A far-infrared mapping spectrometer for SOFIA

We present a concept for a far-IR integral field spectrograph for SOFIA, the Cornell Mapping Spectrometer (CMS). CMS takes advantage of the next generation of large bolometer arrays to deliver moderate resolution spectroscopy in the wavelength range between 40 μm and 220 μm over a large field of view. Two separate wavelength channels with each employing six SCUBA-2-like bolometer arrays will provide 64 element spectra over a 10 × 12 spatial pixel footprint. By matching the pixel size to the beam size the field of view obtained is 190" × 160" at the important [CII] line in the long-λ channel and 77" × 64" at the [OI] line in the short-λ channel. The sliced image is fed into an R2 echelle spectrometer of modest resolving power (R ≈ 2000). The grating will operate in 4th order at 158 μm in the long-λ channel and 4th order at 63 μm in the short-λ channel. This results in nearly matching orders for other important fine-structure lines of [NII] and [OIII]. Filterwheels are placed right in front of the detector arrays to separate the orders. By subdividing the detector assemblies and appropriately placing the 3 × 1 detector sub-assemblies it is possible to observe four fine-structure lines simultaneously. CMS combines the advantages of a FIFI-LS design (instantaneous spatial and spectral coverage) with the sensitivity advantage of bolometers over photo-conductors. This will make CMS the instrument of choice for deep spectroscopic integrations of extended regions in galaxies.