An additional technique that might be useful for this sensor is to 
construct the detector ruling – make it of two sets of rulings, with a 
90 degree offset (1/2 line offset between the sets -- i.e., 1/4 period 
offset.)  Then use two photodetectors, one for each set.  This way you 
can eliminate much of the gain variation due to temperature, etc. 
Additionally the sine wave phase difference from the detectors will 
allow the detection of which way the mirror is moving so you have a 
sense of positive/negative, north/south, etc.
Regards,
Charles R. Patton
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