SPATIAL HETERODYNE SPECTROSCOPY FOR THE EXPLORATION OF DIFFUSE INTERSTELLAR EMISSION-LINES AT FAR-ULTRAVIOLET WAVELENGTHS

Spatial heterodyne spectroscopy (SHS) is a new instrumental technique for interference spectroscopy which promises to extend into the FUV (1200-2000 angstrom) spectral region the large throughput advantage at high spectral resolution usually associated with Fabry-Perot and Michelson interferometers. In addition, SHS systems are compact in size, can be field-widened to increase their throughput advantage even further, have no moving parts, and can be built in all-reflection configurations. SHS appears to be well suited for high resolution, space-based spectroscopy of faint interstellar emission lines in the ultraviolet. This has significant implications for the study of the dynamics and distribution of hot gas within the Galactic disk and halo. For example, a field-widened SHS incorporating 5 x 5 cm gratings could obtain a radial velocity resolved (20 km s-1), 3-degrees angular resolution map of the high-latitude interstellar C IV lambda-1550 emission in less than 1 year. SHS systems built and tested in the laboratory at visible and UV (2500 angstrom) wavelengths have verified the basic concepts and performance characteristics of the technique, and the development of SHS for FUV astrophysics has begun.