ATM 12.5" f/5 Double-Arched Mirror Blank.

[Roger Ceragioli <rogerc@as.arizona.edu>]

I recently wrote a little bit about this mirror blank.  It involves 
taking a standard 12.5" Pyrex mirror blank and light-weighting it by 
generating a deep sphere into the back of the blank, so as to remove a 
lot of glass but still leave a smooth regular shape (like a deep candy 
dish), and by generating away an outer annulus of glass (from 80% to 
100% of the radial distance of glass center to edge) whose absence (so 
to speak) looks like a doughnut.  When you flip the glass face up and 
imagine it cut in section, you would see two arches or spans somewhat 
like those of a bridge, with a gracefully tapered vertical column in 
between at the 70% zone.

This results in a 40% lightweighting.  The blank is nowhere thinner than 
1/2" thick (center and edge).  The "column" at 70% is full thickness 
(2.125").  The blank is structurally very strong.

Recently, I finished all the generating, grinding and polishing of my 
blank and have been figuring it.  This post is just meant to report some 
of the results so far.  I used an interferometer to guide the figuring, 
but did the stroking in the same way that most people do, using a 
variety of pitch laps and rouge.  After that I silvered the glass and 
put in in my Dob for a star test.

A couple of possibly interesting observations: 1) because of 
difficulties with my test setup I was left with a TDE that I didn't see 
in the interferometer.  But I could easily see the problem in star 
testing, and by looking at a star with a Ronchi grating.  Probably 
double-pass autocollimation will also reveal it.  Chaulk one up for 
autocollimation and against the interferometer (as I set up the test)! 
 On the other hand (away from the edge) where the interferometer got a 
good look at the mirror, it's test results are completely vindicated by 
the star test.  Very close match.  No definite astigmatism was seen in 
the star test either.

2) The thermal lag time is definitely much smaller; that is, 
equilibration is definitely faster with the light-weighted blank.  In 
fact so far, I don't see much thermally induced spherical when I mask 
out the bad edge--which I will refigure soon.  But at this time of year 
in Tucson, we don't get huge nighttime temperature changes.  I'm sure 
I'll see worse effects in a few months.

3) The blank is very strong and I had no trouble figuring it on account 
of the light-weighting, same as with the heavily ribbed 10" Hale mirror 
replica which I wrote about a few weeks ago.  Support in the tube is 
also not an issue.

4) Last night I was able to split a double of 0.8 arcsec with two equal 
5th mag. components very clearly (2 or 16 Vulpeculae?).  And I was able 
to see a fine rill in the crater Posidonius, which I think is supposed 
to be about 0.4 arcsec wide.  My 8" achromat was unable to display it 
(although the seeing was so-so, and the comparison wasn't definitive). 
 This was of course with a mask in place to get rid of the bad edge and 
using a Paracorr.  Saturn looked quite nice and sharp at about 350x.  

5) On the down side, the diffraction rings in focus are far more 
noticeable with this Newtonian than in my 8" refractor.  The latter 
still gives esthetically nicer images of double stars, despite the 
secondary spectrum.  And the moon took on a 3 dimensional effect more 
easily with the 8".

So for whatever little it's worth those are my observations.  Now I'm on 
to fixing my turned edge and finishing my 12.5" Dob.

Cheers,
Roger