When the PI mirror controller is powered cycled the reference point (tip/tilt zero) is set to the sensor reading corresponding the centre of the PZT stroke. Due to differences in the thermal expansion of the materials used, the centre of the PZT stroke drifts by approximately 1.5um/K with respect to the reference position. As the the chop throw is symetrical about the reference positon, the total chop throw is reduced by 3um/K of the change in operating temperature since the controller was last powered cycled. 

So, as the maxium stroke of the piezos is 75 um, corresponding to a chop throw of 33.6 arcsec (peak to peak), the maxium chop throw is reduced by approximately 1.4 arcsec/K. The maxium chop throw is also reduced by the head room required by tip/tilt correction approximately 5 arcsec.