The third-generation South Pole Telescope camera (SPT-3G) improves upon its predecessor (SPTpol) by an order of magnitude increase in detectors on the focal plane. The technology used to read out and control these detectors, digital frequency-domain multiplexing (DfMUX), is conceptually the same as used for SPTpol, but extended to accommodate more detectors. A nearly 5× expansion in the readout operating bandwidth has enabled the use of this large focal plane, and SPT-3G performance meets the forecasting targets relevant to its science objectives. However, the electrical dynamics of the higher-bandwidth readout differ from predictions based on models of the SPTpol system due to the higher frequencies used and parasitic impedances associated with new cryogenic electronic architecture. To address this, we present an updated derivation for electrical crosstalk in higher-bandwidth DfMUX systems and identify two previously uncharacterized contributions to readout noise, which become dominant at high bias frequency. The updated crosstalk and noise models successfully describe the measured crosstalk and readout noise performance of SPT-3G. These results also suggest specific changes to warm electronics component values, wire-harness properties, and SQUID parameters, to improve the readout system for future experiments using DfMUX, such as the LiteBIRD space telescope.
The SPT-3G receiver was commissioned in early 2017 on the 10-meter South Pole Telescope (SPT) to map anisotropies in the cosmic microwave background (CMB). New optics, detector, and readout technologies have yielded a multichroic, high-resolution, low-noise camera with impressive throughput and sensitivity, offering the potential to improve our understanding of inflationary physics, astroparticle physics, and growth of structure. We highlight several key features and design principles of the new receiver, and summarize its performance to date.
The South Pole Telescope (SPT) is a millimeter-wavelength telescope designed for high-precision measurements of the cosmic microwave background (CMB). The SPT measures both the temperature and polarization of the CMB with a large aperture, resulting in high resolution maps sensitive to signals across a wide range of angular scales on the sky. With these data, the SPT has the potential to make a broad range of cosmological measurements. These include constraining the effect of massive neutrinos on large-scale structure formation as well as cleaning galactic and cosmological foregrounds from CMB polarization data in future searches for inflationary gravitational waves. The SPT began observing in January 2017 with a new receiver (SPT-3G) containing ~16,000 polarization-sensitive transition-edge sensor bolometers. Several key technology developments have enabled this large-format focal plane, including advances in detectors, readout electronics, and large millimeter-wavelength optics. We discuss the implementation of these technologies in the SPT-3G receiver as well as the challenges they presented. In late 2017 the implementations of all three of these technologies were modified to optimize total performance. Here, we present the current instrument status of the SPT-3G receiver.
The third-generation instrument for the 10-meter South Pole Telescope, SPT-3G, was first installed in January 2017. In addition to completely new cryostats, secondary telescope optics, and readout electronics, the number of detectors in the focal plane has increased by an order of magnitude from previous instruments to ~16,000. The SPT-3G focal plane consists of ten detector modules, each with an array of 269 trichroic, polarization-sensitive pixels on a six-inch silicon wafer. Within each pixel is a broadband, dual-polarization sinuous antenna; the signal from each orthogonal linear polarization is divided into three frequency bands centered at 95, 150, and 220 GHz by in-line lumped element filters and transmitted via superconducting microstrip to Ti/Au transition-edge sensor (TES) bolometers. Properties of the TES film, microstrip filters, and bolometer island must be tightly controlled to achieve optimal performance. For the second year of SPT-3G operation, we have replaced all ten wafers in the focal plane with new detector arrays tuned to increase mapping speed and improve overall performance. Here we discuss the TES superconducting transition temperature and normal resistance, detector saturation power, bandpasses, optical efficiency, and full array yield for the 2018 focal plane.
The desire for higher sensitivity has driven ground-based cosmic microwave background (CMB) experiments to employ ever larger focal planes, which in turn require larger reimaging optics. Practical limits to the maximum size of these optics motivates the development of quasi-optically-coupled (lenslet-coupled), multi-chroic detectors. These detectors can be sensitive across a broader bandwidth compared to waveguide-coupled detectors. However, the increase in bandwidth comes at a cost: the lenses (up to ~700 mm diameter) and lenslets (~5 mm diameter, hemispherical lenses on the focal plane) used in these systems are made from high-refractive-index materials (such as silicon or amorphous aluminum oxide) that reflect nearly a third of the incident radiation. In order to maximize the faint CMB signal that reaches the detectors, the lenses and lenslets must be coated with an anti-reflective (AR) material. The AR coating must maximize radiation transmission in scientifically interesting bands and be cryogenically stable. Such a coating was developed for the third generation camera, SPT-3G, of the South Pole Telescope (SPT) experiment, but the materials and techniques used in the development are general to AR coatings for mm-wave optics. The three-layer polytetra uoroethylene-based AR coating is broadband, inexpensive, and can be manufactured with simple tools. The coating is field tested; AR coated focal plane elements were deployed in the 2016-2017 austral summer and AR coated reimaging optics were deployed in 2017-2018.
