Introduction

The desire for high spectral resolution observations in the near-infrared has been met with three main types of instrument. The traditional way of mapping an object at high spectral resolution is to use a long-slit cooled grating spectrograph, and step the slit across the sky. Although this technique records data at every spectral point simultaneously, it is highly inefficient on extended objects if only a single wavelength (or a small number of wavelengths) are of interest. The Fourier Transform Spectrometer (FTS) works by Fourier transforming an interferogram produced by a two-beam interferometer, and can perform measurements over a large wavelength range (e.g., 0.9 - 5.5 m in the case of the CFHT FTS; Bohlender 1994). The FTS tends, however, to be mechanically large, complex, and expensive, and is also not very efficient for monochromatic applications. Furthermore, every pixel has the noise from the entire continuum in it, and this noise is correlated from pixel to pixel. The Fabry-Perot interferometer, by contrast, is small, has a high throughput (compared to a grating of comparable size and resolving power; Jacquinot 1954), and can deliver consistently high spectral resolution over a wide field, which is imaged directly with an infrared array.

Fabry-Perot etalons have been successfully employed for narrow-band imaging in the optical for many years (e.g., Atherton et al. 1982; Bland & Tully 1989; Jones & Bland-Hawthorn 1997). However, it is only the recent advent of low-noise, large-area detector arrays that has made their use as tunable narrow-band filters for the near-infrared particularly advantageous. There is clearly much to be gained by observing emission lines in the infrared; for example, aside from the reduction in extinction relative to the optical regime, many rotational and vibrational transitions of molecules (such as H) also become accessible. Although many of the brighter Galactic sources can be imaged using filters with fixed, narrow (%) bandpasses, the use of a Fabry-Perot etalon with resolving power confers a number of advantages, including

• the ability to resolve closely spaced lines, or resolve the line of interest from adjacent OH airglow or atmospheric absorption lines;
• the ability to reveal velocity gradients, or even a complete velocity field, when Doppler motions exceed a few tens of km ;
• the reduced sky background and continuum flux passed to the detector. The resultant increase in the line-to-continuum ratio improves the measurement stability, and allows the sky to be sampled less often.

In this paper, we describe one such system, named UNSWIRF (University of New South Wales Infrared Fabry-Perot), which is intended to complement the existing near-infrared imaging and spectroscopic capabilities of IRIS (Allen et al. 1993) at the Anglo-Australian Telescope (AAT), but which could also function as a visiting instrument at other facilities (e.g., MSSSO 2.3 m, UKIRT). In the next section, we give a brief overview of Fabry-Perot systems, and UNSWIRF in particular. We then describe some of the novel approaches taken to calibrate UNSWIRF and process the resultant data, and give illustrations of some of the early scientific results obtained with UNSWIRF.

© Copyright Astronomical Society of Australia 1997