IRIS2 Photometric transformations

Transforms to Other Photometric Systems

The J,H,K,Ks filters are the same MKO infrared filters as those discussed in Alan Tokunaga's 2001 paper. This paper contains links to other work using NSFcam on the IRTF and IRCAM3 on UKIRT to define colour terms for these filters. The colour terms for IRIS2 will be different due to those instruments using InSb (not HgCdTe) detectors, and having different optical trains.

Deriving transformations from the IRIS2 system to other systems require that enough stars spanning a wide range in colour be observed. Only a preliminary transformation to the MKO system is available for the "Mark 1" science-grade array in use between March 2002 and April 2005, based on observations of only a handful of UKIRT Faint Standard stars over 3 separate nights in July 2002:

(J - H)_MK = 0.90 +/- 0.05 (J - H)_IRIS2
(H - K)_MK = 1.05 +/- 0.05 (H - K)_IRIS2
(J - K)_MK = 0.95 +/- 0.05 (J - K)_IRIS2

Perhaps most useful is the transformation between the MKO K band, and the Ks band more often used with IRIS2 for reduced background:

K_MK = Ks_IRIS2 - 0.13 (H - K)_IRIS2
= Ks_IRIS2 - 0.12 (H - K)_MK

i.e., redder stars will appear brighter in K than in Ks.

For the "Mark 2" science-grade array in use since May 2006, a more complete set of transformations to other systems has been derived from observations on the night of 2 Sep 2006 of UKIRT Faint Standards FS 140 and 144; NICMOS standards S279-F, S024-D, S071-D, S808-C, and S889-E; and LCO red stars L547 and BRI2202. The zero-points (magnitude of a star in the MKO system giving 1 ADU/s in a 10 arcsecond diameter aperture at 1 airmass), extinction (mag/airmass), and colour terms for this night are given by the following relations:

JMKO = 22.58 - 2.5 log (I / t) - 0.076 (X - 1) - 0.002 (J - K)MKO
HMKO = 22.79 - 2.5 log (I / t) - 0.048 (X - 1) + 0.038 (H - K)MKO
KMKO = 22.35 - 2.5 log (I / t) - 0.090 (X - 1) + 0.003 (J - K)MKO
Ks2MASS = 22.37 - 2.5 log (I / t) - 0.097 (X - 1) - 0.009 (J - Ks)2MASS

where   I = total integrated ADU in 10" aperture;
             t = integration time in seconds;
and     X = airmass of observation.

Note that we do not attempt to extrapolate to zero airmass, due to the Forbes effect (i.e. the extinction curve is non-linear between 0 and 1 airmass) - see Section 2.4 of Tokunaga & Vacca (2007; astro-ph/0702285).

Although included in the MKO filter set, the Ks magnitude scale is not defined in the MKO system, so 2MASS colours are used instead. Transforms to the 2MASS system are given by:

JMKO - J2MASS = -0.03 - 0.03 (J - Ks)2MASS
HMKO - H2MASS = -0.01 + 0.05 (H - Ks)2MASS
KMKO - Ks2MASS = -0.01 - 0.01 (J - Ks)2MASS

The difference between IRIS2's Ks magnitudes and 2MASS Ks magnitudes was found to be <0.01 over the full colour range.

Transforms to the LCO system are given by:

JMKO - JLCO = -0.02 - 0.05 (J - K)MKO
HMKO - HLCO = -0.02 + 0.06 (H - K)MKO
KMKO - KLCO = 0.00 - 0.03 (J - K)MKO

Transforms to other systems can be derived with the help of Leggett et al. 2006, MNRAS, 373, 781 (astro-ph/0609461).

When absolute photometric accuracy is paramount, we still recommend observers determine their own extinction corrections each night, and when transformation to other infrared photometric systems is required, that a suitable colour range of standards be observed to calibrate their own data.