Re: [ATM] Star test with laser light sources

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

At 07:54 PM 7/27/2004 -0500, Ray@J-Engineering.com wrote:



>When it comes to comparing the inside-of-focus to outside-of-focus images, 
>do you think it maters if the source is monochromatic?  Even if there are 
>a few dozen diffraction rings in the in-focus image?  (sorry for copying 
>the whole thread but I like the history in this case)



Ray

A monochromatic source will help you to see the fresnel rings more clearly

Regards star testing in focus, heres a simple benchmark you can follow.

Take a good look at the diffraction patterns with spherical aberration in 
Suiters book Page 180.

To recreate these patterns, assuming a bright laser source , you'll need 
ideally a double polarizing set up with a polarizer in  the eyepiece and 
another in a holder the eyepiece sits in. This way you'll have full control 
of the attenuation. Assuming good seeing, if the wavefront is better than 
1/4 wave ( well assume here talking about simple spherical aberration ) 
when turning the  eyepiece with filter against the other filter and causing 
variable polarization, 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.

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 practise to recognize a perfect in 
focus  airy  pattern from quarter wave pattern.  Studying Suiter page 180 
will help. Notice with the half wave computer simulation how dominant the 
second ring becomes. With aberrations largely beyond this 3, 4, 5th rings 
rapidly take on a lot of light.

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.

Hope this helps.

Mark 

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