During the process of changing the IMC incident power in the past few days, I noticed that the rotational stage had not been calibrated. So I calibrated it by using old data (see alog 9306).
It now gives a coarse estimation of the angle-power relation. The calibration is, sadly to say, not so accurate any more at the point -- I saw an expected power of 600 mW even though the actual power is now at 1 W. Anyway, it should at least give us which direction to go when we need to rotate it.
=== some details ===
I used a least square fitting from gnuplot.