<title>Differential field-widened spatial heterodyne spectrometer for investigations at high spectral resolution of the diffuse far-ultraviolet 1548-A emission line from the interstellar medium</title>

Abstract

In this paper we describe the design of the Spatial Heterodyne Spectrometer that we are developing for velocity-resolved observations of the diffuse interstellar C IV emission, and we describe a new implementation of the Spatial Heterodyne Spectroscopy (SHS) concept that reduces contamination due to grating scattering and unwanted orders in wide field applications. Laboratory tests of this concept at visible and ultraviolet wavelengths demonstrate that this SHS technique is suitable for the study of faint, diffuse emission lines at any wavelength, particularly in the ultraviolet, where the full advantages of interference spectroscopy have not been realized. Specifically, the development of SHS will provide the opportunity to map both the distribution and radial velocities of the hot (10<SUP>5</SUP> K) component of the interstellar medium. An SHS spectrometer capable of obtaining the first velocity-resolved (20 km s<SUP>-1</SUP>) line profiles of the C IV (lambda) 1548 emission line in the Cygnus loop and in the hot component of the interstellar medium at high Galactic latitude is currently being developed for suborbital observations. Our long term goal is the development of SHS for a radial-velocity-resolved sky survey of the 104+5) K `coronal gas' component of the interstellar medium using a small satellite with an approximately one year operational lifetime.