RUGATE FILTERS FOR OH-SUPPRESSED IMAGING AT NEAR-INFRARED WAVELENGTHS

Ground-based observations at near-infrared wavelengths are severely af fected by atmospheric OH bands. Many authors have recognized the poten tial gains in sensitivity from suppressing these features, Dispersive instruments show some promise but are both expensive and complicated t o build. OH suppression filters using single or periodic notches have the advantage of simplicity but significant gains have not pet been re alized. Rugate filters (with graded index inhomogeneous coatings) offe r key advantages for astronomical imaging. It is possible to produce a transmission profile comprising a series of irregular and sharply def ined bandpasses. We demonstrate through numerical simulation of rugate filters that it should be possible to achieve 95 per cent suppression of the OH features in the J photometric band, while retaining roughly half of the spectral coverage, This would lead to extraordinary gains in sensitivity even for observations of continuum sources, In additio n, these filters allow longer exposures before the detector saturates on the sky background, I- and z-band filters can also be envisaged. In 1-arcsec seeing, a J-band rugate filter used in conjunction with a 4- m telescope would detect a J = 23 continuum source at 5.0 sigma in a s ingle 10-min exposure. In comparison, a conventional J filter requires multiple exposures for a IO-min Integration time and achieves only a 2.5 sigma detection. Fur emission-line sources. the rugate filter has an even bigger advantage over conventional filters, with a fourfold in crease in signal-to-noise ratio possible in certain instances. Astroph ysical studies that could benefit From rugate filters are searches for very low-mass stars and galaxy evolution out to z=3.