<title>Heterodyne instrument for FIRST (HIFI): preliminary design</title>

We describe the preliminary design of the proposed Heterodyne Instrument for FIRST (HIFI). The instrument will have a continuous frequency coverage over the range from 480 to 1250 GHz in five bands, while a sixth band will provide coverage for 1410 - 1910 GHz and 2400 - 2700 GHz. The first five bands will use SIS mixers and varactor frequency multipliers while in the sixth band a laser photomixer local oscillator will pump HEB mixers. HIFI will have an instantaneous bandwidth of 4 GHz, analyzed in parallel by two types of spectrometers: a pair of wide-band spectrometers (WBS), and a pair of high- resolution spectrometer (HRS). The wide-band spectrometer will use acousto-optic technology with a frequency resolution of 1 MHz and a bandwidth of 4 GHz for each of the two polarizations. The HRS will provide two combinations of bandwidth and resolution: 1 GHz bandwidth at 200 kHz resolution, and at least 500 MHz at 100 kHz resolution. The HRS will be divided into 4 or 5 sub-bands, each of which can be placed anywhere within the full 4 GHz IF band. The instrument will be able to perform rapid and complete spectral line surveys with resolving powers from 10<SUP>3</SUP> up to 10<SUP>7</SUP> (300 - 0.03 km/s) and deep line observations.