ATM Foucault Follies and Equilibration Anxieties

[Roger Ceragioli <rogerc@dakotacom.net>]

I have recently been refiguring a 6-in. f/4 mirror for the practice, since
it formerly had a rough surface and a marginal figure, which was 1/4 wave
overcorrected.  Using a Foucault tester with a slit and moving source, I
was able to get what I thought was an excellent figure--at least that's
what the numbers and the analysis were saying.

But then I star tested.  And found the same 1/4 wave overcorrection!  So I
figured out using the "TEX" program and ADMIR what zonal numbers would give
me 1/4 wave UNDERCORRECTION.  And I refigured yet again using these numbers
as my goal.  

Two nights ago here in Tucson it was windy and very balmy.  Star testing
was not easy, but the figure looked good: almost identical patterns inside
and outside focus.  The shadow of the diagonal appeared at almost identical
distances from focus.  I was pleased, but wanted to confirm the results on
a better night.

Then last night at about 4 am, the skies cleared and the seeing was pretty
good without wind.  Earlier last night, I took the scope out, collimated it
and looked at the diffraction patterns before the mirror was fully cooled off.

Amazing!  Significant UNDERCORRECTION was clearly visible (same thing
happened tonight).  The shadow of the diag. was big and dark immediately
inside of focus, smaller and reluctant to appear until for beyond focus.  I
was disturbed but decided to wait for fully equilibration.  At 4 am, I
looked at Pollux and Procyon.  

Low and behold, almost indentical inside and outside focus images!  I was
amazed at how much change in figure occurred during equilibration.  Jupiter
and Saturn were too low to see, but Mars was relatively high and for fun I
looked at it, despite the fact that my mirror is still uncoated.  I got a
sharp image at 245X, despite my 30% obstruction.  I could see what looked
like a polar cap and maybe Syrtis Major.  

Roger Ceragioli