[ATM] Cassegrain Secondary figuring and Star Test Interpretation

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

Hello!  I am in the final process of star testing my 11 inch classical
cassegrain system with a final system F ratio of 28.  The initial appearance
of the star test with a spherical secondary before figuring showed brighter
external rings outside of focus.  I used a star lap for correction.  I
eventually overshot a little.  I've backed off with a normal lap.

 

    I thought I was done after my last 15 minute polishing session.  At
200x, with about 4 waves defocus on either side, the images looked nearly
identical, but the outside of focus image looked a little soft, even though
the structure of the diffraction rings looked similar to the image inside of
focus.  I tweaked the position of the optics, tried different stars, etc,
and finally I realized that the softness in the outside of focus image
seemed to be a dim diffraction ring outside of the bright ring while the
inside of focus image had no such dim outer ring and as a result appeared
sharp.  I could not see a corresponding shadow ring inside of focus, though
I think that if would be difficult in this situation to notice on the inside
of focus image that one of the shadow rings just inside the edge ring was a
bit darker than its neighbors.

 

    I interpret this pattern as a raised ring on the secondary - like an 'S'
zone - from the use of a star lap, probably with the border somewhere
between 80 to 90% out along the radius.  Is this a correct interpretation?  

 

    Finally, should I try and correct it, since it is only visible at 4
wavelengths of defocus?  Thank you and best regards.

 

Mike

 

    

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