Counting Beams

The first step in doing spectral extraction is determining the number of beams to extract. For regular object-sky observations there will be one positive beam and, depending on how large the offsets are or if nodding was done along the slit or not, zero or one negative beams. A nod is considered to be along the slit if the nod angle is within 5 degrees of the slit angle, which represents roughly 1 arcsecond over a 10 arcsecond throw. The length of the slit is not taken into account, so throws to a position off the end of the slit will still count as being along the slit, even though the spectrum will not appear on the detector.

For chopped observations there can be one or two positive beams and zero, one or two negative beams, depending on combinations of chop throw, chop angle, nod throw and nod angle. If the chop throw and nod throw are equal to within 2 arcseconds and the chop and nod are along the slit, there will be one positive beam and two negative beams. If the chop throw and nod throw are equal to within 2 arcseconds and the chop and nod are to sky, there will be two positive beams and two negative beams. If the chop is along the slit then there will be one positive beam and one negative beam. If the chop is along the slit but the offset is to sky, then there will be one positive beam and one negative beam. If the chop is to sky and the nod is along the slit, then there will be one positive beam and one negative beam. If the chop and nod are both to sky, then there will be one positive beam.

For dual-beam polarimetry observations the number of beams is as above, but doubled. For single-beam polarimetry the number of beams is as above.
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