SpicA FAR infrared Instrument, SAFARI, is one of the instruments planned for the SPICA mission. The SPICA
mission is the next great leap forward in space-based far-infrared astronomy and will study the evolution of galaxies,
stars and planetary systems. SPICA will utilize a deeply cooled 2.5m-class telescope, provided by European industry, to
realize zodiacal background limited performance, and high spatial resolution. The instrument SAFARI is a cryogenic
grating-based point source spectrometer working in the wavelength domain 34 to 230 μm, providing spectral resolving
power from 300 to at least 2000.
The instrument shall provide low and high resolution spectroscopy in four spectral bands. Low Resolution mode is the
native instrument mode, while the high Resolution mode is achieved by means of a Martin-Pupplet interferometer.
The optical system is all-reflective and consists of three main modules; an input optics module, followed by the Band
and Mode Distributing Optics and the grating Modules. The instrument utilizes Nyquist sampled filled linear arrays of
very sensitive TES detectors.
The work presented in this paper describes the optical design architecture and design concept compatible with the
current instrument performance and volume design drivers.
In this paper we present the development and verification of feed horn simulation code based on the mode- matching technique to simulate the electromagnetic performance of waveguide based structures of rectangular cross-section. This code is required to model multi-mode pyramidal horns which may be required for future far infrared (far IR) space missions where wavelengths in the range of 30 to 200 µm will be analysed. Multi-mode pyramidal horns can be used effectively to couple radiation to sensitive superconducting devices like Kinetic Inductance Detectors (KIDs) or Transition Edge Sensor (TES) detectors. These detectors could be placed in integrating cavities (to further increase the efficiency) with an absorbing layer used to couple to the radiation. The developed code is capable of modelling each of these elements, and so will allow full optical characterisation of such pixels and allow an optical efficiency to be calculated effectively.
As the signals being measured at these short wavelengths are at an extremely low level, the throughput of the system must be maximised and so multi-mode systems are proposed. To this end, the focal planes of future far IR missions may consist of an array of multi-mode rectangular feed horns feeding an array of, for example, TES devices contained in individual integrating cavities. Such TES arrays have been fabricated by SRON Groningen and are currently undergoing comprehensive optical, electrical and thermal verification. In order to fully understand and validate the optical performance of the receiver system, it is necessary to develop comprehensive and robust optical models in parallel. We outline the development and verification of this optical modelling software by means of applying it to a representative multi-mode system operating at 150 GHz in order to obtain sufficiently short execution times so as to comprehensively test the code.
SAFARI (SPICA FAR infrared Instrument) is a far infrared imaging grating spectrometer, to be proposed as an ESA M5 mission. It is planned for this mission to be launched on board the proposed SPICA (SPace Infrared telescope for Cosmology and Astrophysics) mission, in collaboration with JAXA. SAFARI is planned to operate in the 1.5-10 THz band, focussing on the formation and evolution of galaxies, stars and planetary systems. The pixel that drove the development of the techniques presented in this paper is typical of one option that could be implemented in the SAFARI focal plane, and so the ability to accurately understand and characterise such pixels is critical in the design phase of the next generation of far IR telescopes.
The SAFARI Detector Test Facility is an ultra-low background optical testbed for characterizing ultra-sensitive
prototype horn-coupled TES bolmeters for SAFARI, the grating spectrometer on board the proposed SPICA satellite.
The testbed contains internal cold and hot black-body illuminators and a light-pipe for illumination with an external
source. We have added reimaging optics to facilitate array optical measurements. The system is now being used for
optical testing of prototype detector arrays read out with frequency-domain multiplexing. We present our latest optical
measurements of prototype arrays and discuss these in terms of the instrument performance.
We have characterized the optical response of prototype detectors for SAFARI, the far-infrared imaging spectrometer for the SPICA satellite. SAFARI's three bolometer arrays will image a 2’×2’ field of view with spectral information over the wavelength range 34—210 μm. SAFARI requires extremely sensitive detectors (goal NEP ~ 0.2 aW/√Hz), with correspondingly low saturation powers (~5 fW), to take advantage of SPICA's cooled optics. We have constructed an ultra-low background optical test facility containing an internal cold black-body illuminator and have recently added an internal hot black-body source and a light-pipe for external illumination. We illustrate the performance of the test facility with results including spectral-response measurements. Based on an improved understanding of the optical throughput of the test facility we find an optical efficiency of 60% for prototype SAFARI detectors.
