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The continued improvement in the sensitivity of superconducting far-infrared bolometers necessitates improved designs of cryogenically cooled broadband spectrometers in order to fully exploit the potential of such detectors. While Fourier transform spectrometers (FTS) have an illustrious history in astronomical research, the sensitivity of state-of-the-art detectors is such that the multiplex disadvantage of FTS is prohibitive unless the spectral bandpass can be restricted to less than 1%. One method of achieving this goal, and the one that has been adopted for the SPICA SAFARI instrument, is to use a diffraction grating as the post-dispersing component. Unlike a typical FTS, in which a single detector simultaneously measures a broad spectral band, a post-dispersed detection system requires multiple detectors, each with their own unique spectral, spatial, and temporal responses. Moreover, the narrow spectral band viewed by each detector results in an interferogram having a large coherence length; the signal is heavily modulated, yet truncated. While simulations play a useful role in modeling instrumental performance, there is no substitute for data obtained from a real implementation of an instrument concept. In this paper we describe the development and current status of a cryogenic, far-infrared, postdispersed, polarizing FTS (PDPFTS): a demonstrator for the SPICA SAFARI instrument.

Recent advances in far-infrared detector technology have led to increases in raw sensitivity of more than an order of magnitude over previous state-of-the-art detectors. With such sensitivity, photon noise becomes the dominant noise component, even when using cryogenically cooled optics, unless a method of restricting the spectral bandpass is employed. One method is to use a low-resolution diffraction grating spectrometer to post-disperse the signal from a high-resolution instrument, such as a Fourier transform spectrometer (FTS). This concept has been adopted for the SAFARI instrument on the SPICA mission. This paper discusses the development of a prototype cryogenic grating spectrometer that has been used to evaluate the concept of a post-dispersed polarizing FTS over the range from 285-500 μm.

The sensitivity of state-of-the-art superconducting far-infrared (FIR) detectors is such that wideband spectroscopic observations, particularly those employing Fourier transform spectrometers (FTS), will require techniques to reduce the spectral bandwidth of a detector to limit the photon noise from an astronomical source. The proposed SPICA SAFARI instrument employs grating spectrometers to post-disperse the light that has been modulated by a polarizing FTS onto a detector array, thereby reducing the photon noise on each detector. While the principles of this method are understood, to date an integrated system has not been realized in the laboratory. We present the development of a FIR post-dispersed polarizing FTS (PDPFTS) consisting of a warm FTS and a 4 K grating spectrometer as a first step to a fully cryogenic PDPFTS demonstrator. Realistic astronomical spectra are generated by combining line emission from a tunable THz photomixer source with continuum emission from a variable blackbody source.

The sensitivity of state-of-the-art superconducting far-infrared detectors used in conjunction with cryogenically cooled space telescopes and instrumentation is such that spectroscopic observations are generally limited by photon noise from the astronomical source or by galactic foreground or zodiacal emission within the field-of-view. Therefore, an instrument design that restricts the spectral bandpass viewed by the detector must be employed. One method of achieving background limited, high resolution spectroscopy is to combine a high resolution component such as a Fabry-Pérot interferometer (FPI) with a lower resolution, post-dispersing system, such as a grating spectrometer, the latter serving to restrict the spectral bandpass. The resonant wavelength of an FPI is most often tuned by changing the spacing or medium between the parallel reflecting plates of the etalon. In this paper, we present a novel design for an FPI in which the wavelength is tuned by scanning the angle of incidence on a high refractive index etalon. This concept simplifies the cryomechanical design, actuation, and metrology. The first results from the realized instrument are presented and compared with theory. The effects on the spectral response as a function of the incident angle have been simulated and shown to agree well with the observation.

