When modelling WFPC2 or FOC+COSTAR, why doesn Tiny Tim ask for the date of the observation, like it does for WF/PC-1 or FOC?
The date of observation was used on the aberrated cameras to determine the correct focus position due to desorption and mirror moves. Back then, desorption was significant and required frequent compensation by moving the HST secondary mirror. However, it leveled off enough by the time the repaired cameras were working that it was no longer important. The mirror is now adjusted roughly every six months. Between times of adjustment, the uncertainties in focus caused by breathing and pointing-dependent effects are greater than defocus caused by desorption.
