Natural Science Forum / Physics / Optics / March 2005

Greetings All,

As a hobby, I've fabricated a few dozen Newtonian primaries, and it is
time to try something new.  Since RC Cassegrains seem pretty popular
at the moment, I figured I'd look into the details.

While fabricating and testing the primary for an RC are a little more
extreme than a normal F/4.5ish parabolic newtonian, I think I have the
testing  method for the RC primary understood. (I just need a large
flat, that's all :)

Next on the list is to figure out how to test the Convex hyperbolic
secondary.  This is definitely new territory, and appears to be the
most challenging aspect of the whole scope..

The goal is to test this secondary with the Fizeau interferometer I
built based on Peter Ceravolo's design.  Since testing with Hindle
spheres are often bounced around on various groups, I thought I'd do
my best to design this setup in Oslo EDU.  Below are two files related
to what I came up with (however inaccurate they may be)

#1 A Gif image of the OSLO results (for those not wanting to load up
OSLO)
    http://lerch.no-ip.com/atm/Hindle_Null.gif (116KB)

#2 The actual OSLO len file for those that might be interested

    http://lerch.no-ip.com/.atm/Hindle_Null.len (2.2KB)

The first question is does it appear I actually created a realistic
representation of the test setup?

Secondly, if I did model the test setup correctly, it would appear
that the spacing between the convex and concave optic is EXTREMLY
critical with regards to obtaining a null.  (My current best guess is
somewhere on the order of +/- 0.01mm)    Does this sound reasonable,
or am I missing something important?  

If the previous paragraph is accurate, then the Hindle sphere test is
not something I feel capable of implementing.  Specifically I have no
way of mechanically measuring the distance between two optical
surfaces to within +/- 0.01mm.  Is there some optical 'trick' that can
be implemented?

If the Hindle test is as demanding as it would appear, this leaves me
with a few other options, but none of them appear very attractive
either, with the exclusion of testing the whole RC system in Auto
collimation and fighting with the figure of the secondary to obtain a
null.  

Just Curious, how do the Professional opticians fabricate and figure
their Convex hyperboloids

Thanks for any thoughts, or ideas!

Take Care,
James Lerch
http://lerch.no-ip.com/atm (My telescope construction, Testing, and Coating site)

Press on: nothing in the world can take the place of perseverance.
Talent will not; nothing is more common than unsuccessful men with talent.
Genius will not; unrewarded genius is almost a proverb.
Education will not; the world is full of educated derelicts.
Persistence and determination alone are omnipotent.
Calvin Coolidge