V.B.2.b. Herschel Space Observatory
Herschel is a cornerstone observatory mission of the European Space Agency (ESA) and is projected for launch in 2009.[1] It is designed to study the formation and evolution of galaxies outside of our own galaxy and the energy sources of particularly luminous galaxies in the early universe. Because of the absorption and re-radiation of ultraviolet radiation by dust grains, galaxies emit large fractions (~30 - 100%) of their energy in the far infrared. Additionally, distant galaxies have large red-shifts that further shift this radiation to long wavelength. Within our own galaxy, Herschel will search for proto-stars, study the formation of stars from the interstellar medium, and explore the evolution of planetary systems. This is a much wider range of scientific goals than those of SWAS and as a result Herschel carries a suite of complementary instruments.
Herschel is a much larger (7 m high, 4.3 m wide, with a 3.5 m telescope primary and 3.25 ton weight) instrument than SWAS and represents about an additional 10 years of THz technology development. Herschel will have three instruments, employing photoconductor (PACS - Photoconductor Array Camera and Spectrometer), [2] bolometer (SPIRE -Spectral and Photometric Imaging REceiver), [3] and superconducting mixer detectors (HIFI - Heterodyne Instrument for FIRST). [4] In order to optimally detect the molecular emissions against the cold background, these detectors and parts of their optics and electronics will be cooled to cryogenic temperatures by a superfluid liquid helium cryostat with a 5 year lifetime. As an important by-product of this cooling, much smaller local oscillator powers can be used in the heterodyne instruments and operation to much higher frequencies will result.
The three complementary instruments on Herschel typify systems in the Submillimeter /Terahertz and comparison can illustrate the trade-offs associated with each. The heterodyne system will provide very high spectral resolution from 610 µ (490 GHz) down to 160 µ (1.9 THz). However, local oscillator and single mode matching requirements restrict both its high frequency limit and broad bandedness. In comparison, the two 'optical' systems will provide medium resolution spectroscopy and photometry between 60 µ (5 THz) and 600 µ (500 GHz). Because these systems use 'optical' rather than heterodyne detection, they do not face increasing difficulties in the fabrication of single mode mixers and in the provision of local oscillator power with increasing frequency. Indeed, PACS employs a 16 x 25 stressed Ge:Ga array for imaging photometry and spectroscopy that has a long wavelength cutoff near 210 µ.
V.B.2.b.ii. Photoconductor Array Camera and Spectrometer
V.B.2.b.iii. Spectral and Photometric Imaging Receiver
HERSCHEL summary: The suite of instruments on Herschel is a particularly interesting example of the current technological state-of-the-art in the THz region. While taken as a whole, Herschel admirably addresses the scientific issues at hand, there is clearly still a gap between the extension of microwave-like technology up from long wavelength and optical-like technology down from short wavelength.
It is probably fair to say that the single technological advance which would most impact similar systems in the future would be the development of widely tunable Submillimeter /Terahertz sources with enough power to use as local oscillators for arrays of mixer elements throughout this spectral region. This would make possible the development of widely tunable heterodyne receivers (in contrast to receivers in carefully selected spectral regions) whose mixer arrays would have the spatial multiplex advantage of the 'optical' systems in PACS and SPIRE. This ‘ideal’ system of the future would then be able to use both spatial and frequency multiplex principles to fully utilize the precious and expensive photons collected by the telescope.
Back to V.B.2 Two representative THz telescopes
[4] N. Whyborn, The Far Infrared and Submillimeter Universe, ESA SP-401 (1997) 19-24.