From: bill@bessel.as.utexas.edu (William H. Jefferys)
Newsgroups: sci.astro,sci.space
Subject: Re: Hubble's mirror
Date: 9 Nov 92 22:41:08 GMT
Followup-To: sci.astro
Organization: McDonald Observatory, University of Texas @ Austin

In article <1dh7e9INN481@rave.larc.nasa.gov> claudio@nmsb.larc.nasa.gov (Claudio Egalon) writes:
#Henry Spencer is right. Although it was sometime ago that I read an 
#article in Physics Today- which explained very nicely the real messed 
#up-, I still recall that there was three diferent instruments that were 
#used to measure the curvature of the primary mirror. Two of them 
#used refraction principle and one of them used the reflection 
#principle (don't remember the details so I may commit some  
#mistakes. If so, I apologize). The results of the other two instruments 
#that used the refraction principle to measure the mirror curvature 
#matched up but... the measurements produced by the one that used 
#reflection, reflective null corrector, was different and, since it was 
#thought to be superior to the other two,  no one bottered about 
#finding out why the discrepancy between the measurements... of 
#course untill they found out that there was something wrong with  
#Hubble. 
#
#*However* as  I recall (and I may be wrong) the error in the RNC 
#came along because the WRONG surface in the instrument was 
#providing the reflection and not that because the instrument was 
#built improperly. Aparently there was an scratch in a surface that was 

No, the reflective null was built improperly. Here's what
happened: The field lens was supposed to be aligned to the
cell by using a precisely machined Invar rod, whose length
was traceable to NBS standards. A laser was used to align
the end of the bar to the cell. In order to guarantee that
the laser was reading just the end of the bar (which was
rounded), an end cap with a small hole was placed over the
end of the bar. The end cap extended 1.3 mm further than
the end of the bar, and was painted black so that there
would be no return to the laser from the end cap. Unfortunately,
as it turned out, a fleck of the black paint got scraped off,
and a significant return from the end of the bar was obtained.
Thus, the bar was positioned too far away from the laser, and
the other end of the bar (which was positioned against the
field lens) was 1.3 mm from where it should have been.

In order to misposition the field lens, they people who did
this _had to shim out the lens cell with washers_!!! The
cell had been properly built, but when the lens didn't fit
where they thought it should (in the wrong position), they
kludged it to _make_ it fit.

You are right that the refractive null and the inverse null
agreed with each other, but not with the reflective null. Why
the engineers didn't see the red flag waving by 2 out of
3 vote, and the fact that they had to shim out the lens position
in the cell, is beyond me.

#suposed to be black, for absorbing incoming radiation, and it was this 
#scratch that worked as the WRONG reflecting surface. So,  that how 
#the wrong prescription came about. Another reference for the 
#investigation on the problem of the mirror was the report put out by 
#Lew Allen's review board (I guess I have a copy of it at home and I 
#am confindent that you people can get one from NASA but I do not 
#know whether they  will charge you or not). Maybe, not oddly 
#enough, there were other complaints about the handling of the  
#project  like people did not like to be bothered with criticism (read 
#peer review)... So aparently, the problem was both technical and 
#arrogance.
#
#Claudio O. Egalon

Bill
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