[sci.astro] The end of Hubble

news@helens.Stanford.EDU (news) (07/23/90)

	I have a simple question: when Amateurs grind a mirror of their
own (say 8") they use the "knife edge" test to look for aberrations.

	I suspect the "knife edge" test would not be good enough to catch
the minuscule sorts of errors they were EXPECTING in Hubble's primary. BUT...

I'm very curious to have 3 questions answered:

1) How hard/expensive would the "knife edge" test have been to do on Hubble's
	primary?

2) Would it have caught the more gross spherical aberration suspected?

3) If the answer to (1) is "not very expensive" and to (2) is "yes it
	would have caught the spherical aberration", why wasn't the test
	done as a simple sanity check?



	Well, I guess now Celestron will be able to advertise "our
telescopes' mirrors are guaranteed more accurate than the mirror in
the Space Telescope!".
\    /\    /\    /\/\/\/\/\/\/\.-.-.-.-.......___________
 \  /  \  /  \  /Dept of Geophysics, Stanford University \/\/\.-.-....___
  \/    \/    \/Joe Dellinger joe@hanauma.stanford.edu  apple!hanauma!joe\/\.-._
************** Hello, Comrade! *************************************************

BUNGE@osu-20.ircc.ohio-state.edu (Robert Bunge) (07/23/90)

<Questions about the knife-edge test that amateurs use and HST>

The Foucault test - the knife-edge test that most amateurs use - is cheap and
easy under many conditions.  With HST is another question.  If I had access to
the clean room at PE when the mirror was tilted on it's side, I know I would
have tried to bring a small tester in and run some numbers on it.  However,
that's not to say that gravity might have mis-shapen the mirror to the point
that the test wouldn't have worked without special gear.  In theory, the 
Foucault test would show the errors involved without any trouble if:

1)The mirror was properly supported:
2)The error ARE all in the primary (the Foucault test doesn't like
convex secondary mirrors).

In some respects, the ATM's that I know are confused - not mad - as to why
a simple test like this wasn't done.

Bob Bunge
bunge@osu-20.ircc.ohio-state.edu