Re: [ATM] The 15.5 Ultrathin Project, Astigmatism discussion

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

GARY FUCHS wrote:
 > Could you use a flat (maybe even a common household mirror
 > for a rough view?) to cut the height down to a manageable
 > amount? I may try this when I get home.

A very good flat would work, but its diameter would probably need to 
be a significant percentage of the diameter of the mirror, depending 
on how far from the primary mirror it is placed.  A household mirror 
would generally not be of the quality required for testing.

The errors of the flat are doubled by the arrangement, since the light 
from the tester reflects off it twice.  Personally, I would want the 
flat to be better than 1/8-wave on the wavefront for anything 
approaching critical testing.  Otherwise you have more than a 1/4-wave 
wavefront error from just the flat, which is more than the Rayleigh 
criterion.

vorblesnak@peak.org wrote:
> 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.

I recently made two 13.1" F/4.5 mirrors.  One was 3/4"-thick plate
glass.  One was 7/8"-thick Pyrex.  (Personally this is about as thin
as I want to go with mirrors of this size, given the figuring
techniques I use and the quality I expect from myself.  Thinnner just
flexes too much.)  I ground the backs of both quite flat and then
ground, polished, and figured the pre-generated surfaces.

During testing, the plate glass mirror consistently showed some
potato-chipping, which was noticeable during Foucault testing.
However, the form of the shadows was the same no matter how the mirror
was rotated, so this was due to the test stand.  (Precautions were
taken while polishing/figuring, so I did not expect any astigmatism
from the actual work.)

Improvement was noted when the mirror was leaned back (~10-15
degrees).  The tester was tipped down to align it with the optical
axis.  The astigmatism was reduced.  During this I supported the
bottom of the mirror with pile-type weatherstripping on a block cut to
conform to the outer radius of the mirror.  The block was ~5" wide,
covering ~35 degrees of the edge of the mirror.  The weatherstripping
has very fine strands, like a miniature version of shag carpet, and
spreads the support out along the bottom of the mirror.  Some time
must be allowed for the mirror to sink into this support.

Interestingly, the Pyrex mirror NEVER showed any indications of this
potato-chipping with Foucault or Ronchi gratings.  It was only 1/8"
thicker, and I thought it would.

The plate glass mirror was later tested uncoated in a telescope,
supported by a 6-pt cell.  It performed excellently, and showed no
astigmatism.  It is now coated and in the hands of another list member
who hopefully will finish building the scope for it soon.

So, my recommendations are to lean the mirror back on a conforming
support that will "float" it, and support the edge gently.  Then, if
the astigmatism is not too bad, just test through it.

I have never used a sling in a telescope or in my testing.  I don't
think they can be positioned the same every time, and that will cause
different bending of the mirror each time you test.  The
weatherstripping support was consistent.

By the way, have you checked it for strain?

	Mike Lockwood


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