FIRS The Smithsonian Astrophysical Observatory
Far-Infrared Spectroscopy Group
March 11, 1999

The FIRS-2

The FIRS-2 is a Fourier transform spectrometer operating in the far- (80-220 cm-1, recently extended to 340 cm-1) and mid-infrared (330-700 cm-1, recently extended to 1220 cm-1). Interferograms are mostly one sided, and the transformed spectra have an apodized resolution of 0.008 cm-1. In the figure below we show sample spectra at 4 different frequency scales. All spectra are from the 1997 Alaska flight, recorded at a tangent height of roughly 33 km, and represent 6 minutes of integration time. Panels A and B are background subtracted and arbitrarily normalized to a peak value of 1 (not corrected for detector response). Panels C and D are normalized to the intensity of a 277 K blackbody.

Sample FIRS-2 balloon spectra

We have operated the spectrometer from both balloon and aircraft platforms. In the balloon configuration the spectrometer takes roughly 6 minutes to complete a pair of up/down scans; the scan rate is 5 times faster in the aircraft configuration. The FIR field of view (FOV) is 0.22 degrees, or 1.3 km at the limb; the MIR FOV is 0.11 degrees, or 0.65 km at the limb. We typically sample the limb at 4 km intervals (a compromise between temporal and vertical resolution) and including a calibration scan it takes 48 minutes to complete a limb scanning sequence.

A reprint of our most recent instrument paper is available in both postcript and PDF formats.

David Johnson (