The Accuracy of Infrared Photometry With Arrays

The photometric accuracy and repeatability of an InSb array in the infrared camera IRCAM is investigated. Measurements of standard stars are described; these were made using the United Kingdom Infrared 3.8-m Telescope (UKIRT) over five nights in May 1988, as part of an observational program on globular clusters. Observations were made at wavelengths of 1.25 (J) and 2.2 microns (K). The results show clearly that the camera-array combination is capable of a photometric accuracy of better than 1 percent when the standard-star images obtained with short on-chip exposure (0.5 sec) are flat fielded with long on-chip exposure (7-50 sec) sky flats. Linearity corrections determined in the laboratory were applied to the data and there was no evidence of residual nonlinearity of greater than 2 percent over the magnitude range 6.5-13.0 m. The range of mean nightly zero points at K during the five nights was 0.03 m, showing that both the site and the camera system were very stable over this period. At J the zero points were less well determined and had twice this range during three nights.