Date: Wed, 25 Nov 92 05:07:38    
From: Space Digest maintainer <digests@isu.isunet.edu>
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Subject: Space Digest V15 #456
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Space Digest                Wed, 25 Nov 92       Volume 15 : Issue 456

Today's Topics:
                           Hubble's mirror

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Date: 25 Nov 92 05:15:05 GMT
From: "William H. Jefferys" <bill@bessel.as.utexas.edu>
Subject: Hubble's mirror
Newsgroups: sci.astro,sci.space

In article <BxxLo9.I6H@zoo.toronto.edu> henry@zoo.toronto.edu (Henry Spencer) writes:
#In article <1992Nov17.121839@cs.man.ac.uk> mario@cs.man.ac.uk (Mario Wolczko) writes:
#>Any idea why an end-to-end test would have been more susceptible to
#>gravitational problems than the null corrector test?  Couldn't both be
#>performed with the primary flat on its back?  And why are there
#>more risks of surface contamination?
#
#Adding tests means moving the mirrors around and doing work in their
#vicinity, which automatically increases the risk of contamination.
#The problem there isn't the nature of the test, but simply the fact that
#it's yet more fiddling with the mirrors, when you would really like to

#handle them as little as possible between manufacturing and launch.

Contamination was a major concern because even a small
amount of organic contamination (a little oil, human
evaporant, or whatever) would seriously degrade UV
performance.

#I would guess that the null-corrector tests were done with the mirror
#flat on its back; it would seem the obvious approach.  The problem with
#gravitational distortion -- I would think -- is simply that it requires
#doing the test in a vertical orientation, which considerably complicates
#the test facility (if for no other reason, because you need a vertical
#shaft of considerable height to mount everything in).  That is, it's not
#something you could expect the P-E optics shop to be able to rig up with
#equipment on hand; it might even need a special building (although there
#are reports that the USAF already has one).

Right. The interferometric tests with the null corrector were
done with the mirror horizontal, in a large vacuum chamber.
The mirror was mounted on a special "metrology mount" that
compensated for gravitational effects by specially adjusted
weights. I doubt that an _in vacuuo_ two-mirror test could
have been performed[*]. A crude "sanity check" test could have
been done; such a test, if planned (it was not) would have
given the whole test process more visibility & might have 
forced the powers that be to recognize that there was a
problem. The problems with the null correctors were too
easily swept under the rug.

Bill

[*] for reasonable cost, that is.

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End of Space Digest Volume 15 : Issue 456
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