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