Use of a Near UV Spatial Heterodyne Spectrometer for Interstellar [OII] Emission Line Studies: Characteristics, Calibration, and Atmospheric Foreground Characterization

This paper describes the use of a newly developed Spatial Heterodyne Spectrometer (SHS) designed to observe radial velocity resolved profiles of diffuse [OII] 3726 Å\ and 3729 Å\ emission lines from the warm (104 K), low-density (10-1 cm-3), ionized component of our Galaxy\u27s interstellar medium (WIM). The [OII] SHS combines interferometric and field-widening gains to achieve sensitivities much larger than conventional grating instruments of similar size and resolving power, and comparable to the Wisconsin H\alpha Mapper (WHAM) Fabry-Perot, but in the near UV where WHAM cannot observe. The high spectral resolution and sensitivity of the SHS allowed us to spectrally isolate for the first time Galactic from terrestrial [OII] emission. We were able to identify the terrestrial [OII] foreground emission and other nearby airglow lines in directions toward very low intensity Galactic [OII] emission regions. The terrestrial [OII] \lambda3729/\lambda3726 line intensity ratio was measured to be 0.47 ±.05:1, compared to an emission ratio of 1.5:1 predicted (and observed) for the interstellar [OII] emission lines in the low density limit. Atmospheric foreground characterization, spectral calibration and absolute intensity calibration are discussed