Re: [ATM] 12.5 inches BVC advice for parabolizing

["Mel Bartels" <mbartels@bbastrodesigns.com>]

> According to Suiter in his book Star Testing
> Astronomical Telescopes in the Appendix A. Other Tests
> page 294 ,295
> He computes the pattern for a perfect F4.5 mirror
> using a 150 line ronchi screen placed at -.06 inside
> COC and a pattern for a 1/2 wave under-corrected
> mirror at -.050.  They are identical.  His point being

I see significant differences between the two images at .05 and .05 offset.

> Suiter also talks about his experience with a 10 inch
> F6 mirror that "passed the Ronchi test but gave soft
> images and failed the star test in a stunning manner."
> Once Foucault tested on the bench it proved to be 1/2
> wave undercorrected.

Well, I've seen lots of mirrors that 'passed the Foucault test' and failed 
stunningly too.  That kind of argument where the original test cannot be 
duplicated is anecdotal and not suitable.

I've use the comparative Ronchi test for 25 years with great results.  It 
has startling advantages.  Namely, it can be taught and applied quickly, it 
has a super rapid turnaround so that the mirror can tested, the change in 
figure noted, and new corrective strokes determined in the same period of 
time someone else is still setting up the Foucault test.  For faster larger 
mirrors it certainly should be thought of as a preliminary test, but the 
nice thing is that because it helps so much in obtaining a smooth figure, 
the final test can be the star test for adjusting spherical aberration.

That's how I've done all my mirrors.  At any time, anyone is welcome to test 
my 20 inch in particular.  It has a perfect star test and I regularly use 
it, seeing permitting, at > 1000x.  I've never done a Foucault or Caustic or 
any other type of measured test on that mirror.

Given pratical results from typical mirror makers, I'd rather have a 
smoothly corrected surface as generated from comparative Ronchi with 
spherical aberration adjusted in the star test, than a zoned mirror with 
passing Foucault readings.

Mel Bartels 

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