Re: [ATM] Astig. due to gravity

["Scott Milligan" <starzkey@charter.net>]

Ok, now that explosives have been mentioned...

Seriously, what is wrong with this protocol as a reasonable alternative:

1. Use PLOP to model what you expect to get when using mirror on the sky.
2. Put 99% of your energy into making your test set up REPEATABLE under the
condition that you will gather more than one set of test data, with the
mirror rotated through a known azimuth between data sets.
3. Counter-rotate the data sets that you obtained with the mirror rotated
and average all data sets to obtain map of what rotates with the optic.
4. Improve it, or use it.

I claim no originality for the above procedure, but will state for the
record that it can produce thin mirrors with verifiable (via the RMS error)
Strehls within the range .90 - .96.  Of course, it helps mightily that you
have access to a test method that can create 2D maps of the surface, such as
interferometry, 2D Hartmann, Roddier, (add your favorite here).

By following this routine, I was able to determine that a significant
portion of the astigmatism appearing in a 16:1 aspect ratio optic with a 30%
center perf. was in fact introduced by the Ross Null lens I was using to
conduct the test; what put me wise was the fact that the error did not
remain consistent as the mirror was rotated, indicating a sum of static and
dynamic contributions to the error.  The "coup de grace" occurred when I
observed the inverse pattern upon rotating the Ross lens.  I discovered that
the key to making this scheme work is REPEATABILITY, much more than
"simulation in situ". A key observation; if your mirror rests on 2 pegs and
is not on the verge of "rocking" either backwards or forwards, then there is
an excellent chance you are not contacting the edge through a plane that
contains the COG.  Minimizing self-weight deflection in an axis horizontal
set up demands this, whereas repeatability would like to see everything
locked down, and so a well executed yet simple set up will walk a fine line
between these two conflicting requirements.

For what it's worth, I realize that makers of really large, thin optics do
have bonafide reasons for preferring the vertical test tunnel, but these
guys also have the budget to make it happen.  This is often where the ATM
conversation gets stuck in the mud, so in a round-about way, I guess I'm
asking the question has anyone considering constructing a vertical test
space truly done all they can do to extract truth from the vastly simpler,
cheaper, and quicker good ole "flathead V8" they already have?  And what
about learning something about finite element analysis, or simply asking one
of several contributors to this list who already have such experience to run
a couple of scenarios for you before taking Bob's plan at face value and
invoking a visit from our already overextended Dept of homeland
(in)security?  Darn, I'm sinking into politics now, gotta go...


Scott Milligan


-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Bob May
Sent: Thursday, March 06, 2008 8:13 PM
To: atm@atmlist.net
Subject: Re: [ATM] Astig. due to gravity

You forgot those of us that don't have basements!
You need to use the chainsaw to dig out the concrete and dig a
hole in the ground for the necessary distance plus stairs to get
down there.  You also need to watch out for high groundwater
(mine is three feet down!)and rocks which can make the digging
more difficult.  Use of explosives is not recommended even though
you can get the formulas for many of them off of the web as the
whole house may not enjoy the results with the commimentment
frying pan upside the head from the wife.
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net


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