3.4.5 Incorrect Location of Image Pieces in Some PFIs

Observing regions (OR) are composed of 6464 PFI ``blocks.'' The simplest observing region is 6464, but quite often larger observing regions are used. For example, when the observing region is 256192, there are 4 ``blocks'' in the E/W direction and 3 in the N/S. Because of the way that the SXT telemetry buffers work, it is necessary to take three separate exposures for a 256192 observing region. The assembly of these ``blocks'' into an observing region is performed properly for over 99% of the observing regions, however, there is a special case when a problem can occur.

When the telemetry mode is FFI dominant (the full frame images get the larger portion of the telemetry), it requires four major frames to telemeter a single 6464 PFI ``block''. If any of the first portions of the ``block'' (which is 6416) is missing, it will cause the image to be assembled incorrectly because of a lack of telemetry information. Be aware of occasional PFI portions which are not aligned properly.