[ATM] Classical cassegrain secondary

["C Abarca" <ccabar@bigpond.com>]

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? I have seen petal laps on several sites but I am not sure if this would work on an oversized secondary.

Regards Cristian
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