The Primordial Inflation Explorer is an Explorer-class mission to open new windows on the early universe through measurements of the polarization and absolute frequency spectrum of the cosmic microwave background. PIXIE will measure the gravitational-wave signature of primordial inflation through its distinctive imprint in linear polarization, and characterize the thermal history of the universe through precision measurements of distortions in the blackbody spectrum. PIXIE uses an innovative optical design to achieve background-limited sensitivity in 400 spectral channels spanning over 7 octaves in frequency from 30 GHz to 6 THz (1 cm to 50 micron wavelength). Multi-moded non-imaging optics feed a polarizing Fourier Transform Spectrometer to produce a set of interference fringes, proportional to the difference spectrum between orthogonal linear polarizations from the two input beams. Multiple levels of symmetry and signal modulation combine to reduce systematic errors to negligible levels. PIXIE will map the full sky in Stokes I, Q, and U parameters with angular resolution 2.6° and sensitivity 70 nK per 1° square pixel. The principal science goal is the detection and characterization of linear polarization from an inflationary epoch in the early universe, with tensor-to-scalar ratio r < 10−3 at 5 standard deviations. The PIXIE mission complements anticipated ground-based polarization measurements such as CMB- S4, providing a cosmic-variance-limited determination of the large-scale E-mode signal to measure the optical depth, constrain models of reionization, and provide a firm detection of the neutrino mass (the last unknown parameter in the Standard Model of particle physics). In addition, PIXIE will measure the absolute frequency spectrum to characterize deviations from a blackbody with sensitivity 3 orders of magnitude beyond the seminal COBE/FIRAS limits. The sky cannot be black at this level; the expected results will constrain physical processes ranging from inflation to the nature of the first stars and the physical conditions within the interstellar medium of the Galaxy. We describe the PIXIE instrument and mission architecture required to measure the CMB to the limits imposed by astrophysical foregrounds.
The third generation receiver for the South Pole Telescope, SPT-3G, will make extremely deep, arcminuteresolution maps of the temperature and polarization of the cosmic microwave background. The SPT-3G maps will enable studies of the B-mode polarization signature, constraining primordial gravitational waves as well as the effect of massive neutrinos on structure formation in the late universe. The SPT-3G receiver will achieve exceptional sensitivity through a focal plane of ~16,000 transition-edge sensor bolometers, an order of magnitude more than the current SPTpol receiver. SPT-3G uses a frequency domain multiplexing (fMux) scheme to read out the focal plane, combining the signals from 64 bolometers onto a single pair of wires. The fMux readout facilitates the large number of detectors in the SPT-3G focal plane by limiting the thermal load due to readout wiring on the 250 millikelvin cryogenic stage. A second advantage of the fMux system is that the operation of each bolometer can be optimized. In addition to these benefits, the fMux readout introduces new challenges into the design and operation of the receiver. The bolometers are operated at a range of frequencies up to 5 MHz, requiring control of stray reactances over a large bandwidth. Additionally, crosstalk between multiplexed detectors will inject large false signals into the data if not adequately mitigated. SPT-3G is scheduled to deploy to the South Pole Telescope in late 2016. Here, we present the pre-deployment performance of the fMux readout system with the SPT-3G focal plane.
Detectors for cosmic microwave background (CMB) experiments are now essentially background limited, so a
straightforward alternative to improve sensitivity is to increase the number of detectors. Large arrays of multichroic
pixels constitute an economical approach to increasing the number of detectors within a given focal plane area. Here, we
present the fabrication of large arrays of dual-polarized multichroic transition-edge-sensor (TES) bolometers for the
South Pole Telescope third-generation CMB receiver (SPT-3G). The complete SPT-3G receiver will have 2690 pixels,
each with six detectors, allowing for individual measurement of three spectral bands (centered at 95 GHz, 150 GHz and
220 GHz) in two orthogonal polarizations. In total, the SPT-3G focal plane will have 16140 detectors. Each pixel is
comprised of a broad-band sinuous antenna coupled to a niobium microstrip transmission line. In-line filters are used to
define the different band-passes before the millimeter-wavelength signal is fed to the respective Ti/Au TES sensors.
