Re: [ATM] Classical cassegrain secondary

[Mike Lockwood <melockwo@uiuc.edu>]

Cristian,

C Abarca wrote:
> Hi everyone. I'm in the process of making the secondary mirror for
> a classical cassegrain telescope (the primary is 250mm F4.5). I am
> doing an interference test using a test plate. The test plate
> mirror is hyperbolic. the actual mirror is pyrex and has been
> polished out. Both mirrors are 10cm diameter. The actual size I
> need is 7cm. I intend to cut it once I'm done. Both pieces of glass
> have been polished out to a diameter of 9cm. When I do the
> interference test I get concentric circles. The circles are wider
> at the centre an get closer together as they move away from the
> centre. when I press the centre the circles move towards the
> centre. My question is what stroke pattern do I make to get the
> circles to turn into reasonably straight lines. Can I do the same
> strokes that I did when making the test plate?

Here's an older post I wrote about fringe testing:
   http://astro.umsystem.edu/atm/ARCHIVES/NOV05/msg00045.html

It explains how to determine relative convexity, concavity, and where 
the high spots are.

All your work depends on your hyperbolic test plate having an accurate 
figure, so be sure that it's right.

As far as figuring, petal laps, even those just made by pressing in 
patterns cut out freehand, are very effective.  Use them gently and 
let a small turntable do much of the work.  Move them around a bit to 
prevent zoning.  Be patient, and eventually the high spots will go away.

	Mike L.



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