Re: Re[2]: Rayleigh Water Test (Was: Re: [ATM] 30" Flats)

[Jarvis Krumbein <jkoptic@juno.com>]

I think a little clarification is required here.  The thread has been
dicussing two different tests.  The first is using a liquid flat to null
test a primary mirror.  The second is using a flat liquid surface to
fringe test a flat.  Both are valid methods for doing two seperate
things.

At one point about 40 years ago, we (Long Island Observers- now defunct)
built a test stand to do the first of these.  We used mercury for the
flat with a very thin film of silicon oil on the surfsce to both dampen
vibrations and to prevent the fumes from doing us in.  We were able to
test mirrors up to 12" in diameter.  These were all full thickness pyrex
blanks from Corning so there were no sagging problems because of thin
glass.  Vibration was a problem but was mostly from the construction of
the stand.  We ended up with much wire cross bracing and eventually
suceeded. The pan for the mercury was about 16" in diameter so there were
no edge problem.  One of our members had borrowed the mercury for our use
so it eventually had to be returned.  It was quite a sizeable amount and
weighed a lot.  We also used heavy motor oil in the same setup and were
able to test and evaluate mirrors that had already been coated.

We also tried to do the water test for flats but were never sucessful.

The spinning of mercury in a pan was never used for flats but was
successfully used to make fairly long focus parabolic mirrors and I
believe one of these may still be in use in Canada.

Jarvis Krumbein
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