We have measured the optical response of detectors designed for SAFARI, the far-infrared imaging spectrometer for the SPICA satellite. To take advantage of SPICA's cooled optics, SAFARI’s three bolometer arrays are populated with extremely sensitive (NEP~2×10-19 W/√Hz) transition edge sensors with a transition temperature close to 100 mK. The extreme sensitivity and low saturation power (~4 fW) of SAFARI’s detectors present challenges to characterizing them. We have therefore built up an ultra-low background test facility with a cryogen-free high-capacity dilution refrigerator, paying careful attention to stray-light exclusion. Our use of a pulse-tube cooler to pre-cool the dilution refrigerator required that the SAFARI Detector System Test Facility provide a high degree electrical, magnetic, and mechanical isolation for the detectors. We have carefully characterized the performance of the test facility in terms of background power loading. The test facility has been designed to be flexible and easily reconfigurable with internal illuminators that allow us to characterize the optical response of the detectors. We describe the test facility and some of the steps we took to create an ultra-low background test environment. We have measured the optical response of two detectors designed for SAFARI’s short-wave wavelength band in combination with a spherical backshort and conical feedhorn. We find an overall optical efficiency of 40% for both, compared with an ideal-case predicted optical efficiency of 66%.
SPICA is an infra-red (IR) telescope with a cryogenically cooled mirror (~5K) with three instruments on board, one of
which is SAFARI that is an imaging Fourier Transform Spectrometer (FTS) with three bands covering the wavelength of
34-210 μm. We develop transition edge sensors (TES) array for short wavelength band (34-60 μm) of SAFARI. These
are based on superconducting Ti/Au bilayer as TES bolometers with a Tc of about 105 mK and thin Ta film as IR
absorbers on suspended silicon nitride (SiN) membranes. These membranes are supported by long and narrow SiN legs
that act as weak thermal links between the TES and the bath. Previously an electrical noise equivalent power (NEP) of
4×10-19 W/√Hz was achieved for a single pixel of such detectors. As an intermediate step toward a full-size SAFARI
array (43×43), we fabricated several 8×9 detector arrays. Here we describe the design and the outcome of the dark and
optical tests of several of these devices. We achieved high yield (<93%) and high uniformity in terms of critical
temperature (<5%) and normal resistance (7%) across the arrays. The measured dark NEPs are as low as 5×10-19 W/√Hz
with a response time of about 1.4 ms at preferred operating bias point. The optical coupling is implemented using
pyramidal horns array on the top and hemispherical cavity behind the chip that gives a measured total optical coupling
efficiency of 30±7%.
SRON is developing an electronic system for the multiplexed read-out of an array of transition edge sensors (TES) by
combining the techniques of frequency domain multiplexing (FDM) with base-band feedback (BBFB). The astronomical
applications are the read-out of soft X-ray microcalorimeters and the far-infrared bolometers for the SAFARI instrument
on the Japanese mission SPICA. In this paper we derive the requirements for the read-out system regarding noise and
dynamic range in the context of the SAFARI instrument, and demonstrate that the current experimental prototype is
capable of simultaneously locking 57 channels and complies with these requirements.
Finlines are planar structures which allow broadband and low loss transition from waveguide to planar circuits.
Their planar structure and large substrate makes them ideal for integration with other planar circuits and
components, allowing the development of an on chip polarimeter. We have developed a method of extending the
employment of finlines to thick substrates with high dielectric constants by drilling or etching small holes into
the substrate, lowering the effective dielectric constant. We present the results of scale model measurements at
15GHz and cryogenic measurements at 90GHz which illustrate the excellent performance of finline transitions
with porous substrates and the suitability of this technique for extending the bandwidth of finline transitions.
