<title>Imaging Spectrometers In Optical Astronomy</title>

Abstract

IntroductionAn imaging spectrometer is a device for examining the 3 dimensional parameter space (x, y, X) which characterizes a major part of the information of interest about astrophysical sources. There are many ways in which this 3D space may be examined, and the techniques used vary across the electro-magnetic spectrum according to the wavelength, and hence the energy of a photon which dictates the telescope diffraction limit and the relevant detector and spectrometer technology. In the visible region of the spectrum the advent of high quantum efficiency, linear, panoramic detectors has led to the development of spectrometers coupled to these detectors, and consequently a large gain in the speed with which the parameter space may be explored. There are three basic types of spectrometer in general use in the visible, based on diffraction gratings, and Fabry -Perot and Michelson interferometers. The last two possess the well known Jacquinot advantage, allowing a much greater throughput of light at a given resolution than a grating of similar size. However, when we are interested in the spectral as well as the spatial information this advantage needs to be reassessed. The use of 2D detectors allows us to access 2 of the three dimensions simultaneously. In the first part of this paper we compare these three types of imaging spectrometer (see Fig. 1) in terms of their ability to obtain both spectral and spatial information, and discuss practical limitations. The second part describes the the new Imaging Fabry Perot being designed for the 4.2 m on La Palma. The final section deals with the conditions under which optimum use is made of the different devices. The Imaging SpectrometersIn this section we will compare the diffraction grating spectrograph with the Fabry Perot and two beam (or Fourier Transform) interferometers in terms of the speed and quality with which they can map the 3D parameter space (x, y, X). Imaging spectrometers based on the prism will not be discussed, since the grating is superior (higher R -L product) for most astronomical applications. (Jacquinot (1954)). Prism -type imaging spectrometers are increasingly being used in earth -resource studies from satellites, under conditions where long -term stability, low spectral resolution and very large spectral range are required ). Figure of MeritThere are numerous problems involved in comparing spectrometers (see e.g. Meaburn 1976, Harwit and concerning the relative merits of the instrumental profile, sensitivity to noise seeing etc. etc. This discussion will be confined simply to the ability to obtain spectral and spatial information over a wide bandwidth and a wide field to the same approximate resolution and signal to noise ratio and using the same size detector. The conventional figure of merit was defined by Jacquinot (1954) as the luminosity (L) i.e. the ability to accept light from a range of angles where L = B.ti.A.Sdwhere B is the source function,i the efficiency, A is the area of the collimated beam, Q is the solid angle subtended by the ...