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|PHOENIX POINT SOURCE SENSITIVITY (S/N=5 PER SPECTRAL PIXEL IN 1 HOUR, W/O OVERHEADS)|
Nod size: Like all infrared spectrographs, observations are taken in pairs (ab) or quads (abba) in order to remove sky emission. For small sources (angular dimensions are much less than the slit length of 14 arcsec), the telescope is nodded back and forth along the slit. For large sources the telescope is nodded between the source and blank sky. For pointlike sources nods are thus very small.
Due to its very high spectral resolution exposure times with Phoenix can be very long. In the JHK windows spectroscopic exposure times of 10,000 seconds on a tenth magnitude star would be needed to saturate the array. Since we are currently unable to abort an exposure in progress, we typically limit the exposure time to 900s.
In the thermal infrared (LL'M) saturation times on the sky and telescope background are 300 seconds near 3µm and 30 seconds near 5µm and limit the individual exposure times.