Re: ATM Interferometry analysis of 25"

[Anthony Stillman <atmer@flash.net>]

Mike,


>...if the tests by Haig and Burton showed a
>"just-noticeable-difference" at 0.87, then don't we want out
>mirrors (ideally) to be just above that threshold,

Without presenting the math, I'm tired of being told its boring, Marechal
(1947) derived an expression for the Strehl ratio and suggested a value of
0.8 or larger for when image degradation  due to the presents of
aberrations would no longer be noticable.  This contrasts with the mean
value 0.87 published by Haig and Burton forty years later.  Who's right?
Marechal suggestions has been around longer and is more widely accepted.
Haig and Burton had better equipment.


>...the Galaxy mirror is good enough to do the job, but not
>necessarily "fantastic".

I'll conceed the mirror may not be fantastic.  I'de want to see the
Zernike's and know under what conditions it was tested to say definitively.
And, I challange any glass pusher to make a large mirror that tests to
those metrics.  The fact is, most big glass isn't even close.  Factor in
typical seeing conditions and who buys most big glass and you understand
why.  A good number of research astronomers would kiss the ground you walk
on for a meter class instrument with a Strehl of 0.7 all to themselves.


>... fantastic...doesn't leave much room for...Dodgen, LOMO, Goff, and others

Astounding, incredible, magic, all come to mind.



>...Are we wasting our money (for) RMS numbers up in the 1/28 wave

It depends.  Wavefront error is cumulative and the mirror isn't the only
component.  Further certain applications, like bragging rights, demand
perfection.  Will you notice a difference?  Possibly.  Not if you were
bragging to me, I'm always impressed by an ATM's low residuals, but for
sure if you were braging to one ATM turned pro I know.  There are
situations where such high tolerances are required, but for the needs of
the typical amateur, if there is such a beast, it may be nice but just
isn't necessary.



>Also, I've heard several people say that John Hudek masks off the
>outer edge of the mirror before doing interferometer tests

Here's a quote I'll never forget.

"Only amateurs make optics to the edge"
                      Bob Kestner


It use to be I could turn an edge just by looking at it.  About five years
ago I went through a period where my surfaces were good out to the bevel.
Lately, I've been turning the edges again.  Turning them, taking them out,
going back and forth just isn't worth the effort.  So now I plan in a
little free board.  Not as much as the pro's, but they'll edge the blank
after its done if that's needed.


>...Should the Galaxy numbers be taken with a grain of salt

All numbers should be taken with a grain of salt.  How well the system
performs for you, and whether or not you are satisfied is the only worthy
criteria.

Reputable optical shops in general do a good job.  They have a reputation
to protect.  All will charge a premium if the work must meet a difficult
specification.  And even in what has arguably been called the world's
preeminent optical shop, numbers for surfaces that can only be described as
magical, get fudged.


Anthony