ON IMPROVING IR PHOTOMETRIC PASSBANDS

In response to recommendations made by a Working Group on Infrared Ext inction and Standardization of IAU Commission 25, we have compared the past and present versions of the passbands in the Johnson JHKLMNQ bro adband photometric system used at a number of observatories with the a tmospheric window transmissions calculated by MODTRAN. The existing pa ssbands are very diverse; contrary to a widespread misperception, ther e is no semblance of a standard set in use. We have used a family of s olar-composition model stellar fluxes from Kurucz (1991 private commun ication) to model the atmospheric extinction under different water-vap or, height, and airmass conditions. Thus, we have simulated extinction curves for the infrared passbands used at several observatories. A fi gure of merit related to the curvature of the extinction line describe s the sensitivity of each response function to variations in water-vap or 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 f or reproducibility and transformability of photometric results. These have similar effective wavelengths to the existing systems, but are sl ightly 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 therma l emission on the passbands. The improved passbands axe less affected by atmospheric thermal emission than existing ones, and should provide similar signal/noise ratios.