[Raytrace] Dall Null

[Michael Peck]

At 08:52 07-09-02 -0500, John D. Upton wrote:
>At 09:33 AM 9/6/2002 -0700, Tom Wicker wrote:
>>the text window the P-V OPD and RMS OPD are slightly
>>different (larger) than what is shown in the graphics window.
>
>     I have wondered about this myself for a long time.  I don't know why 
> this happens.  The differences are usually small enough that it hasn't 
> prompted me to dig into the cause of the difference.  Any list

I don't *know*, but I guess what's happening is different analysis routines 
are sampling the pupil at different numbers of points. Try this exercise 
with any setup you happen to be working on (a single mirror or simple lens 
will work):

In the "Surface Data" spreadsheet click on the button labelled "Setup". 
Towards the bottom of the table that comes up there's a line that starts 
"Aperture divisions across pupil.." That controls how densely the pupil is 
sampled, and the default value is fairly low. Try setting that to a larger 
value, like 90. For some reason when you click on the box a list of choices 
will pop up with a selection of non-integer values. You can ignore the 
list, I think and just type in a number.

Now, in the "Wavefront Analysis" graphic right click in the interior of the 
window. That will bring up a pop-up menu. Select the item "Re-calculate 
using new parameters..." That will bring up another dialog box, with the 
second line reading "Number of lines". Set that value to the same number 
you set in the surface data setup box. When I do that I get the same 
numbers in the text based wavefront analysis as on the graphic.

This is all for ver. 6.1 by the way. The same parameters were in version 
5.4 but they were set in different ways. If you check the help files there 
are ways to set these from the command line. Probably there's some way to 
change the startup defaults too, but I haven't looked into it. I have no 
idea why the default sampling strategies differ for different routines, but 
the differences in values are usually no more than a few percent. Denser 
sampling is more accurate of course, but graphics and computations are 
slower too.

Mike Peck

It's good to see some traffic on this list again. I think the only raytrace 
messages I got for 6 months were monthly reminders that I was still subscribed.


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Michael Peck
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