On improving IR photometric passbands

In response to recommendations made by a Working Group on Infrared Extinction and Standardization of IAU Commission 25, we have compared the past and present versions of the passbands in the Johnson JHKLMNQ broadband photometric system used at a number of observatories with the atmospheric window transmissions calculated by MODTRAN. The existing passbands are very diverse; contrary to a widespread misperception, there is no semblance of a standard set in use. We have used a family of solar-composition model stellar fluxes from Kurucz (1991 private communication) to model the atmospheric extinction under different water-vapor, height, and airmass conditions. Thus, we have simulated extinction curves for the infrared passbands used at several observatories. A figure of merit related to the curvature of the extinction line describes the sensitivity of each response function to variations in water-vapor extinction. All the existing wideband infrared systems are severely compromised by curve-of-growth effects in the Earth's atmosphere. On the basis of the simulations and the figure of merit, we recommend the improved set of passbands described in Table 3, which are optimized for reproducibility and transformability of photometric results. These have similar effective wavelengths to the existing systems, but are slightly narrower, greatly reducing the effects of molecular absorptions, and allowing the use of a linear extinction curve with much smaller errors. Finally, we discuss the effects of aurorae, airglow and thermal emission on the passbands. The improved passbands are less affected by atmospheric thermal emission than existing ones, and should provide similar signal/noise ratios.