Re: [ATM] Star test with laser light sources

["Ray@J-Engineering.com" <Ray@J-Engineering.com>]

Hi All,

Mark Suchting wrote:

> At 07:54 PM 7/27/2004 -0500, Ray@J-Engineering.com wrote:
>
> A monochromatic source will help you to see the fresnel rings more 
> clearly 

Yes, when I compare my monochromatic fresnel rings the those in Suiter, 
mine look much thinner and the minima between them is very distinct even 
when defocused by 10 or more waves.  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? 

> Snip... you'll quickly obliterate all of the extraneous rings leaving 
> the airy disc and one ring. At half a wave the second ring will be 
> much closer in brightness to the first and be very obvious when all 
> the outer rings are attenuated from visibility with filtration. If you 
> attenuate further , both second and first ring will disappear closely. 

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.

> A perfect system and a 1/4 wave system will look very similar in the 
> in focus diffraction pattern but more intensity in the airy disc  and  
> less in the ring. It needs a lot of practice Snip..

Hopefully with the help of the list, I'll be able to practice instead of 
simply trying to set up the test.  Obtaining in-focus views has been the 
hard part *so far*.

> Star testing in focus like this teaches you that, aside from all the 
> `nit picking' we are so fond of, the biggest improvement in optical 
> performance happens when we nail the so called Rayleigh criterion, and 
> beyond that the improvements in image quality are a little less 
> tangible under average conditions.  1/4 wave wavefront for smooth 
> spherical aberration at best focus is the fundamental and most 
> important benchmark beginner opticians should be aiming for I think. 

Without grinding, polishing, figuring, bench testing, and star testing a 
mirror, I might not have understood your statement as clearly as I do 
now.  My mirror  tested quite good in foucault tests, but if I can't 
tell the quality when using the scope, what's the point?  I need to 
*see* that the mirror works well, not tests well.  As a beginner, it has 
not been easy to see that the mirror works as well as the bench testing 
suggests.

> Hope this helps. 

Tremendous help.

Ray J

_______________________________________________
ATM mailing list http://www.atmlist.net/