Detector patterns of a celestial source near the optical axis of SPI. Shown is the typical 5 × 5 rectangular dithering strategy with SPI, marked with dots and sequenced by number above the insets. Each dot represents one pointing with a field of view of 16° ×16° and is 2.1° away from the next/neighbouring pointing. The celestial source is marked with a blue star symbol at the position (l/b) = (35.8 / − 3.9°). In each inset panel, ranging from 0 to 2.75, the relative detector pattern of how the source would be seen by SPI is shown. The dashed lines in each sub-panel indicate the differences in the expected sky pattern and how it changes from pointing to pointing with respect to a flat 1 : 1 : … : 1-ratio pattern. From Siegert (2017).
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