Working group on infrared astronomy

The evolution of ground-based infrared astronomy into a precise and accurate tool for astronomical studies continued during the past triennium. The limitation of photometric precision as practiced at the time were described and discussed in Milone (1989) and in Young, Milone, and Stagg (1994), partial solutions provided. The limitation in precision was shown to be due primarily to Atmospheric features within the passbands and the use of the edges of the atmospheric windows to define those passbands. The saturation of portions of the passbands high in the atmosphere means, especially for the longer wavelength passbands, a large difference between a linearly extrapolated zero-air mass magnitude and the actual value. The rapid curvature of the extinction curve between 1 and 0 air mass (more properly, a water-vapor mass, notwithstanding the contributions of carbon dioxide, ozone, etc.) is known as the Forbes effect. Since the widths of the atmospheric windows vary with altitude and the circumstances of each site, different observatories have responded to the problem in the past by redefining the Johnson system from J to Q to suit the needs of the site. The result was a proliferation of systems. As a rule, filters were selected for maximum throughput and so were not optimally placed, shaped, and narrowed to minimize the effects of the absorption bands of the terrestrial atmosphere. Given this situation and in light of the tremendous promise of high precision presented by infrared photometry (see Milone 1989), it was clear that something needed to be done to properly standardize the infrared system.