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We had expected that the IMAPS optical axis would not be exactly
coincident with the axis of the Astro-SPAS startracker. What came as a
surprise was that the magnitude of the offset was so large. To obtain a
spectrum over the nominal orders between 950-1150Å, we needed a
correction in pointing along the cross disperser direction.
To get the best blaze efficiency of the echelle grating, the pointing
offset in the x direction had to be about . During the
mission, we operated on the side of the echelle blaze function that was
closer to the instrument axis, to reduce losses from the mechanical
collimator. As a result, echelle positions 1 and 2 show a left to right
gradient in sensitivity (with position 1 showing the most prominent
effect), while positions 3 and 4 were near the center of the blaze. We
have not yet identified which alignment procedure led to the error
between the IMAPS and Astro-SPAS startracker axes.