We have designed and built a high spectral resolution cryogenic double Fabry-Perot infrared spectrometer to survey the diffuse emission of ionized hydrogen (Br γ line) in the Galactic plane. In the initial survey, presented here, we employed the non-imaging mode of the spectrometer to achieve the best possible sensitivity for the faint diffuse emission. The emission measure detection limit was about 300 cm^-6pc. The resolving power of the instrument is 10⁴ and the simultaneous velocity range is approximately 200 km s^-1. The spectrometer consists of a liquid nitrogen cryostat, which accommodates Fabry-Perot interferometers and a liquid helium cryostat for the InSb 256 x 256 detector. Each Fabry-Perot interferometer is located in a separate chamber in the cryostat. Tuning of the spectrometer is accomplished by varying the pressure of dry nitrogen gas in the chamber. The cold shields of the spectrometer and the detector cryostat are connected. Stray radiation is reduced by use of a cold stop, with the appropriate relay optics, located between the cryostats. This design limits stray background radiation and thermal radiation from the spectrometer itself, greatly improving the detection limit of the system. All of these design features maximize experiment sensitivity, which is limited by thermal emission from outside of the spectrometer, and by variation of the atmospheric emission lines.

Date Published: 7 March 2003
PDF: 10 pages
Proc. SPIE 4841, Instrument Design and Performance for Optical/Infrared Ground-based Telescopes, (7 March 2003); doi: 10.1117/12.461750

Published in SPIE Proceedings Vol. 4841:
Instrument Design and Performance for Optical/Infrared Ground-based Telescopes
Masanori Iye; Alan F. M. Moorwood, Editor(s)