[ATM] Test for Convex Secondary - Star Test

["M/S Heald" <maheald@octa4.net.au>]

Hello!  I’ve read the posts of the recent discussion regarding a convex
secondary.  I’ve been waiting to post the results of my testing for my 11
inch cassegrain because I am finishing the Bartelizing of the scope.

 

In any case, the method I found easiest to use was the Star Test.  I
originally planned a D-K.  At first, I planned the primary as f3 and the
secondary to be 65mm with an ROC of 300mm.  I decided to make the secondary
first, and star test the primary.  With such an extreme ROC and small lank,
polishing continuously gave me a severe turned up edge of the secondary.
After a lot of trial, I managed to obtain a spherical test plate and a
secondary tested by interference fringe testing (I scratched the blank twice
during the process, and had to re-grind twice with fine grit!).

 

After discussion with others, questions posed to the list, etc, and after
evaluating my desires for the scope, I changed to a classical cassegrain
design.  I polished the primary first.  For the secondary, I purchased a ¼
wave singlet convex lens from Melles-Griot 90mm in diameter for $70.00.  I
wanted to use through the back testing and the Foucault test and then trepan
the blank to 65mm in order to avoid an edge effect (I know trepanning has
its own edge effect, but I thought I could minimize it).

 

    In any case, because of the severe ROC, I couldn’t get the Foucault test
to work.  As a result, I went to Star testing.  I used a petal lap on the
secondary, and tested my system frequently until I had a good star test.  I
actually spent about 4 to 5 hours polishing, overshot, and had to back off
with a full lap with about 1 hour of polishing to return to a hyperbolic
figure.

 

    In summary, for a classical cassegrain, the primary can be tested
normally.  The secondary can be tested by star testing the complete system.
No additional Hindle spheres, fringe testers, etc., are required.  The only
problem is starting the secondary from a good sphere.  I avoided this
problem by purchasing a plano-convex lens of sufficient quality such that I
could start figuring directly.  It was much easier than I originally
envisioned.

 

Best regards.

 

Mike

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