Finding a Common Best Focus Position for High & Medium Resolution Modes


Last week I finished running scans of both Medium and High Resolution Zemax models for different settings of collimator distances. I used my Medium Res results to pick a detector distance of 95mm from the turning mirror and 35 turns on the Echelle Collimator.  The blue straight line on the graph is shows Medium Res best focus position for different positions of the Echelle Collimator.  The brown curve shows the Medium Res best focus geometric spot radius for those different Echelle Collimator positions.

  Then I went to my High Res results, found where the detector distance of 95mm and Echelle Collimator setting of 35 turns coincided (roughly) with an Echelon Collimator setting of 10 turns. The individual straight lines represent best focus detector positions for different Echelle Collimator positions at one Echelon Collimator position. The extrapolation on the High Res graph indicates about 13 turns needed on the Echelon Collimator to coincide with the Medium Res best focus position.

 So... going to my Zemax models and setting these conditions I get an RMS spot size (radius) of ~18microns in Medium Res (vs Airy radius 167microns) and an RMS spot size (radius) of ~101microns in High Res (vs Airy radius 143microns.)  The best setting for the Echelon Collimator was about 11.4 turns instead of 10 or 13 ... not wildly different considering how I got all of these numbers.

The logarithmic PSFs for both modes are essentially the same with both having FWHM of 200 microns or 8 pixels at 13.7 microns wavelength:

 

In other words, this method works pretty well - in the models.  Also, my value of 95mm is, I think, indistinguishable with the current measured value of 93mm.

Now - how do we know when we've set the real collimators at some 'n' number of turns with respect to the nominal Lacy model positions?