[Raytrace] wavefront analysis

[Michael Peck]

At 15:57 07-09-02 -0700, Tom Wicker wrote:
>are smaller with the f/6) cause the difference. This
>makes me wonder which is more relevant to knife edge
>testing. I would guess transverse aberation, ie., min
>spot size. Anyone have any input on this? Can anyone
>recommend a good optics text that goes into this?

Assuming the optical system you're working on is intended to be 
"diffraction limited" you want to minimize RMS OPD. That will maximize the 
Strehl ratio, and at least approximately maximize the total amount of 
energy in the Airy disk.

A good way to demonstrate this is with the old standby of the almost 
diffraction limited sphere, say a 6" f/8 (with object at infinity). After 
setting it up just cycle through all the analysis graphics with the focus 
set to minimize RMS OPD and RMS spot size. What you especially want to 
compare are the diffraction PSFs - you'll probably want to maximize the 
window size to see the differences. Also you should compare the diffraction 
PSFs to the geometric spot diagrams.

There's a third focus position that's relevant, which is the one that 
minimizes the maximum transverse aberration. That's at 1.5 times the focus 
position that minimizes RMS OPD. For example for a 150mm spherical mirror 
with rc = -2400mm the RMS OPD minimizing focus is at about .294mm, RMS spot 
size focus is at .392mm, and the min max TA position is at .441mm. This 
last one is what you find by trial and error in Texereau's foucault data 
reduction worksheet. It's also the basis for the "Millies-Lacroix" 
tolerance criterion. Look at the geometric spot diagram and you should see 
a circle uniformly filled with points - at either of the other two 
positions you'll see a tighter core with a spray of points farther out. It 
helps to turn on the airy disk for this exercise. You can do that by right 
clicking in the spot diagram window and setting "Show Airy disk" in the 
change parameters dialog. Be sure to compare the diffraction PSF with the 
geometric spot diagram and the PSFs at the other focus positions.

Chapter 11 of Smith's "Modern Optical Engineering, 3rd edition" gives a 
pretty good discussion of the differences between geometric and diffraction 
based optical quality criteria. It's a little bit technical, but less so 
than anything else in my small library.

Mike Peck


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Michael Peck
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