6.1.4 Movement between Exposures

  Small movements of the orders were noted in successive exposures that were supposed to be exact repeats of each other (i.e., ones with no commanded movement of the echelle tilt or pointing of the IMAPS optical axis). While this was noted in the quick-look images that were sent to the ground during the mission, the magnitude and character of the motions were not fully known until the final data reduction of all images had been completed. The motions were erratic: sometimes there was no movement between one exposure and the next, and other times large jumps were apparent. A discussion of how the movements were measured and compensated in the data reduction is given in §7.7

Initially, we had suspected that the motions were caused by either (1) malfunctions in the Astro-SPAS attitude control system, resulting in random glitches in the null position or (2) the response of the structure to changes in temperature, where mechanical stresses might be suddenly relieved by random frictional slippages in joints somewhere in the long chain of structures between the startracker (mounted on the ORFEUS telescope) and the IMAPS payload, much like the snapping of a stove pipe as it heats up. Post-flight analyses of the attitude control system's telemetry records showed neither functional anomalies that could explain alternative (1) above, nor, to confirm hypothesis (2), any registration by the sensitive spacecraft's gyros of any impulsive reaction torques that were large enough to explain movements of the entire IMAPS instrument by the observed angular displacements of the spectra. Later, we discovered that there was a statistically significant correlation between the echelle position number and the average sizes and frequencies of the motions. This correspondence put the blame on the mechanisms used to tilt the echelle. Later inspection of the instrument showed that the pivot bearing was unusually sticky. Thus, after each intentional movement of this bearing, small angular motions in both in x and y were the result of random releases of the stresses that had built up.

Motions along the echelle dispersion direction (x coordinate) that occurred during the middle of any exposure will result in a loss of resolution. These motions are generally much smaller than the typical displacements from one exposure to the next because the exposure time of 39 seconds is less than the 110 second interval between one exposure and the next.