ATM Hindle Sphere

["Bennett, Thomas D" <thomas.d.bennett@lmco.com>]

Jim,

I had something easier in mind. Don't I already have a Hindle sphere? Isn't
the primary in my C9.25 a short focus sphere (F2 - F3 I think). Better yet -
could I purchase a dropped/damaged SCT OTA cheap, remove the primary and use
it as a Hindle sphere to figure the secondary for a DK Cass? Just a thought.

        Tom 



At 10:14 98-06-12 -0600, Bennett, Thomas D wrote:

>Texeraeu's book (just got it) mentions a Method of Hindle that involve the
>fabrication of a spherical test disk of focal length equal to the Cass
>primary. Looks like a foucault test is performed on a double-pass beam
which
>strikes the secondary twice thereby increasing its sensitivity. Texereau
>states that the characteristics of the Hindle sphere are not "stringent".
>Bratislav - would this get me the sensitivity needed for an excellent
>secondary?

Let me chime in here.  I made a 10" f/1 Hindle sphere for testing an RC
cass secondary and solved the problem of turning it into a reference by
doing a spline fit to the surface obtained by a very careful Foucault test
(with a modified tester and 5 mm zones).  Then one can find normals to the
surface and calculate where rays hit the surface, questions that turn up in
testing a secondary.  Sort of "solve it with software", trading computation
for null testing against a super surface.

        -- Jim Burrows          phone 206.244.2933, fax .0294
        -- Email                burrjaw@halcyon.com
        -- Seattle              N47.4722, W122.3661 (WGS84)