Current and future astronomical detectors based on Transition Edge Sensors (TESs) need to achieve theoretically
predicted current noise performance determined by the sum of contributions from thermal noise in the link to the
heat bath, Johnson noise in the sensor itself and noise in the electrical readout circuit. Present TES geometries
can have noise levels significantly above this limit. Our Mo/Cu bilayer TESs are fabricated on long, narrow,
thermally isolating silicon nitride structures and are designed for operation at 360 or 200 mK. We briefly review
the likely sources of the additional noise sources in this geometry and show results of measurements and modelling
of the noise sources as the TES geometry is modified for TESs operated at both temperatures.
We have fabricated TES bolometers with finline transitions for the CℓOVER project. We have measured the
optical response of CℓOVER's first prototype 97-GHz detectors and find that they have a detection efficiency
close to 100%. We have also investigated the effects of misalignment of the finline in the waveguide and of
thinning the substrate. The prototype detectors have dark NEPs as low as 1.5 x 10-17W/√Hz and satisfy
the requirement of photon-noise limited operation on CℓOVER. We describe the optical tests of CℓOVER's
prototype 97-GHz detectors and discuss their implications for the design of the science-grade detectors.
CℓOVER is a multi-frequency experiment optimised to measure
the Cosmic Microwave Background (CMB) polarization, in
particular the B-mode component. CℓOVER comprises two
instruments observing respectively at 97 GHz and 150/225 GHz.
The focal plane of both instruments consists of an array of
corrugated feed-horns coupled to TES detectors cooled at 100
mK. The primary science goal of CℓOVER is to be sensitive to
gravitational waves down to r ~ 0.03 (at 3σ)in two years of operations.
SCUBA-2 is a new wide-field submillimeter camera under construction for the James Clerk Maxwell Telescope
on Mauna Kea in Hawaii. SCUBA-2 images simultaneously at 450 and 850 μm using large-scale arrays of
superconducting bolometers, with over five thousand pixels at each wavelength. Time division multiplexed
readouts and cryogenic amplifiers, both based on superconducting quantum interference devices (SQUIDs), are
also used in the design. The SCUBA-2 detector arrays must be well shielded against magnetic fields, since the
performance of the bolometers can be seriously affected by the presence of a strong field, and the SQUIDs are
themselves sensitive magnetometers. This shielding is to be provided by a combination of high-permeability and
superconducting layers on both the ambient temperature and cryogenic stages of the instrument. To optimise
and demonstrate the effectiveness of the shielding design, a finite-element modelling method was employed, using
the Ansoft(R) Maxwell 3DTM package. Although a number of approximations had to be made in the modelling,
the finite-element results allow a good estimation of the effectiveness of the shielding at attenuating external
magnetic fields to be made. This paper describes the modelling process, outlines the key results and summarises
the final shielding design.
CℓOVER is an experiment which aims to detect the signature of gravitational waves from inflation by measuring
the B-mode polarization of the cosmic microwave background. CℓOVER consists of three telescopes operating
at 97, 150, and 220 GHz. The 97-GHz telescope has 160 horns in its focal plane while the 150 and 220-GHz
telescopes have 256 horns each. The horns are arranged in a hexagonal array and feed a polarimeter which
uses finline-coupled TES bolometers as detectors. To detect the two polarizations the 97-GHz telescope has 320 detectors while the 150 and 220-GHz telescopes have 512 detectors each. To achieve the required NEPs the
detectors are cooled to 100 mK for the 97 and 150-GHz polarimeters and 230 mK for the 220-GHz polarimeter.
Each detector is fabricated as a single chip to guarantee fully functioning focal planes. The detectors are
contained in linear modules made of copper which form split-block waveguides. The detector modules contain
16 or 20 detectors each for compatibility with the hexagonal arrays of horns in the telescopes' focal planes. Each
detector module contains a time-division SQUID multiplexer to read out the detectors. Further amplification of
the multiplexed signals is provided by SQUID series arrays. The first prototype detectors for CℓOVER operate
with a bath temperature of 230 mK and are used to validate the detector design as well as the polarimeter
technology. We describe the design of the CℓOVER detectors, detector blocks, and readout, and give an update
on the detector development.
