Dithering Sensitivity Degradation

The instrumental sensitivities given in Figures and are for a source on axis and do not take into account dithering. As stated earlier, observations with SPI should not be done in staring mode, since this makes the identification and removal of the background impossible. Dithering on the other hand has the disadvantage that the source is not observed for the full integration time in the center of the fully coded field of view. The SPI response falls off towards the edge of the field of view, and therefore dithering will degrade the sensitivity of the instrument somewhat. The hexagonal dither (a central pointing with six surrounding pointings in hexagonal pattern, all 2 apart) only samples the central part of the SPI fully coded FoV. Therefore no reduction in the sensitivity is noticeable. The 5 by 5 dither however samples closer to the edge of the fully coded FoV. In this case the sensitivity is degraded by a factor 0.84.