An autocollimator (AC) should show (0,0) if a retroreflector is placed in front of the AC.
However, the AC may have an offset. Also the retroreflector may not reflect the beam back with an exact parallelism.
To calibrate these two errors, the autocollimator is calibrated. The retroreflector was rotated by 0, 90, 180, 270 deg
while the reticle position are monitored. The images of the autocollimator were taken by my digital camera looking at the eyepiece of the AC.
Note that 1 div of the AC image corresponds to 1arcmin.
Basically the rotation of the retroreflector changed the vertical and horizontal positions of the reticle pattern by 0.6mdeg and 0.1mdeg
(2 and 0.4 arcsec). Therefore the parallelism of the retrorefrector is determined to be less than an arcsec. This is negligibly good for our purpose.
The offset changes by ~1div in a slanted direction if the knob of the AC, whose function is unknown, is touched.
So the knob should be locked, and the offset should be recorded before we start the actual work every time.