Nominal zeropoints for a 5 arcsecond aperture at an airmass of 1.0 are given below. A larger field stop was installed in February 2003 leading to less vignetting at L’ and M’, and larger zeropoints. An ARC controller with a different gain was installed in Oct. 2006, resulting in a difference in zero points. Note that the zeropoint is a function of airmass, can vary from night to night, and will be smaller than these values if the seeing is very poor.

Filter

Zeropoint mag

	Oct 2002 – Feb 2003	Mar 2003 – Oct 2006 Larger Field Stop	29 Oct 2006 – May 2008 (New controller)	June 2008 – Present (row-reset in thermal)
J	21.12	21.15	22.37	22.37
H	21.47	21.50	22.62	22.62
K	20.77	20.80	21.84	21.84
L’	19.40	19.50	20.50	20.51
M’	17.13	17.29	18.29	18.33