The Herschel Space Observatory, to be launched to the Earth–Sun second Lagrangian point, L2, by the European Space Agency in 2008, will carry with it a 3.5-meter passively cooled silicon carbide telescope. Together with European and Canadian teams, US contributioning scientists include co-Investigators on the instrument payload as well as Mission Scientists and the Optical System Scientist. The Observatory will be used to study the formation and evolution of stars and galaxies, the physics and chemistry of the interstellar medium, and solar system bodies.
The self-emission of the Cassegrain telescope’s reflecting surfaces is expected to be the dominant source of noise for two of the three Observatory instruments. Thermal models of the telescope–spacecraft system predict an equilibrium operating telescope temperature in the range 70–90 K. Both the Photodetector Array Camera & Spectrometer PACS and the Spectral and Photometric Imaging Receiver SPIRE, which together provide both imaging and moderate resolution spectroscopy over the 55–670-micron spectral range, are expected to be background-limited by the thermal self-emission of the telescope’s reflecting mirrors. Together, the telescope’s equilibrium temperature in space and the emissivity of the mirror surfaces will determine the far-infrared–submillimeter background and thus the sensitivity of these instruments if stray-light levels can be kept low relative to the mirror emission. A third instrument, the Heterodyne Instrument for the Far Infrared HIFI, is expected to achieve resolving powers up to R ~ 107. The HIFI covers the frequency range from 480 to 1250 GHz in five bands and the range from 1410 to 1910 GHz in two bands, by use of seven mixers and 14 local oscillator (LO) subbands. With a smooth and stable baseline, its detection sensitivity is expected to be within a factor of 3 of the theoretical quantum noise limit.