Ground-based observations at near-infrared wavelengths are severely affected
by atmospheric OH bands. Many authors have recognized the potential gains in
sensitivity from suppressing these features. Dispersive instruments show some
promise but are both expensive and complicated to build. OH suppression filters
using single or periodic notches have the advantage of simplicity but
significant gains have not yet been realised.
Rugate filters (with graded index inhomogeneous coatings) offer key
advantages for astronomical imaging. It is possible to produce a transmission
profile comprising a series of irregular and sharply defined bandpasses. We
demonstrate through numerical simulation that it should be possible to achieve
95% 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 addition, these
filters allow much longer exposures before the detector saturates on the sky
background. (I and z-band filters can also be envisaged.)
In 1" seeing, a J-band rugate filter used in conjunction with a 4m telescope
would detect a J=23 continuum source at 5.0-sigma in a single 10 min exposure.
In comparison, a conventional J filter requires multiple exposures for a 10
minute integration time and achieves only a 2.5-sigma detection. For emission
line sources, the rugate filter has an even bigger advantage over conventional
filters, with a fourfold increase in signal:noise ratio possible in certain
instances. Astrophysical studies which could benefit from rugate filters are
searches for very low mass stars and galaxy evolution out to z=3.Comment: 13 pages; Latex; 11 encapsulated postscript figures; submitted to
MNRA