This shift should be corrected in the motor program so that equation 2.3 can be used to calculate the slit width and motor positions. 2.10 shows the shutter photodiode signal as a function of motor steps. Note that the region where the signal is 0 has been trimmed off. Create a similar measurement and save your trimmed data. X mark the calibration data, triangles mark the fitted shutter diode signal.
Lines mark the fitted parabolas.
They all lie reasonably well on top of each other.