Compact imaging spectrograph for broadband spectral simultaneity

Abstract

We report on the design of a small spectrograph that is capable of imaging several thousand angstrom simultaneously at a moderate spectral resolution. The prototype instrument included a number of developmental items that were used to assess their utility in this and other applications. Some we would recommend using again, some we would not. In the configuration that was built and tested, the instantaneous wavelength range was chosen to be 3700-11,700 Å. However, the wavelength range could be selected for a lower wavelength, as low as ~ 1200 Å. The spectral imaging was achieved with an intensified-CCD focal-plane detector. The broad wavelength coverage was achieved with a matrix of four diffraction gratings and a custom-designed photocathode system. The photocathode was specially built to provide a response over the chosen broad wavelength range by use of a single image intensifier. The theoretical spectral resolution of the instrument varied from 12 to 20 Å depending on waveleng th segment. A higher spectral resolution can be selected at the expense of total wavelength coverage. The optical system was designed to be moderately fast (f/6) when considered at the level of each of the four optical subchannels and suitable for use on relatively weak airglow signals. The instrument was designed to be readily portable, weighing 15 kg, with an envelope of 37 cm × 37 cm × 48 cm. The advantages and weaknesses of such an instrument are discussed, and improvements are suggested for specific applications. This study represents a stepping stone in the evolution of electronic spectrographs and leads to later designs that are currently being evaluated.