Re: [ATM] Star test with laser light sources

[Mark Suchting <masuch@cia.com.au>]

At 09:14 AM 7/28/2004 -0500, Ray@J-Engineering.com wrote:


>  I would guess, help me out here, that since we need to compare the 
> brightness between the first and second fresnel rings, wider rings might 
> make it easier to judge small differences in brightness?

  I was referring to studying in- focus diffraction rings ( first and 
second ) . For a reasonable mirror, fresnel rings can tell you about 
surface smoothness and seeing but not a lot else easily ( without some sort 
of computer analysis )  . I know someone out there is using software to 
deconvolve pictures of fresnel rings to measure wavefront.

>If  I see several dozen diffraction rings, should I assume my 
>monochromatic source is way too bright?  I'll work on a smaller pin hole 
>and perhaps turn down the brightness before I run out and get some 
>polarizing material.  I like the idea of attenuating the source without 
>walking back to the source, however.

You need precise control over attenuation to be able to see patterns like 
in Suiters book. Its done from your viewing eyepiece with a polarizer 
screwed into the bottom of your eyepiece. You can still get a lot of 
polarization cut off with a single polarizer and a laser source.

This kind of star testing will all tell you instantly more about the state 
of symmetry of your optics than any amount of multiple  chord foucault 
testing. Large mirrors for example frequently show complex mixes low and 
higher order astigmatism thats hard to diagnose and visualize on by any 
other means than a direct star test ( in a null situation ).

Mark



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