Under some circumstances the positions of X-ray sources can appear to drift by 0.1 to 0.4 arcsec during an observation. This occurs due to thermal alignment changes in the fiducial transfer system periscope related to overall spacecraft heating.
As of DS 8.4, a drift correction is applied to the aspect solution using the periscope gradients telemetry in the standard data processing pipeline. However, temporal drifts have continued to increase with thermal variation of the spacecraft; therefore, the Aspect team suggests that to accomplish science related to sub-arcsecond source structure, users should follow the CIAO thread to correct residual drift induced by the periscope. This requires a relatively bright, on-axis source (within a few arcminutes off-axis angle) to perform a "self-calibration" of the aspect solution.