The Imaging FTS for Herschel SPIRE
Mach-Zehnder Fourier Transform Spectrometer for sub-millimetre astronomy
Bruce M. Swinyard, Kjetil Dohlen, Didier Ferand, et al. — Rutherford Appleton Laboratory & Laboratoire d'Astrophysique de Marseille
Overview
The design of the Fourier Transform Spectrometer for the Herschel sub-millimetre Spectral and Photometric Imaging Receiver (SPIRE) is described. This innovative design uses intensity beam splitters in a Mach-Zehnder configuration rather than traditional polarising beam splitters. It achieves a resolution of 0.04 cm⁻¹, a 2.6 arcmin circular field of view, and covers a large wavelength range from 200 to 670 microns — all with a single scanning mirror mechanism.
Key Highlights
0.04 cm⁻¹ Resolution
Maximum resolving power of 1000 at 250 µm
Mach-Zehnder Design
Intensity beam splitters replace traditional polarising beam splitters
Single Mechanism
One scanning mirror drives both interferometer arms
4× Path Folding
Roof-top mirrors fold optical path, doubling resolving power per unit travel
<11 K Operation
Focal plane cooled below 11 K; bolometers at ~300 mK
200–670 µm Coverage
Large sub-millimetre wavelength range with 2.6 arcmin FOV
Optical Configuration
After reflection from common input mirrors, the spectrometer beam is picked off at an intermediate field image and sent through the optical bench to a parallel plane 170 mm from the photometer plane. The input relay mirror focuses the beam to an image plane just after the first beam splitter, after which the beam is collimated and sent vertically toward the moving mirror assembly using roof-top mirrors. The roof-top shifts the beam and sends it toward the camera mirror. A fold mirror takes the beam out of plane, allowing a single mirror mechanism to serve both arms and achieve a factor-of-four folding of optical path difference relative to actual mirror movement.
Cryogenic Architecture
All optical components except final filters, lenses, and detector assemblies are mounted from the common optical bench at <5 K. The detectors, lenses, and final filters are mounted in a thermally isolated box strapped to the Herschel cryostat liquid helium tank at <2 K. NTD Germanium bolometer feedhorn arrays operate at ~300 mK, provided by a ³He sorption cooler. Pupil images at three positions in each arm control unwanted rays; the final fold mirror acts as a reflective stop.
Telecentricity Solution
Limited space made it impossible to make the final beams telecentric — a problem because feedhorn detectors require a flat focal plane. Off-axis detector beams would miss the pupil image and lose efficiency, exacerbated by beam shear during optical path difference changes. A lens was added to the detector assembly, moving the pupil image effectively to infinity and making the optics appear telecentric. This eliminated fringe-contrast loss during scanning.
The Imaging FTS for Herschel SPIRE — PDF
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