RE: [ATM] some largish thin mirror questions -> Really Long

["Peter" <peter@kitgear.com>]

Hi,

I have recently worked on two large ultra thin mirrors and both turned out
surprisingly well. In both cases I experienced astigmatism, I got rid of it,
induced it back and got rid of it again. The first is a 16" f/5.25 mirror,
which is only 0.735" thick. The second is a 25" f/4.8, which is only 0.910"
at the edge. The larger mirror turned out to be a nice 1/4 wave front optic
and the smaller was a good sphere for a future flex scope. Although thin
mirrors are not a popular topic of consideration on this list I may be able
to add some useful info to someone. I believe the trend and the future of
ATM is going to be larger and thinner mirrors placed into telescopes which
will be lighter, faster to setup and lower cost. Ultra thin mirrors
absolutely need to be considered by ATMers at some point in time.

I do have a web page outlining this work which is out of date. In short I
found that the following works:

1)	I assume you are using plate glass so you must ensure your glass is
decent quality and stress free. Richard Schwartz has a web page showing how
to perform a stress test using polarizers. You should do this before
starting this project.

2)	Ensure you have thick bottom support for your mirror. A thick piece of
carpet wrapped in a garbage bag is what I used. The plastic wrap allows you
to wash the bag between grit changes. Make sure the working board you
support the mirror does NOT have a measurable amount of warp otherwise the
carpet is useless.

3)	Grind the back flat. I did not do this on the 25" and I could not make a
mirror out the glass until I went back and performed this critical step. I
cannot stress how important this is!! Don’t assume the back is flat. I used
a 50% tile tool and #60 grit. A metal edge or flat is used to test the back
surface. When I could not slide a piece of paper anywhere between the glass
and my metal edge I stopped. The total working time was about 1.5 hrs. The
back does not have to be perfectly flat as any even figure of revolution
will suffice.

4)	Your fine grinding tool must be light enough to prevent severe bending of
the glass. This is where I differ from most of what you've read. The tool I
used for my 25" mirror was 3/4 size sub-diameter - 20" round and only 0.75"
thick. Please make sure the tool is round. I attached a central post to the
back of the tool using pitch which I used to move the tool and I never
applied any downward force just side to side and rotational. If you induce
astigmatism at this stage then no amount of polishing will remove it. The
tile tool is rotated in a normal fashion as you go around the barrel. The
mirror is constantly turned a varying amount between 1/3 to 1/4 turns for
each rotation around the barrel. One wet lasted about 3-4 minutes of working
time. During fine grinding the tool never left the glass until it was time
to change grit sizes. This reduces the probability of scratches as well as
tiles coming off.

5)	MOT or TOT? I used MOT to polish out the 16" and I found that if I did
not do this critical step then astigmatism worked it's way back. I used a
central polish post as I described above. This method worked very well and
the mirror polished out fast. On the 25" I tried polishing it by hand but it
ended up with astig so I reground the back, then the front and I machine
polished it out at a friend's place in Sudbury Ontario. Stathis would be
very disappointed if he hears this! The mirror polished out quickly using a
50% tool TOT working the 50% zone. The mirror still came out slightly
astigmatic so... read on...

6)	To remove astigmatism we worked the 25"er by hand TOT a 2/3 sub diameter
tool using figuring strokes. We performed mirror and tool rotations just as
I described in the fine grinding stage above. Use very little downward
pressure and slow strokes. Make sure the tool has lots of drag and use lots
of cerium oxide.

7)	When testing the mirror you'll need a sling. The mirror shows terribly
astigmatic unless you do this. Once you set the mirror on the sling you will
have to relieve the sling pressure then set the mirror a second time
otherwise you'll cause strange deformations. Use a pinhole test as described
in the archives then a standard Foucault test. The measurements were
perfectly repeatable for both mirrors.

8)	When mounting an ultra thin mirror you will need lots of bottom  points
of support and a ball bearing edge support. A sling is fine for thick
mirrors but not thin.

Those are my 8 secrets (so far) to successfully making large ultra thin
mirrors.


Regards,

Peter

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