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Zero Points

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A zero point is defined as the magnitude of an object that would yield 1 ADU/sec at an airmass of 1. Zero points depend on instrument and telescope transmittances as well as detector electronics. They tend to be stable at the 3% level over many months. Measured values need to be corrected for variation of atmospheric extinction with airmass. The following table contains zero points for GNIRS with its short (0.15 arc sec/pix) camera and broad band "photometric" filters. Extinction values are taken from Tokunaga, Simons & Vacca (2002 PASP 114, 180) for 2mm of precipitable water vapor. See their Table 2 for extinction values for other amounts of precipitable water.

Filter Central wavelength
Zero point magnitude
(for 1 ADU/s)
Typical extinction
Y 1.02 24.2  
J 1.25 24.22 0.015
H 1.64 24.07 0.015
K 2.20 23.40 0.059