak@peak.org
>Subject: [ATM] The 15.5 Ultrathin Project, Astigmatism
discussion
>To: atm@atmlist.net
>
>A number of you have suggested the support of the mirror on
the test stand
>could be causing the astigmatism. This was most recently
suggested by
>Dale Eason, but his is just the latest in a group of several that
have
>sent responses and emails in the last 24 hours asking about
this. To be
>honest, I had not given it much credence, because of the
shape of the
>mirror and the weight of the glass. However, as Dale
questioned, does the
>astigmatism rotate with the glass? That is a hard question to
answer as
>the surface is so complex. The simple answer is no. At least
the gross
>astigmatism is always vertically oriented. But, that does not
release me
>from the grips of this problem. There are axis where there is
no
>astigmatism. If the astigmatism is induced by the stand,
either at that
>rotation point two errors cancel each other, OR the mirror is
consistently
>setting correctly on the stand at that rotation point.
>
>It is obvious that I should support the mirror in a cell while
testing. I
>just happen to have one I can use. I normally support the
mirror's edge
>with two points at the bottom, but I will experiment with a
strap.
>
>Another suggestion has been that the glass is just too flexible,
>regardless of it's shape, and so can never be reliably
supported with a
>simple cell mount. That could be, but I will play with the glass
a while
>longer before I resign myself to that conclusion.
>
>Here is a very good article on grinding and finishing thin
mirrors, that
>can be found on the web at Newport Glass...
>
>http://www.newportglass.com/grind1.htm
>
>I have clipped out the piece on astigmatism and posted it
here. I have
>never met Bob Kestner, the author of the piece, but I would
like to some
>day. Here then is his take on astigmatism. I notice that like
most
>others he suggests going back to a fine grind for most cases,
though he
>at least offers a possibility of polishing out a minor case. He is
using
>the pinhole test.
>
>David Davis
>Toledo, OR 97391
>
>Abridged from an article appearing in Telescope Making #13
>Provided: Courtesy of Astronomy Magazine
>Article's Author: Bob Kestner
>
>" ... If you don’t see a round image, make sure your eyepiece
is square
>on, then rotate the eyepiece to make doubly sure it is not the
fault of
>the eyepiece. Tilt your head to make sure it is not your eyes.
>Make a note as to the shape and orientation of the problem
and which side
>of focus you’re on. Then take the mirror off the sling, and
without
>rotating it, reposition it on the sling again making sure it hangs
freely
>and evenly against the carpet. Inspect the image again. If the
problem now
>looks significantly different, then your mirror is not well
supported on
>the test stand.
>
>If the image still looks the same, rotate the mirror 45 degrees
in the
>sling and again observe the out of focus image. If you still see
>astigmatism but it has not rotated with the mirror, then it is
due to
>bending in the vertical position. If this is the case, you can
probably
>make the astigmatism come and go depending on just how
the mirror rests in
>the sling. Develop a technique for putting the mirror in its sling
to
>minimize the effect - and then ignore it. Since it will probably
be in the
>horizontal and vertical directions, it will not significantly affect
your
>testing of the mirror.
>
>With thin mirrors, this bending problem sometimes takes a
form not quite
>like astigmatism. Sometimes called "potato chipping, " it
means the bottom
>of the mirror is bent, but the top is not. You may see round
images with a
>flat spot on the bottom outside focus image, the bottom area
having more
>light in it.
>
>Even if you see a round image apparently free of astigmatism,
rotate the
>mirror anyway to make sure that astigmatism in the mirror
has not canceled
>astigmatism due to the support system.
>
>The bad news starts if the problem rotates reliably with the
mirror. If
>the astigmatism is very slight or difficult to detect, there is a
chance
>it will polish out. I do not recommend trying. Polishing out
astigmatism
>by hand may prove fruitless and you can waste a lot of time
and a great
>deal of energy.
>
>Make one final check before despairing. Allow the mirror to
come to
>thermal equilibrium, at least 3 to 4 hours, and then retest it.
Some
>portholes become astigmatic while cooling.
>
>If it is astigmatic, the solution is to go back to #400 or 12
micron grit,
>whichever you used, and grind again. Be extremely
scrupulous in applying
>all the precautions outlined in part 1. Make sure the back is
flat. If you
>are using a porthole and you did not grind the back flat, do so.
Did you
>forget and let the carpet pile get smashed down? If the tool
was on the
>thin side, maybe it was too thin. Examine your rotating
techniques to
>insure you are rotating by random amounts, and not repeating
positions.
>
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