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6.1.2 Misalignment

  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 $5\arcmin$ 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 $15\arcmin$. 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.