Re: [ATM] ronchi accuracy

[Jason Hissong <jhissong@gmail.com>]

Thanks for your comments Mel,

I used both for my recent (and first fully completed mirror).  The 
Foucault really helped me understand what was going on with my mirror.  
I got pretty good at reading the zones.  I used the Ronchi to verify if 
my numbers were close to reality.  When I looked at both, it really gave 
me a better confidence level of what I was seeing.  I have two testers, 
one for Ronchi and one for Foucault.  It made it easy to go back and forth.

I don't think I could have done it with just the Ronchi.  I needed the 
quantitative numbers to show me what to do next.  Maybe with experience, 
I can rely more on the Ronchi tests.  You're right though, I instantly 
knew what my mirror was doing just by looking at it.

Best Regards,
Jason Hissong

Mel Bartels wrote:

>I've been using the Ronchi test for 25 years.  I make better mirrors using
>the Ronchi test compared to using other tests.
>
>Mirror testing serves one main purpose: producing a good mirror.  Testing
>also serves secondary purposes of learning what figuring strokes do, and of
>learning about optics.
>
>Before we proceed with tests, we need to describe good mirrors.  I'll
>confine my thinking to a variety of Newtonians and the occasional
>Cassegrain.  
>
>1) A good mirror is properly corrected, that is, it's fully parabolized such
>that spherical aberration is absent at the focal plane.  
>2) A good mirror has no zones.  
>3) A good mirror is smooth, that is, there are no flat, deep, high, or low
>portions.  
>4) A good mirror is fully polished out.  
>5) Finally, a good mirror has no 'primary' or third order astigmatism, where
>a vertical slice of the mirror has a different best focus than a horizontal
>slice of the mirror.
>
>Not only do I need to test for all the above, but I also need a test that is
>reliable, consistent, accurate, inexpensive, easy and fast to use.
>Particularly in mirror making classes, the bottleneck is testing, when
>students line up, waiting for their mirror to be tested and analyzed.
>
>There's an amazing variety of tests to select from.  Some are easier to use,
>some are more predictable, some are more accurate, and some are more
>expensive.  Many tests use math.  Most Americans are mathematically
>illiterate.  To recommend a math intensive test is to tell many that mirror
>making is beyond them.
>
>I'll only discuss the Ronchi so as to stay on the thread's topic.  It's
>non-mathematical, and it's quick, cheap, and easy to use.  It does a super
>job testing for zones (#2 above), and testing for smoothness (#3 above).
>
>Very few tests adequately test 'stig (#5 above), so the Ronchi test is at no
>particular disadvantage.  The long time knock is testing for overall
>correction (#1 above), which is Suiter's reference.
>
>The Ronchi can be used to test for 'in the ballpark' correction (#1 above).
>I do this by comparing Ronchigrams of mirrors that are 1/4 wavefront under
>and overcorrected to the ideal shape.  On large fast mirrors, the
>differences are slight.  On small slow mirrors, the differences are obvious.
>The Ronchi grating does need to be moved precisely up and down the optical
>axis of the mirror, typically to quarter of a millimeter (hundredth of an
>inch).  So some sort of sliding stage mechanism needs to be built.
>
>For precision correction, the Ronchi suffers.  But so do most other tests,
>specifically Foucault in the hands of the average mirror maker.  It's my
>experience based on teaching mirror making classes over the years that the
>starting amateur attains an overall correction using the Foucault that is no
>better than the starting amateur obtains using the quantitative Ronchi test.
>Additionally, the slowness of the Foucault means one or two tests an evening
>whereas the Ronchi equipped amateur can run through a half dozen figuring
>spells in the same time.  And then there's still the issue of 'stig (#5
>above).
>
>Consequently, I use and recommend to students in my mirror making classes to
>conclude with the star test.  It tests (in many cases, overly sensitive) for
>spherical aberration (#1 above), and it tests for 'stig (#5).  It exposes
>problems with mirror mounts.  Mirror makers also get a sense of how the
>in/out focus star test relates to the figure of their mirrors.  And that's
>very worthwhile beyond simple mirror making.
>
>Mel Bartels
>
>
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>
>  
>


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"A long journey starts with the first step and an understanding spouse."
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