Re: ATM Foucault testing an hyperbolic mirror?

["James Lerch" <jlerch1@tampabay.rr.com>]

I can't offer any pointers to documentation, but I can recount a recent
experience with a 6" F/2 sphere I tried testing.

Using a slitless tester, the problem I ran into was I could not get the
entire mirror to be illuminated.  I could only get the center 50%
illuminated, which showed an obvious null at ROC, but only for the center
50% of the mirror surface!  It was almost like my light source wasn't big
enough (anyone have any suggestions? I'm using a 5mm LED behind the knife)

I'm currently thinking I'm going to need a point source light, like a
LED/Laser pointer reflected of a Ball bearing (Or something) to test such a
fast mirror surface.

Of course this brings up an interesting question in my mind.  The cause of
the  inability to fully illuminate the mirror at F/2, does this translate
over to any possible error for slower mirrors?

As for testing the hyperbolic surface, if you can get the mirror illuminated
and get some zonal readings, you can use either Figure45 or Sixtests to
reduce the data.  Just enter the appropriate conic constant in either
program.  (BTW, if you enter the zonal measurements into Sixtests, the
program will spit out a best fit conic constant as well, which I found
rather handy for figuring an Oblate concave mirror I was making with a b of
+2.3)

Best of Luck!
James Lerch
http://lerch.no-ip.com/atm (My Telescope Construction Page)