From 2.27 to 2.45 microns (the K_dark band) there is a natural gap in the airglow distribution, where one may hope to reach the background level (Harper 1989). The existence of this window was confirmed during 1994 at the South Pole by two independent experiments under the auspices of CARA: IRPS (Ashley et al. 1995), developed by the University of New South Wales (UNSW), and the 60-cm SPIREX telescope and GRIM camera, from the University of Chicago. The results (Ashley et al. 1996; Nguyen et al 1996) show that the window exists, and that the sky flux is between 20 and 100 times less than at temperate latitude sites. For reasons that are still unclear, the sky emission is a factor of ~5 above the original prediction (Harper 1989).
During June 1996, the SPIREX telescope observed the edge-on spiral galaxy NGC 5907, searching for near-infrared halo emission. In a total of 7.3 hours of observing (3.3 hours on source, 4 hours on sky) SPIREX was able to perform surface photometry to the extraordinarily deep limit of 25 mag arcsec (Rauscher et al. 1998).
At longer wavelengths, 2.9-4.2 microns, the IRPS showed a factor of 20-40 lower sky emission at the South Pole. This wavelength region has received relatively little scientific attention in the past due to the poor atmospheric transmission at typical observatories, and the large flux from the telescope and sky. The South Pole has the dual advantages of wider, more stable windows, and low background flux. This allows wider broad-band filters to be used without adding noise from the sky, and also allows longer integration times before the limit of detector saturation is reached. For these reasons, the gains in the L and M bands could surpass those in the 2.27-2.45 micron window.Back to the main site testing page.