From: "Jerry" 
To: "'Michael Peck'" ,
        
Reply-To: "Jerry" 


        I sure won't add any "facts" to this but here is what I recall
hearing at the time from equally unreliable sources as myself. Story was
that it had 8 waves of spherical aberration (I assume this was regarding
the entire system) and the source of the error was the washers mounting the
reflective null corrector being on the wrong side of its attach points
causing an error of the thickness of the washers.

        I doubt any particular accuracy to what I heard but it at least is
some ways matches what seems to be more detailed explanations by some who
claim to have been involved in the project at the time. It seems that those
directly responsible for the mirror and its shape don't comment about it in
public. And I guess I understand why.

        I believe it is another urban legend regarding the Foucault test
being required, planned, or ever even contemplated for the HST mirror.  And
I agree that the error is not something that would have been eminently
obvious had a simple Foucault test been done. Maybe if it had really been 8
wave out.  But then any properly done test suitable to the task would have
shown the error. The problem is not that they didn't Foucault test.  It is
that they didn't resolve contradictory test data that they did have.

Jerry


-----Original Message-----
From: owner-atm{at}shore.net [mailto:owner-atm{at}shore.net] On Behalf Of Michael
Peck
Sent: Sunday, April 27, 2003 11:56 AM To: atm{at}shore.net
Subject: Re: ATM Fw: to parabolize or not......II


At 21:23 4/26/2003 -0500, Michael Peck wrote:


>Hmm... Is this the start of a new urban legend, or is there actual
>documentation for any of this?

Now that I've had time to Google around a bit and look for files buried on
my laptop, here are some reasons I'm suspicious of these stories:

1) The HST primary ended up *overcorrected*, not undercorrected. The target
conic constant was -1.0023 (just barely hyperboloidal). The actual cc ended
up at -1.0132. That's enough to produce about .5 wave RMS of spherical
aberration at the focal plane.

2) Here's a recent post to sci.astro.research (that's the moderated and
relatively non-nutty usenet group) from someone who claims to know from
second hand information what really went wrong, which was a spacing error:
http://groups.google.com/groups?dq=&hl=en&lr=&ie=UTF-8&selm=mt2.0-1881-1040
164914%40star.bris.ac.uk&rnum=7>.

Scroll past the Gordon Pusch rant to the original material to get to the
substantive discussion.

3) I don't recall seeing any mention of any form of non-null test being
done on the HST primary, but I wasn't there. So, is it plausible to think
the error could have been detected with Foucault or some variant test?
Here's a few factoids to ponder. Measured in the vicinity of the center of
curvature the HST primary had about 150 waves P-V of spherical aberration
(44 waves RMS) - that's with source & knife edge set to null the 70%
zone. So to detect the error that was present you'd have to measure the
mirror's shape to within about 1%, and to decide if it had the right shape
to within original design tolerances you'd have to improve your
measurements by another order of magnitude. The total longitudinal
aberration in a moving source test would be about 65mm. Again, you'd have
to measure that to within 1% to detect the actual error that was present.
The same 1% accuracy would be required for a transverse aberration
measurement, so a caustic test wouldn't be any easier.

I'm not going to say that's impossible, but I will say there's no way you
could just eyeball the mirror with a simple test rig and conclude that it
had a problem.

Mike Peck

------
Michael Peck
mpeck1{at}ix.netcom.com

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