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Zero Points

A zero point is defined as the magnitude of an object that would yield 1 ADU/sec at an airmass of 1. They depend on instrument and telescope transmittances as well as detector electronics. Zero points 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 NIRI with its f/6 camera and broad band 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.

For more detailed information on the NIRI zero points please see the Instrument Performance Monitoring project web page.

Filter Central wavelength
(µm)
Zero point magnitude
(for 1 ADU/s)
Sky background
(mag/arcsec)
Typical extinction
(mag/airmass)
Y 1.02 22.98    
J 1.25 23.97 15 to 16 0.015
H 1.64 24.04 14 to 14.5 0.015
Kprime 2.12 23.68 13.7 to 14.2 0.059
Kshort 2.15 23.40 13.6 to 14.1 0.043
K 2.20 23.43 13.5 to 14.0 0.033
L' 3.77 22.21 3.5 0.104
M' 4.68 20.1 0.3 0.223