The SPIRE Instrument for Herschel
Updated SPIRE instrument design for the Herschel Space Observatory
M. J. Griffin, B. M. Swinyard, L. Vigroux — Cardiff University, Rutherford Appleton Laboratory, CEA Service d'Astrophysique
Overview
This paper presents the refined SPIRE instrument design for ESA's Herschel Space Observatory. SPIRE provides three-band photometry from 250–500 µm and imaging Fourier Transform Spectroscopy from 200–670 µm using NTD spider-web bolometers cooled to 0.3 K. A beam steering mirror (BSM) supported by flexural pivots enables spatial modulation, while the FTS scanning mirror drive provides smooth, frictionless motion for high-resolution spectroscopy.
Key Highlights
Updated for Herschel
Refined design from the FIRST mission concept for the Herschel Space Observatory
4×8 arcmin FOV
Simultaneous photometry at 250, 350, and 500 µm with diffraction-limited resolution
FTS Resolution
Adjustable 0.04–2 cm⁻¹ (λ/Δλ = 20–1000 at 250 µm) with continuous scan mode
Flex Pivot BSM
Beam Steering Mirror on flexural pivots for frictionless cryogenic spatial modulation
0.3 K Detectors
NTD germanium bolometers in feedhorn-coupled spider-web configuration
Three Temperature Stages
4 K, 2 K, and 300 mK — all without lubricants or sliding surfaces
Scientific Objectives
SPIRE on Herschel targets the investigation of galaxy formation and star formation at high redshift. Galaxies emit 30–100% of their energy in the far infrared due to reprocessing of stellar UV by interstellar dust, with the far-IR peak redshifted into the SPIRE wavelength range for z > 1. SPIRE's unmatched sensitivity enables deep imaging photometry and moderate-resolution spectroscopy of these sources.
Flexural Pivot Mechanisms
Two key mechanisms in SPIRE use flexural pivots: the Beam Steering Mirror (BSM) for photometric spatial modulation, and the FTS scanning mirror drive. Both operate at cryogenic temperatures (4 K and below) where conventional bearings with lubricants would fail. Flexural pivots provide friction-free, wear-free, maintenance-free motion — eliminating lubricant outgassing that would contaminate the cold optics.
Photometer Design
The photometer field of view is 4×8 arcminutes, observed simultaneously in all three bands through fixed dichroic beam-splitters. The 4 K optical elements are mounted directly from the main optical bench panel; the 2 K detector enclosures are suspended by stainless steel blades. Three array modules are bolted to the outer wall of the 2 K box, with detector arrays, feedhorns, and filters thermally isolated by Kevlar wires and cooled by a thermal strap to the ³He refrigerator.
FTS Architecture
The Mach-Zehnder FTS uses novel broadband intensity beam dividers and a single constant-speed scanning mirror mechanism to vary optical path difference in both arms simultaneously. A 4× path folding via roof-top mirrors doubles the resolving power per unit of mirror travel. The maximum OPD provides 0.04 cm⁻¹ resolution (λ/Δλ = 1000 at 250 µm). A lens was added to the detector assembly to address telecentricity, eliminating fringe-contrast loss during scanning.
The SPIRE Instrument for Herschel — PDF
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