From: Guy Brandenburg 
To: Jim Burrows 
CC:  atm{at}shore.net
Reply-To: Guy Brandenburg 


I have heard a lot of stories about the famous errors in the Hubble mirror,
and they all seem to differ tremendously. I have heard that the mirror was
too thin to test normally on Earth with a foucault knife-edge test or a
star test, because it would flex, so they had do do these other tests. Who
knows the truth, first hand about the causes of the error? Guy B

Jim Burrows wrote:
>
> At 18:08 2003-04-26 -0400, Dave McCarter wrote:
>
>> Not so. Perkin Elmer did not use any form of Foucault test on the
>> Hubble mirror, as
>> they were using a much more sensitive interferometric test that gave
>> them absolutely
>> total confidence in the surface they prepared. Unfortunately they were
>> using the same
>> set up they had developed for the KH11 Keyhole Spy satellites, which
>> use not
>> parabolic mirrors, but slightly elliptical mirrors. After the error
>> was discovered by
>> star testing, another test most good ATMs use before coating and
>> certainly before
>> "flying" their scopes, PE put out the story that a small washer
>> misplaced the
>> reference element in their test set up, hence the error. NOT! Don't
>> believe it for a
>> second!
>>
>> Had they done a Foucault test they would indeed have noticed the
>> error. But they had
>> misplaced confidence in their high tech solution. As someone on this
>> list is fond of
>> saying, always question "conventional wisdom", for
therein lies folly
>> and error.
>
>
> This from Bob Goff:
>
>  >>The Hubble was figured using a botched null test.  A beginning
> amateur with
>  >>a wood Foucault test rig could have seen the spherical
aberration if too
>  >>many optical experts weren't standing around giggling. 
>  >
>  >
>  >Actually, the opticians working on the Space telescope had noticed that
>  >there was a great discrepancy between the test results with the
> catadioptric
>  >null tester and the Offner type null tester and the Foucault device. The
>  >offner and Foucault agreedexactly, showing gross undercorrection.
>  >Management made an executive decision without having any experience
> that the
>  >Catidioptric null tester was the best that could be had and didn't
>  >understand the evidence. The MAN said go with the Perfect one; the Cat.
>  >WRONG.
>  >Basically, all of the people who were reall qualified and really
> understood
>  >the project had retired due to the various delaying tactics of Congress.
>  >The Large Space Telescope was to have been 120" o.d.
>  >That's another story.
>
>         -- Jim Burrows
>         -- mailto://burrjaw{at}earthlink.net
>         -- http://home.earthlink.net/~burrjaw
>         -- Seattle N47.4723 W122.3662 (WGS84)
>
>

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