NUMERICAL EVALUATION OF OH AIRGLOW SUPPRESSION FILTERS

The presence of OH lines in the near-infrared spectrum of the night sk y currently limits sensitivities out to 2.2 mu m. Attempts to rectify this situation have so far only concentrated on elaborate grating spec trographs, such as those described by Iwamuro et al. (1994, PASJ, 46, 515) and Content and Angel (1994, Proc. SPIE, 2198, 757). This is beca use appreciable gains using filter-based solutions have so far proven to be elusive (Herbst 1994, PASP, 106, 1298). An alternative solution, described in this paper, explores the use of a multiple bandpass filt er (MBF) to selectively remove those wavelengths at which the brightes t OH lines occur. A numerical evaluation of MBFs as used in the J and H bands is presented, the results from which demonstrate that signific ant improvements in signal to noise are only possible from such a filt er (and indeed, any method of OH line blocking), if more than the brig htest similar to 50% of the line population is removed. We show here t hat relative to broadband J and H, gains in signal to noise greater th an unity are difficult to achieve for a continuum source. Over narrowe r bandpasses, gain factors of around 1.8 (J band) and 1.5 (H band) are possible for the same continuum source and can be 2 to 6 for a source with line emission. We conclude that the ordinary rectangular wave pr ofile of an MBF is most suited to blocking over narrow and intermediat e bandwidth filters, as might be used for imaging of emission-line sou rces or searches for such objects in wide-field surveys. Significant g ains in continuum imaging will require more complex filter technologie s (e.g., ''rugate'' coating technology), which are still evolving.