[ATM] The 15.5 Ultrathin Project, Astigmatism discussion

[vorblesnak@peak.org]

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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