Typical extinction values measured at UKIRT are given in the table below. Extinction can vary from night to night and if the difference in airmass between target and calibrator is significant then the value for that night should be determined. Historical information on extinction as a function of wavelength is also available.

Filter	Average Extinction mag/airmass	Range in Extinction mag/airmass
UFTI I	0.05	0.00 – 0.09
UFTI Z	0.02	0.00 – 0.08
J MKO	0.05	0.00 – 0.10
H MKO	0.03	0.00 – 0.12
K MKO	0.05	0.00 – 0.12
L’ MKO	0.11	0.08 – 0.15
M’ MKO	0.23	0.18 – 0.29

Zeropoint Measurements for 5″ Aperture at Airmass=1.0

The zeropoint listed below is for 1.0 airmass and is defined as

    (true magnitude) = (instrumental magnitude) + zeropoint 

A larger zeropoint implies a higher throughput. Small changes in zeropoint can be expected due to sky, telescope and instrumentation throughput variations. The zeropoint will be smaller than the values listed below if the seeing is very poor, unless the software aperture is increased. Observers should always determine their own zeropoint (and extinction if necessary) for the night.

WFCAM Zeropoints (ZY not yet well determined)

Filter	Zeropoint, mag
WFCAM Z	22.98
WFCAM Y	22.81
J	23.00
H	23.24
K	22.55

UIST Zeropoints

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

UFTI Zeropoints

Filter	Zeropoint, mag
	1999 – Feb 2001; Feb 2002 Controller#1	Mar 2001 – Jan 2002; Mar 2002 – Present Controller#2
I	22.03 – 22.24
Z	22.59 – 22.86
Y		22.7+/-0.07 (from May ’07)
J	22.19 – 22.52	22.50 – 22.78
H	22.35 – 22.60	22.75 – 22.90
K	21.92 – 22.10	22.20 – 22.44

IRCAM/TUFTI Zeropoints

Filter	Zeropoint, mag
J	22.98 – 23.21
H	22.96 – 23.18
K	22.27 – 22.53
L’	21.02 – 21.22
M’	18.88 – 19.13