The SPIRE Instrument for FIRST
Bolometer instrument design for ESA's Far Infrared and Sub-millimetre Telescope
Matthew Griffin, Bruce Swinyard, Laurent Vigroux — Queen Mary & Westfield College London, Rutherford Appleton Laboratory, CEA-Service d'Astrophysique
Overview
SPIRE (Spectral and Photometric Imaging Receiver) is a bolometer instrument for ESA's FIRST satellite. Designed for deep extragalactic and galactic imaging surveys and spectroscopy of star-forming regions, it comprises a three-band imaging photometer (250–500 µm) and an imaging Fourier Transform Spectrometer (200–670 µm). A beam steering mirror enables spatial modulation, and novel broadband intensity beam dividers are used in the FTS in a Mach-Zehnder configuration.
Key Highlights
Three Photometer Bands
250, 350, and 500 µm simultaneously with dichroic beam dividers over a 4×8 arcmin FOV
FTS Coverage
200–670 µm with adjustable resolution of 0.04–2 cm⁻¹ (λ/Δλ = 20–1000)
Beam Steering Mirror
Internal BSM or drift scanning for spatial modulation — flex-pivot supported mechanism
0.3 K Bolometers
NTD spider-web germanium bolometers cooled by a ³He sorption refrigerator
Novel Beam Dividers
Broadband intensity beam splitters in Mach-Zehnder configuration for high efficiency
Dual Output Ports
Spatially separated input/output ports enable simultaneous sky and calibration measurements
Scientific Goals
SPIRE's primary goals are investigating galaxy formation statistics and physics at high redshift, and studying the earliest stages of star formation. These require large-area deep photometric imaging surveys and follow-up spectroscopy. SPIRE exploits FIRST's large-aperture cold telescope, complete absence of atmospheric emission, and access to the poorly explored 200–700 µm range.
Flexural Pivot Beam Steering Mirror
The photometer's internal Beam Steering Mirror (BSM) provides spatial modulation of the telescope beam on the detector arrays. The BSM mechanism uses flexural pivots to achieve frictionless, repeatable angular positioning without wear or lubrication — critical for a cryogenic instrument where lubricants would outgas or freeze. Alternatively, the telescope can drift-scan for confusion-limited deep surveys.
FTS Mirror Drive Mechanism
The Fourier Transform Spectrometer operates in continuous scan mode with a constant-speed mirror drive mechanism varying the optical path difference between the two interferometer arms. This scanning mirror mechanism — also flex-pivot supported — must provide extremely smooth, low-friction motion to achieve the 0.04 cm⁻¹ maximum spectral resolution corresponding to λ/Δλ = 1000 at 250 µm.
Cryogenic Architecture
The focal plane unit operates at three temperature stages: 4 K and 2 K (provided by the FIRST cryostat) and 300 mK (from the internal ³He refrigerator). The main optical bench is supported from the 10 K cryostat bench by stainless steel blade mounts. Detector arrays and final optics are enclosed in 2 K boxes, cooled by a thermal strap to the ³He cooler.
The SPIRE Instrument for FIRST — PDF
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