Detectors are read out using a 64x frequency domain multiplexing (fMux) scheme. The microfabrication of the SPT-3G
detector arrays involves a total of 18 processes, including 13 lithography steps. Together with the fabrication process, the
effect of processing on the Ti/Au TES’s Tc is discussed. In addition, detectors fabricated with Ti/Au TES films with Tc
between 400 mK 560 mK are presented and their thermal characteristics are evaluated. Optical characterization of the
arrays is presented as well, indicating that the response of the detectors is in good agreement with the design values for
all three spectral bands (95 GHz, 150 GHz, and 220 GHz). The measured optical efficiency of the detectors is between
0.3 and 0.8. Results discussed here are extracted from a batch of research of development wafers used to develop the
baseline process for the fabrication of the arrays of detectors to be deployed with the SPT-3G receiver. Results from
these research and development wafers have been incorporated into the fabrication process to get the baseline fabrication
process presented here. SPT-3G is scheduled to deploy to the South Pole Telescope in late 2016.
The Primordial Inflation Explorer is an Explorer-class mission to measure the gravity-wave signature of primordial inflation through its distinctive imprint on the linear polarization of the cosmic microwave background. PIXIE uses an innovative optical design to achieve background-limited sensitivity in 400 spectral channels spanning 2.5 decades in frequency from 30 GHz to 6 THz (1 cm to 50 micron wavelength). Multi-moded non-imaging optics feed a polarizing Fourier Transform Spectrometer to produce a set of interference fringes, proportional to the difference spectrum between orthogonal linear polarizations from the two input beams. Multiple levels of symmetry and signal modulation combine to reduce the instrumental signature and confusion from unpolarized sources to negligible levels. PIXIE will map the full sky in Stokes I, Q, and U parameters with angular resolution 2.6 deg and sensitivity 0.2 µK per 1 deg square pixel. The principal science goal is the detection and characterization of linear polarization from an inflationary epoch in the early universe, with tensor-to-scalar ratio r < 10-3 at 5 standard deviations. In addition, PIXIE will measure the absolute frequency spectrum to constrain physical processes ranging from inflation to the nature of the first stars to the physical conditions within the interstellar medium of the Galaxy. We describe the PIXIE instrument and mission architecture with an emphasis on the expected level of systematic error suppression.
KEYWORDS: Polarization, Sensors, Telescopes, Physics, Receivers, Galaxy groups and clusters, Antennas, Signal detection, Signal to noise ratio, Bolometers
We describe the design of a new polarization sensitive receiver, spt-3g, for the 10-meter South Pole Telescope (spt). The spt-3g receiver will deliver a factor of ~20 improvement in mapping speed over the current receiver, spt-pol. The sensitivity of the spt-3g receiver will enable the advance from statistical detection of B-mode polarization anisotropy power to high signal-to-noise measurements of the individual modes, i.e., maps. This will lead to precise (~0.06 eV) constraints on the sum of neutrino masses with the potential to directly address the neutrino mass hierarchy. It will allow a separation of the lensing and inflationary B-mode power spectra, improving constraints on the amplitude and shape of the primordial signal, either through spt-3g data alone or in combination with bicep2/keck, which is observing the same area of sky. The measurement of small-scale temperature anisotropy will provide new constraints on the epoch of reionization. Additional science from the spt-3g survey will be significantly enhanced by the synergy with the ongoing optical Dark Energy Survey (des), including: a 1% constraint on the bias of optical tracers of large-scale structure, a measurement of the differential Doppler signal from pairs of galaxy clusters that will test General Relativity on ~200Mpc scales, and improved cosmological constraints from the abundance of clusters of galaxies
The SPTpol camera is a two-color, polarization-sensitive bolometer receiver, and was installed on the 10 meter South Pole Telescope in January 2012. SPTpol is designed to study the faint polarization signals in the Cosmic Microwave Background, with two primary scientific goals. One is to constrain the tensor-to-scalar ratio of perturbations in the primordial plasma, and thus constrain the space of permissible in inflationary models. The other is to measure the weak lensing effect of large-scale structure on CMB polarization, which can be used to constrain the sum of neutrino masses as well as other growth-related parameters. The SPTpol focal plane consists of seven 84-element monolithic arrays of 150 GHz pixels (588 total) and 180 individual 90 GHz single- pixel modules. In this paper we present the design and characterization of the 90 GHz modules.