The sensitivity of state-of-the-art superconducting far-infrared detectors is such that astronomical observations at these wavelengths are limited by photon noise from the astronomical source unless a method of restricting the spectral bandpass is employed. One such method is to use a high resolution Fabry-Perot interferometer (FPI) in conjunction with a lower resolution, post-dispersing system, such as a grating spectrometer. The resonant wavelength of an FPI is typically tuned by changing the spacing or medium between the parallel reflecting plates of the etalon. We previously reported on a novel design in which the wavelength is tuned by scanning the angle of incidence, which simplifies the cryo-mechanical design, actuation and metrology. Here we present first light results from the realized instrument.

Over half of the light incident on the Earth from the Universe falls within the Far-Infrared (FIR) region of the spectrum. Due to the deleterious effects of the Earth's atmosphere and instrument self-emission, astronomical measurements in the FIR require space-borne instrumentation operating at cryogenic temperatures. These instruments place stringent constraints on the mechanical and thermal properties of the support structures at low temperatures. With high stiffness, tensile strength, strength-to-mass ratio, and extremely low thermal conductivity, carbon fibre reinforced polymers (CFRPs) are an important material for aerospace and FIR astronomical applications, however, little is known about their properties at cryogenic temperatures. We have developed a test facility for exploring CFRP properties down to 4 K. We present results from our ongoing study in which we compare and contrast the performance of CFRP samples using different materials, and multiple layup configurations. Current results include an evaluation of a cryostat dedicated for materials testing and a custom cryogenic metrology system, and preliminary cryogenic thermal expansion measurements. The goal of this research is to explore the feasibility of making CFRP-based, lightweight, cryogenic astronomical instruments.

We present the design and performance of a cryogenic, angle-scanned Fabry-Perot interferometer for far infrared astronomical spectroscopy. Novel features of the design are discussed, and the spectral response of the instrument is modeled. Experimental methods being developed to validate the spectral response are presented.

Nature is such that observations at far-infrared wavelengths are optimal for exploring both the nearby and distant Universe. The minute amount of energy carried by far-infrared photons, however, requires extremely sensitive instrumentation for their detection. Moreover, the instrumentation itself must be cooled to

The SPICA mission will employ ultra-sensitive superconducting detectors. In this case the multiplex disadvantage of Fourier transform spectroscopy dictates the use of a post-dispersed, diffraction grating detection system. The challenges of reconstructing astronomical spectra will be reviewed.

The sensitivity of state-of-the-art superconducting far-infrared detectors is such that astronomical observations at these wavelengths are limited by photon noise from the astronomical source unless a method of restricting the spectral bandpass is employed. One such method is to use a high resolution Fabry-Pérot interferometer (FPI) in conjunction with a lower resolution, post dispersing system, such as a grating spectrometer. The resonant wavelength of an FPI is typically tuned by changing the spacing or medium between the parallel reflecting plates of the etalon. We present a novel design in which the wavelength is tuned by scanning the angle of incidence, which simplifies the cryo-mechanical design, actuation and metrology. The effects on the spectral response as a function of incident angle have been simulated and shown to be manageable.

The principles and practice of astronomical imaging Fourier transform spectroscopy (FTS) at far-infrared wavelengths are described. The Mach-Zehnder interferometer design has been widely adopted for current and future imaging FTS instruments; we compare this design with two other common interferometer formats. Examples of three instruments based on the Mach-Zehnder design are presented. The techniques for retrieving astrophysical parameters from the measured spectra are discussed using calibration data obtained with the Herschel SPIRE instrument. The paper concludes with an example of imaging spectroscopy obtained with the SPIRE FTS instrument., 10 pages; 9 figures. Accepted for publication in the Canadian Journal of Physics. v2: minor typographical changes to match published version

Future far-infrared space astronomy missions will feature cryogenically cooled telescopes that will enable over 100x increase in sensitive over previous missions. The effects of pointing errors on FTS observations in this environment are discussed.

Cryogenic optical equipment will become a standard requirement for future far infrared observatories. Here we outline the design of a cryogenic test bed for optical metrology and discuss its role in regards to component testing of future space-borne instruments.

