UKIRT – Faint Standards

UKIRT – Faint Standards

UKIRT Faint Standards tramsformed to the (new) Mauna Kea Observatory (MKO-NIR) system

Both UKIRT cameras (UFTI and IRCAM) are now equipped with filters from the Mauna Kea Consortium (MKO-NIR) filter set (see the papers “The Mauna Kea Observatories Near-Infrared Filter Set. I. Defining Optimal 1-5 Micron Bandpasses” by Simons & Tokunaga 2002 PASP 114, 169 and “The Mauna Kea Observatories Near-Infrared Filter Set. II. Specifications for a New JHKL’M’ Filter Set for Infrared Astronomy” by Tokunaga et al. 2002 PASP, 114, 180). These filters have been designed to be a better match to the atmospheric windows, offering maximum sensitivity together with minimum dependance on altitude and atmospheric water-vapour content, and are also of excellent optical quality.

The MKO-NIR filter set has been in UFTI since commissioning in October 1998, replaced the old JHK set in IRCAM/TUFTI prior to commissioning of its new plate scale in September 1999, and were installed in UIST prior to its 2002 commissioning. Both the J and H filters are significantly different from the old filters and hence the magnitudes for the Faint Standards on the old IRCAM3 system are no longer on the natural systems of the imagers. Colour transformations have been derived in two ways: one empirically based on UFTI photometry and the other calculated by convolving the known filter profiles with spectroscopic data for a representative set of red stars. The two determinations agree reasonably well but the actual measurements should be used in preference to the transformed values given below. Note that the transformations for IRCAM/TUFTI with the same filter set should be effectively identical. 

The transformations between the old IRCAM3 system and the new MKO-NIR system at H and K are well behaved and single-valued. However for the J filter different terms have to be applied depending whether or not the standard star has intrinsic water absorption features. This is due to the fact that the new J filter cuts off shorter than the old filter, specifically to avoid water absorption in the terrestrial atmosphere. We have determined that for stars with no intrinsic water features the colour transformations are:

K(old) = K(new) + 0.020[+/-0.005](J-K)(new)
(J-H)(old) = 1.040[+/-0.010](J-H)(new)
(H-K)(old) = 0.830[+/-0.010](H-K)(new)
(J-K)(old) = 0.960[+/-0.010](J-K)(new)

K(new) = K(old) - 0.020[+/-0.005](J-K)(old)
(J-H)(new) = 0.960[+/-0.010](J-H)(old)
(H-K)(new) = 1.205[+/-0.010](H-K)(old)
(J-K)(new) = 1.040[+/-0.010](J-K)(old)

however for stars with significant water absorption, stars with spectral type M4 through to the L class (but not including the T class with methane absorption, see Stephens & Leggett 2004 PASP 116, 9):

K(new) = K(old) - 0.020[+/-0.005](J-K)(old)
(J-H)(new) = 0.870[+/-0.010](J-H)(old)
(H-K)(new) = 1.205[+/-0.010](H-K)(old)
(J-K)(new) = 0.980[+/-0.010](J-K)(old)

We have applied the appropriate transformations to the UKIRT Faint Standards and the Table below lists the combined fundamental and extended standards list with the photometry given in the cameras’ current natural system, which is defined by the Mauna Kea consortium JHK filters. The errors include both observational error and system transformation uncertainty. 

For more information on e.g. selection of standard stars see the following sections on either the original Fundamental List of UKIRT standards or the later Extended List.