In late 1998, W. Voges and S. Doebereiner (MPE) were working on the implementation of the WFC star tracker data for the aspect solution required for observations between the 1998Apr loss of the second tracker and the 1998Sep destruction of the HRI detector. During this investigation, they developed an aspect solution routine and comparing photon positions to those in archival SASS output, found an average offset of 3.5 pixels for a test observation. This effect vanished if a time shift was introduced into the aspect table. The value of the time shift was close to the fractional part of aspect times. At SAO, we found a bug in the SASS code which explains the results obtained at MPE.
The bug occurs because the fractional time (an integer) assigned to the aspect time was divided by 8192 to get decimal seconds to add to whole integer seconds, whereas it should have been divided by 64. This means that the time associated with each aspect solution is wrong by up to one second (for a particular observation, the magnitude of this error is fixed). When a photon arrives, and does the interpolation to find the x,y.roll to apply to itself, it will often be interpolating between wrong entries (i.e. the wrong interval) and it will always get the fraction of the interval wrong. Thus the events have a wrong position, with mean offset of 3.5 pixels for one observation. Evaluation of this observation (Cen A) indicates an improvement in the width of the PRF by 10%, with an similar value for the increase in peak intensity (i.e. a sharpening of the PRF). For Cen A, the bug contributes something of order 3'' as an added error term to the PRF; i.e. FWHM(bad)= .
The origin of this bug is unclear. The code was written over 10 years ago. Since it leads to only a mild smearing of the PRF, it had gone undetected.