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This paper discusses the development of a cryogenic grating spectrometer that has been used to evaluate the performance of a post-dispersed polarizing FTS over the range of 285-500 µm.

The performance of a prototype far-infrared post-dispersed polarizing FTS has been measured using a source module consisting of unresolved emission and absorption lines and the results are compared with theoretical simulations.

We describe the engineering development unit of a novel cryogenic scan mechanism suitable for post-dispersed polarizing FTS instruments. Compliance to the stringent SPICA mission requirements is demonstrated through analyses and results from proof-of- concept activities.

This paper describes the development and status of a cryogenic, far-infrared, post- dispersed, polarizing FTS (PDPFTS), a candidate instrument concept for the next generation of far- infrared astronomical space telescopes.

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.

The high spectral resolution mode of the SpicA FAR-infrared Instrument (SAFARI) is enabled by inserting a Fourier Transform Spectrometer (FTS), based on a Martin-Puplett interferometer, into the signal path of the instrument. The cryogenic mechanism (FTSM) enables linear scans of two back-to-back rooftop mirrors sharing a common apex. ABB Inc. is under contract with the Canadian Space Agency to develop and test at 4 K an FTSM Engineering Demonstration Unit (EDU) for TRL-5 demonstration. The main SAFARI FTSM performance drivers are the stringent mechatronic demands (position stability of roof-top mirrors

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.

We provide a detailed description of the Herschel-SPIRE Fourier Transform Spectrometer (FTS) Spectral Feature Finder (FF). The FF is an automated process designed to extract significant spectral features from SPIRE FTS data products. Optimising the number of features found in SPIRE-FTS spectra is challenging. The wide SPIRE-FTS frequency range (447-1568 GHz) leads to many molecular species and atomic fine structure lines falling within the observed bands. As the best spectral resolution of the SPIRE-FTS is ~1.2 GHz, there can be significant line blending, depending on the source type. In order to find, both efficiently and reliably, features in spectra associated with a wide range of sources, the FF iteratively searches for peaks over a number of signal-to-noise ratio (SNR) thresholds. For each threshold, newly identified features are rigorously checked before being added to the fitting model. At the end of each iteration, the FF simultaneously fits the continuum and features found, with the resulting residual spectrum used in the next iteration. The final FF products report the frequency of the features found and the associated SNRs. Line flux determination is not included as part of the FF products, as extracting reliable line flux from SPIRE-FTS data is a complex process that requires careful evaluation and analysis of the spectra on a case-by-case basis. The FF results are 100% complete for features with SNR greater than 10 and 50-70% complete at SNR of 5. The FF code and all FF products are publicly available via the Herschel Science Archive., Comment: 20 pages, 8 figures, 8 tables, final version accepted by MNRAS June 2020

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 high resolution spectroscopic mode of the SPICA SAFARI instrument will be achieved using a Martin-Puplett interferometer. The key science and technology requirements of the spectrometer will be reviewed and the current instrumental concept presented.

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.

Imaging applications at terahertz frequencies are, in general, limited to relatively low spatial resolution due to the effects of diffraction. By using a subwavelength aperture in the near-field, however, it is possible to achieve subwavelength resolution, although low transmission through the aperture limits the sensitivity of this approach. Plasmonic lenses in the form of bullseye structures, which consist of a circular subwavelength aperture surrounded by concentric periodic corrugations, have demonstrated enhanced transmission, thereby increasing the utility of near-field imaging configurations. In this paper, the design, fabrication, and experimental performance of plasmonic lenses optimized for 300 GHz are discussed. While nanofabrication techniques are required for optical applications, microfabrication techniques are sufficient for terahertz applications. The process flow for fabricating a double-sided bullseye structure using a precision micromilling technique is described. Transmission and beam profile measurements using a customized terahertz testbed are presented.

The FP7 project, FISICA (Far Infrared Space Interferometer Critical Assessment), called for the investigation into the suitability of Carbon fiber Reinforced Plastic (CFRP) for a 2m primary mirror. In this paper, we focus on the major challenge for application, the development of a mirror design that would maintain its form at cryogenic temperatures. In order to limit self-emission the primary is to be cooled to 4K whilst not exceeding a form error of 275nm PV. We then describe the development of an FEA model that utilizes test data obtained from a cryogenic test undertaken at the University of Lethbridge on CFRP samples. To conclude, suggestions are made in order to advance this technology to be suitable for such an application in order to exploit the low density and superior specific properties of polymeric composites.

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.

A cryogenic FTS is being developed at the University of Lethbridge based on a commercial rotary arm system. Measurement of optical path difference is accomplished using a fiber-fed laser interferometer system internal to the cryostat.

Imaging applications at terahertz (THz) frequencies are limited to relatively low spatial resolution due to the effects of diffraction. A subwavelength aperture can be used to improve the resolution at the cost of low transmission. Plasmonic lenses in the form of bullseye structures, consisting of a single subwavelength circular aperture surrounded by concentric periodic corrugations, have shown enhanced transmission and beam confinement. In this paper, we discuss the design and performance of plasmonic lenses optimized for transmission at 325 GHz., 2015 40th International Conference on Infrared, Millimeter, and Terahertz waves (IRMMW-THz), August 23-28,2015, Hong Kong, China

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.

We present an update on the ongoing commissioning of FTS-2, the imaging Fourier transform spectrometer ancillary instrument for use with the SCUBA-2 camera at the James Clerk Maxwell Telescope.