- E. F. Milone, Chair

The Working Group has continued to pursue the improvement of IR passbands following the recommendations of the joint meeting of Commissions 25 and 9 at the Baltimore IAU (Milone 1989). The work has also continued to be funded by research grants from the University of Calgary and Canadian NSERC. As previously noted, the passbands of the Johnson JHKLMNQ broadband photometric system along with atmospheric window transmissions were calculated by MODTRAN, and used a series of stellar flux models from Kurucz (1991 private communication) to probe the atmospheric extinction under different water vapor, height, and airmass conditions. The curvature of the resulting extinction line describes the sensitivity of each response function to variations in water-vapor extinction. On the basis of our simulations, and an extensive set of experiments to optimise S/N without seriously degrading the reproducibility and transformability, and subject to field trials, we recommended a set of passbands which should improve both extinction and standardization from all sites, and enable transformable infrared photometry from lower altitude sites than is currently the case, at least for the shorter wavelength passbands. The basis for the work, the procedures, and the recommendations were summarized in Young, Milone, and Stagg (1993, 1994), and in Milone, Young and Stagg (1993, 1995); a paper presenting the extinction curves is in preparation. The importance of the new passbands for studies of binary stars in clusters was reviewed briefly in Milone (1996).

Since the previous report, a small subcommittee of the working group, viz., A. T. Young and Milone, has continued to work on the problem, and has computed the emission expected through each of the tested passbands. First, the atmospheric model used for numerical simulations was upgraded to a recent release of MODTRAN3. Second, numerical experiments confirmed that the new filters have two orders of magnitude less emission in the iK passband over the traditional K filter and more than one order of magnitude improvement over one of the better K filters, in use at the RAO. The experiments also indicated a reduction in emission of 1/3 over the Wainscot-Cowie filter, and nearly a factor 2 improvement over the 2Mass filter for the U.S. Standard atmosphere at a site at 2 km altitude. For the L passbands, the improved iL filter was a factor 6 better than a modern L filter, which is cut somewhat to the red of the original Johnson L filter.

Filters within the specifications of the Young et al. (1994) recommendations have been produced by Custom Scientific of Phoenix and are now available for purchase and testing. Six of these filters (iz, iH, iJ, iK, IL, and iLp) were placed in a dewar equipped with an InSb detector at the Rothney Astrophysical Observatory of the University of Calgary and were undergoing testing in Fall, 1996. Initial trials indicated that the filters were successful at reducing the sensitivity of IR photometry to water vapour, but more trials were needed to check the behaviour of these filters compared to versions of the Johnson filters, and the reliabilty, i.e., self transformation, under different atmospheric conditions. Future acquisition and testing of longer passband filters (iM, iN, in, and iQ) are planned.

The WG would like to express its appreciation to David Marcus of Custom Scientific for undertaking the manufacture of these new filters, at reasonable cost, and for extensive discussion and helpfulness. Andy Young provided improvements to the draft versions of this report.

References