[Raytrace] Dall Null

[Tom Wicker]

That was it! I refocused using min RMS OPD
monochromatic and got 0.1037 P-V.  I had been using
focus to minimum spot size monochromatic.   

I also had noticed previously that the wavefront error
had a minimum wrt null lens angle off axis (TLA). It
decreased from 0.5 degree to a min at 1 degree then
increased.  Thought this was very strange.  After
focusing with min RMS OPD. I checked this, and now I
get a smoothly increasing wavefront error with offset
angle, no minimum as I would expect. What is the
difference in these two focusing techniques? I'm using
version 6.1. Thanks for your help!

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