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.