The SPTpol camera is a dichroic polarimetric receiver at 90 and 150 GHz. Deployed in January 2012 on the South Pole Telescope (SPT), SPTpol is looking for faint polarization signals in the Cosmic Microwave Background (CMB). The camera consists of 180 individual Transition Edge Sensor (TES) polarimeters at 90 GHz and seven 84-polarimeter camera modules (a total of 588 polarimeters) at 150 GHz. We present the design, dark characterization, and in-lab optical properties of the 150 GHz camera modules. The modules consist of photolithographed arrays of TES polarimeters coupled to silicon platelet arrays of corrugated feedhorns, both of which are fabricated at NIST-Boulder. In addition to mounting hardware and RF shielding, each module also contains a set of passive readout electronics for digital frequency-domain multiplexing. A single module, therefore, is fully functional as a miniature focal plane and can be tested independently. Across the modules tested before deployment, the detectors average a critical temperature of 478 mK, normal resistance RN of 1.2Ω , unloaded saturation power of 22.5 pW, (detector-only) optical efficiency of ~ 90%, and have electrothermal time constants < 1 ms in transition.
SPTpol is a dual-frequency polarization-sensitive camera that was deployed on the 10-meter South Pole Telescope in January 2012. SPTpol will measure the polarization anisotropy of the cosmic microwave background (CMB) on angular scales spanning an arcminute to several degrees. The polarization sensitivity of SPTpol will enable a detection of the CMB “B-mode” polarization from the detection of the gravitational lensing of the CMB by large scale structure, and a detection or improved upper limit on a primordial signal due to inationary gravity waves. The two measurements can be used to constrain the sum of the neutrino masses and the energy scale of ination. These science goals can be achieved through the polarization sensitivity of the SPTpol camera and careful control of systematics. The SPTpol camera consists of 768 pixels, each containing two transition-edge sensor (TES) bolometers coupled to orthogonal polarizations, and a total of 1536 bolometers. The pixels are sensitive to light in one of two frequency bands centered at 90 and 150 GHz, with 180 pixels at 90 GHz and 588 pixels at 150 GHz. The SPTpol design has several features designed to control polarization systematics, including: singlemoded feedhorns with low cross-polarization, bolometer pairs well-matched to dfference atmospheric signals, an improved ground shield design based on far-sidelobe measurements of the SPT, and a small beam to reduce temperature to polarization leakage. We present an overview of the SPTpol instrument design, project status, and science projections.
One of the most exciting targets for cosmic microwave background (CMB) polarization measurements is the faint signal from the primordial gravity waves predicted by inflationary models. Currently existing experiments and those under construction would constrain or detect such a signal at around r = 0.01, where r is the tensor to scalar ratio. In order to further improve the measurement, experiments for the next generation have to combine the following three: 1) excellent sensitivity, 2) multi-frequency measurement for the removal of galactic foregrounds, and 3) well-controlled systematics. We propose the Multimoded Survey Experiment (MuSE), which uses highly multimoded polarization-sensitive bolometers developed at NASA Goddard Space Flight Center (GSFC). MuSE, consisting of 69 pixels, will achieve a sensitivity equivalent to several thousand single-moded bolometers. Each pixel can be configured to be sensitive to a different frequency band, allowing very wide frequency coverage by a single focal plane. This enables us to clean galactic synchrotron and dust components with our data alone. MuSE achieves an effective array sensitivity to the CMB of 8 μK√s even after accounting for the sensitivity degradation from foreground removal and reaches a 2-σ error on r of 0.009 with two years of operation.
In January 2012, the 10m South Pole Telescope (SPT) was equipped with a polarization-sensitive camera, SPTpol, in order to measure the polarization anisotropy of the cosmic microwave background (CMB). Measurements of the polarization of the CMB at small angular scales (~several arcminutes) can detect the gravitational lensing of the CMB by large scale structure and constrain the sum of the neutrino masses. At large angular scales (~few degrees) CMB measurements can constrain the energy scale of Inflation. SPTpol is a two-color mm-wave camera that consists of 180 polarimeters at 90 GHz and 588 polarimeters at 150 GHz, with each polarimeter consisting of a dual transition edge sensor (TES) bolometers. The full complement of 150 GHz detectors consists of 7 arrays of 84 ortho-mode transducers (OMTs) that are stripline coupled to two TES detectors per OMT, developed by the TRUCE collaboration and fabricated at NIST. Each 90 GHz pixel consists of two antenna-coupled absorbers coupled to two TES detectors, developed with Argonne National Labs. The 1536 total detectors are read out with digital frequency-domain multiplexing (DfMUX). The SPTpol deployment represents the first on-sky tests of both of these detector technologies, and is one of the first deployed instruments using DfMUX readout technology. We present the details of the design, commissioning, deployment, on-sky optical characterization and detector performance of the complete SPTpol focal plane.
