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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. 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 broad band filters. Extinction values were obtained from the UKIRT web pages.

Filter Center wavelength (µm) Zero point magnitude
(for 1 ADU/s)		 Sky background
(mag/arcsec)		 Typical extinction
(mag/airmass)
J 1.25 24.05 15 to 16 0.03
H 1.64 24.0 14 to 14.5 0.061
Kprime 2.12 23.68 13.7 to 14.2 0.09
Kshort 2.15 23.40 13.6 to 14.1 0.09
K 2.20 23.4 13.5 to 14.0 0.09
L' 3.77 22.21 3.5 .10
M' 4.68 20.1 0.3 0.44

Last update 2006 January 11; Andrew Stephens & Tom Geballe