SCUBA-2, which replaces SCUBA (the Submillimeter Common User Bolometer
Array) on the James Clerk Maxwell Telescope (JCMT) in 2006, is a
large-format bolometer array for submillimeter astronomy. Unlike previous detectors which have used discrete bolometers, SCUBA-2 has two dc-coupled, monolithic, filled arrays with a total of ~10,000 bolometers. It will offer simultaneous imaging of a 50 sq-arcmin field of view at wavelengths of 850 and 450 microns. SCUBA-2 is expected to have a huge impact on the study of galaxy formation and evolution in the early Universe as well as star and planet formation in our own Galaxy. Mapping the sky to the same S/N up to 1000 times faster than SCUBA, it will also act as a pathfinder for the new submillimeter interferometers such as ALMA. SCUBA-2's absorber-coupled pixels use superconducting transition edge sensors operating at 120 mK for performance limited by the sky background photon noise. The monolithic silicon detector arrays are deep-etched by the Bosch process to isolate the pixels on silicon nitride membranes. Electrical
connections are made through indium bump bonds to a SQUID time-domain multiplexer (MUX). We give an overview of the SCUBA-2 system and an update on its status, and describe some of the technological innovations that make this unique instrument possible.
SCUBA-2 is the next-generation replacement for SCUBA (Sub-millimetre
Common User Bolometer Array) on the James Clerk Maxwell Telescope. Operating at 450 and 850 microns, SCUBA-2 fills the focal plane of the telescope with fully-sampled, monolithic bolometer arrays. Each SCUBA-2 pixel uses a quarter-wave slab of silicon with an implanted resistive layer and backshort as an absorber and a superconducting transition edge sensor as a thermometer. In order to verify and optimize the pixel design, we have investigated the electromagnetic behaviour of the detectors, using both a simple transmission-line model and Ansoft HFSS, a finite-element electromagnetic simulator. We used the transmission line model to fit transmission measurements of doped wafers and determined the correct implant dose for the absorbing layer. The more detailed HFSS modelling yielded some unexpected results which led us to modify the pixel design. We also verified that the detectors suffered little loss of sensitivity for off-axis angles up to about 30 degrees.
SCUBA-2 is a second generation, wide-field submillimeter camera under development for the James Clerk Maxwell Telescope. With over 12,000 pixels, in two arrays, SCUBA-2 will map the submillimeter sky ~1000 times faster than the current SCUBA instrument to the same signal-to-noise. Many areas of astronomy will benefit from such a highly sensitive survey instrument: from studies of galaxy formation and evolution in the early Universe to understanding star and planet formation in our own Galaxy. Due to be operational in 2006, SCUBA-2 will also act as a "pathfinder" for the new generation of submillimeter interferometers (such as ALMA) by performing large-area surveys to an unprecedented depth. The challenge of developing the detectors and multiplexer is discussed in this paper.
The XRS instrument on Astro-E is a fully self-contained microcalorimeter x-ray instrument capable of acquiring, optimally filtering, and characterizing events for 32 independent pixels. We have recently integrated a full engineering model XRS detector system into a laboratory cryostat for use on the electron beam ion trap (EBIT) at Lawrence Livermore National Laboratory. The detector system contains a microcalorimeter array with 32 instrumented pixels heat sunk to 60 mK using an adiabatic demagnetization refrigerator. The instrument has a composite resolution of 8 eV at 1 keV and 11 eV at 6 keV with a minimum of 98% quantum efficiency and a total collecting area of 13 mm2. This will allow high spectral resolution, broadband observations of plasmas with known ionization states that are produced in the EBIT experiment. Unique to our instrument are exceptionally well characterized 1000 Angstrom thick aluminum on polyimide infrared blocking filters. The detailed transmission function including the edge fine structure of these filters has been measured in our laboratory using a variable spaced grating spectrometer. This will allow the instrument to perform the first broadband absolute flux measurements with the EBIT instrument. The instrument performance as well as the results of preliminary measurements of Fe K and L shell at fixed electron energy, Fe emission with Maxwellian electron distributions, and phase resolved spectroscopy of ionizing plasmas will be discussed.