The Japanese SPace Infrared telescope for Cosmology and Astrophysics (SPICA), a 3 m class telescope cooled to ~ 6 K, will provide extremely low thermal background far-infrared observations. An imaging Fourier transform spectrometer (SAFARI) is being developed to exploit the low background provided by SPICA. Evaluating the performance of the interferometer translation stage and key optical components requires a cryogenic test facility. In this paper we discuss the design challenges of a pulse tube cooled cryogenic test facility that is under development for this purpose. We present the design of the cryostat and preliminary results from component characterization and external optical metrology.

The Spectral and Photometric Imaging Receiver (SPIRE) is one of three scientific instruments on board the European Space Agency's Herschel Space Observatory which ended its operational phase on 29 April 2013. The low to medium resolution spectroscopic capability of SPIRE is provided by an imaging Fourier transform spectrometer (iFTS) of the Mach-Zehnder configuration. With their high throughput, broad spectral coverage, and variable resolution, coupled with their well-defined instrumental line shape and intrinsic wavelength and intensity calibration, iFTS are becoming increasingly common in far-infrared space astronomy missions. The performance of the SPIRE imaging spectrometer will be reviewed and example results presented. The lessons learned from the measured performance of the spectrometer as they apply to future missions will be discussed.

The design and performance of a dual polarizing bolometer detector system for use with a polarizing Fourier transform spectrometer to conduct broadband astronomical spectroscopy at submillimeter wavelengths is presented. The system features a fully differential electronics design which virtually eliminates common mode noise. The optical design efficiently rejects unwanted radiation, both spectrally and spatially, while minimizing the effects of resonant optical cavities. The system is cooled by an efficient, closed cycle 4He–3He refrigerator which is cycled under computer control. The noise performance of the system is determined from analysis of electrical, optical, and spectral measurements, and the results are compared with a theoretical bolometer model.

We present early results from the commissioning of FTS-2, the imaging Fourier transform spectrometer for use with SCUBA-2 at the James Clerk Maxwell Telescope.

The non-linear behaviour of bolometric detectors can lead to significant radiometric errors in Fourier spectroscopy if left uncorrected. We discuss preliminary investigation of this effect and its correction.

We present the recent developments and current design and of an imaging Fourier transform spectrometer (IFTS) for use with SCUBA-2, the second generation, wide-field, submillimetre camera currently under development for the James Clerk Maxwell Telescope (JCMT). The spectrometer will offer variable resolution with resolving powers ranging from R ~10 to 5000. The IFTS uses a folded Mach-Zehnder configuration with novel intensity beam dividers and dual input ports for continuous atmospheric cancellation. This system, which is planned for operation in 2006, will provide simultaneous, broadband, intermediate spectral resolution imaging across both the 850 and 450 μm bands. The optics, observing modes, and projected telescope performance of the IFTS are discussed.

We present the conceptual design of an imaging Fourier transform spectrometer (IFTS) for use with SCUBA-2, the second generation, wide-field, submillimeter camera currently under development for the James Clerk Maxwell Telescope (JCMT). This system, which is planned for operation in 2006, will provide simultaneous, broadband, intermediate spectral resolution imaging across both the 850 and 450 µm bands. The spectrometer will offer variable resolution with resolving powers ranging from R ~10 to 5000. When operated at low resolution, the IFTS will provide continuum measurements, well suited to spectral index mapping of molecular clouds, as well as bright nearby galaxies. The IFTS uses a folded Mach-Zehnder configuration and novel intensity beamdividers. The preliminary design, projected telescope performance and scientific impact of the IFTS are discussed. The preliminary design, novel observing modes, projected telescope performance and scientific impact of the IFTS are discussed.

The design of a dual polarizing bolometer detector system for use with a polarizing Fourier transform spectrometer to conduct broadband astronomical spectroscopy at submillimeter wavelengths is presented. While inclement weather precluded astronomical observations during the commissioning run, the initial performance of the system, as determined from observing laboratory calibration sources, is presented, and the implications for astronomical spectroscopy discussed.