Cross correlation to compute shifts among exposures cannot be easily automated, since not all the exposures have enough features or a good signal to noise ratio, and because of the presence of cosmic rays. Jitter files are normally used to monitor the telescope pointing stability and the trends in the telescope and instrument performance as the orbital environment changes. By correlating the HST Mission Schedule and the time-tagged engineering telemetry data stream downloaded from HST it is possible to reconstruct with great accuracy the pointing sequence of any given scientific observation. Since the initial production of jitter files, different formats have been adopted. On the other hand the relative accuracy is better than 10 mas within the same HST visit, that is, if the telescope didn't have to re-acquire a guide star during the sequence of observations.
Problems arise if the observation is taken in parallel mode.
While the velocity aberration is corrected for the primary instrument, the differential velocity aberration causes the secondary instrument aperture to experience a drift on the sky. The effect can be as high as 50 mas for a full orbit observation, depending on the relative position on the focal plane of the primary and secondary instruments. Assigning a jitter quality flag to each exposure depending on the telemetry keywords and on the standard deviations. Computing the shifts and in PC pixels Identifying those exposures in the association which are well registered within a tenth of a PC pixel and placing them in sub groups for cosmic ray removal. The overall mosaic of the association can also be requested.