The Astro-E High Resolution X-ray Spectrometer (XRS) was developed jointly by the NASA/Goddard Space Flight Center and the Institute of Space and Astronomical Science in Japan. The instrument is based on a new approach to spectroscopy, the x-ray microcalorimeter. This device senses the energies of individual x-ray photons as heat with extreme precision. A 32 channel array of microcalorimeters is being employed, each with an energy resolution of about 12 eV at 6 keV. This will provide spectral resolving power 10 times higher than any other non-dispersive x-ray spectrometer. The instrument incorporates a three stage cooling system capable of operating the array at 60 mK for about two years in orbit. The array sits at the focus of a grazing incidence conical mirror. The quantum efficiency of the microcalorimeters and the reflectivity of the x-ray mirror system combine to give high throughput over the 0.3- 12 keV energy band. This new capability will enable the study of a wide range of high-energy astrophysical sources with unprecedented spectral sensitivity. This paper presents the basic design requirements and implementation of the XRS, and also describes the instrument parameters and performance.
The x-ray spectrometer (XRS) on the Japanese Astro-E observatory is the first ultra low temperature space borne instrument. The system utilizes a 900g ferric ammonium alum adiabatic demagnetization refrigerator (ADR) with 3He gas gap heat switch to cool the detector assembly to 0.060 K. The system operates in a 'single shot' configuration allowing the system to remain at its operating temperature for about 40 hours in the lab before executing a recharge cycle. The on-orbit performance is expected to be about 36 hours with a 97 percent duty cycle. The detector assembly for XRS consists of a 32 channel microcalorimeter array, bias electronics, thermometry, and an anti-coincidence detector that are attached to the cold stage of the ADR. To thermally isolate the detector system from the superfluid helium reservoir, the detector system is suspended by Kevlar cords and electrical connection is made by 130, 20 micron diameter, tensioned NbTi leads. The detectors are read out in a source-follower arrangement using FET amplifiers operating at 130 K mounted in nested, thermally-isolated assemblies that also use Kevlar suspension and stainless steel wiring. The design and thermal performance of this system will be discussed.
We describe the transmission calibration of the Astro-E XRS blocking filters. The XRS instrument has five aluminized polymide blocking filters. These filters are located at thermal stages ranging from 200 K to 60 mK. They are each about 1000 angstrom thick. XRS will have high energy resolution which will enable it to see some of the extended fine structure around the oxygen and aluminum edges of these filters. Thus, we are conducting a high spectral resolution calibration of the filters near these energies to resolve out extended fine structure and absorption lines.
XRS is the microcalorimeter x-ray detector aboard the US- Japanese ASTRO-E observatory, which is scheduled to be launched in early 2000. XRS is a high resolution spectrometer - with less than 9 eV resolution at 3 keV and better than 14 eV resolution over its bandpass ranging from about 0.3 keV to 15 keV. Here we present the results of our first calibration of the XRS instrument. We describe the methods used to extract detailed information about the detection efficiency and spectral redistribution of the instrument. We also present comparisons of simulations and real data to test our detector models.
We describe the signal processing system of the Astro-E XRS instrument. The Calorimeter Analog Processor provides bias and power for the detectors and amplifies the detector signals by a factor of 20,000. The calorimeter digital processor performs the digital processing of the calorimeter signals, detecting x-ray and risetime determination. We also discuss performance, including the three event grades, anticoincidence detection, counting rate dependence, and noise rejection.
The XRS instrument has an array of 32 micro-calorimeters at the focal plane. These calorimeters consist of ion-implanted silicon thermistors and HgTe thermalizing x-ray absorbers. These devices have demonstrated a resolution of 9 eV at 3 keV and 11 eV at 6 keV. We will discuss the basic physical parameters of this array, including the array layout, thermal conductance of the link to the heat sink, operating temperature, thermistor size, absorber choice, and means of attaching the absorber to the thermistor bearing element. We will present representative performance data, though a more detailed presentation of the results of the instrument calibration is presented elsewhere in these proceedings. A silicon ionization detector is located behind the calorimeter array and serves to reject events due to cosmic rays. We will briefly describe this anti-coincidence detector and its performance in conjunction with the array.