CASIMIR, the Caltech Airborne Submillimeter Interstellar Medium Investigations Receiver, is a far-infrared
and submillimeter heterodyne spectrometer, being developed for the Stratospheric Observatory For Infrared Astronomy,
SOFIA. CASIMIR will use newly developed superconducting-insulating-superconducting (SIS) mixers.
Combined with the 2.5 m mirror of SOFIA, these detectors will allow observations with high sensitivity to be
made in the frequency range from 500 GHz up to 1.4 THz. Initially, at least 5 frequency bands in this range
are planned, each with a 4-8 GHz IF passband. Up to 4 frequency bands will be available on each flight and
bands may be swapped readily between flights. The local oscillators for all bands are synthesized and tuner-less,
using solid state multipliers. CASIMIR also uses a novel, commercial, field-programmable gate array (FPGA)
based, fast Fourier transform spectrometer, with extremely high resolution, 22000 (268 kHz at 6 GHz), yielding
a system resolution > 106. CASIMIR is extremely well suited to observe the warm, ≈ 100K, interstellar medium,
particularly hydrides and water lines, in both galactic and extragalactic sources. We present an overview of the
instrument, its capabilities and systems. We also describe recent progress in development of the local oscillators
and present our first astronomical observations obtained with the new type of spectrometer.