KEYWORDS: Control systems, Sensors, Telescopes, Data archive systems, Antennas, Human-machine interfaces, Bolometers, Data acquisition, Detection and tracking algorithms, Data storage
We present the software system used to control and operate the South Pole Telescope. The South Pole Telescope is
a 10-meter millimeter-wavelength telescope designed to measure anisotropies in the cosmic microwave background
(CMB) at arcminute angular resolution. In the austral summer of 2011/12, the SPT was equipped with a new
polarization-sensitive camera, which consists of 1536 transition-edge sensor bolometers. The bolometers are read
out using 36 independent digital frequency multiplexing (DfMux) readout boards, each with its own embedded
processors. These autonomous boards control and read out data from the focal plane with on-board software
and firmware. An overall control software system running on a separate control computer controls the DfMux
boards, the cryostat and all other aspects of telescope operation. This control software collects and monitors
data in real-time, and stores the data to disk for transfer to the United States for analysis.
The Primordial Inflation Explorer is an Explorer-class mission to measure the gravity-wave signature of primordial
inflation through its distinctive imprint on the linear polarization of the cosmic microwave background. PIXIE
uses an innovative optical design to achieve background-limited sensitivity in 400 spectral channels spanning 2.5
decades in frequency from 30 GHz to 6 THz (1 cm to 50 μm wavelength). Multi-moded non-imaging optics
feed a polarizing Fourier Transform Spectrometer to produce a set of interference fringes, proportional to the
difference spectrum between orthogonal linear polarizations from the two input beams. The differential design
and multiple signal modulations spanning 11 orders of magnitude in time combine to reduce the instrumental
signature and confusion from unpolarized sources to negligible levels. PIXIE will map the full sky in Stokes I,
Q, and U parameters with angular resolution 2.°6 and sensitivity 0.2 μK per 1° square pixel. The principal
science goal is the detection and characterization of linear polarization from an inflationary epoch in the early
universe, with tensor-to-scalar ratio r < 10-3 at 5 standard deviations. We describe the PIXIE instrument and
mission architecture needed to detect the signature of an inflationary epoch in the early universe using only 4
semiconductor bolometers.
The Primordial Inflation Explorer (PIXIE) is an Explorer-class mission to map the absolute intensity and linear
polarization of the cosmic microwave background and diffuse astrophysical foregrounds over the full sky from
frequencies 30 GHz to 6 THz (1 cm to 50 μm wavelength). PIXIE uses a polarizing Michelson interferometer with 2.7 K
optics to measure the difference spectrum between two orthogonal linear polarizations from two co-aligned beams.
Either input can view either the sky or a temperature-controlled absolute reference blackbody calibrator. The multimoded
optics and high etendu provide sensitivity comparable to kilo-pixel focal plane arrays, but with greatly expanded
frequency coverage while using only 4 detectors total. PIXIE builds on the highly successful COBE/FIRAS design by
adding large-area polarization-sensitive detectors whose fully symmetric optics are maintained in thermal equilibrium
with the CMB. The highly symmetric nulled design provides redundant rejection of major sources of systematic
uncertainty. The principal science goal is the detection and characterization of linear polarization from an inflationary
epoch in the early universe, with tensor-to-scalar ratio r << 10-3. PIXIE will also return a rich data set constraining
physical processes ranging from Big Bang cosmology, reionization, and large-scale structure to the local interstellar
medium.
The next generation of Cosmic Microwave Background (CMB) experiments probing for signals of inflation and
small angular scale polarization anisotropies require higher sensitivity and better control of systematics. We are
developing monolithic arrays of orthomode transducer (OMT) coupled transition edge sensor (TES) polarimeters
designed for operation at 150 GHz to address these requirements. OMT coupling allows for simultaneous and
independent detection of two orthogonal linear polarization states incident on a single pixel. We present measurements
of optical efficiencies ηop of single pixels with on-chip band-defining filters, with ηop = 57±4 stat±9 sys %.
We also provide evidence for an out-of-band blue leak and address possible sources as well as mitigation techniques.
Additionally, we discuss methods for increasing efficiency being implemented in the next generation of
pixels, currently in fabrication. Still under development, these pixels are produced as monolithic polarimeter arrays
and are slated for use in the Atacama Cosmology Telescope Polarization (ACTpol) and South Pole Telescope
Polarization (SPTpol) experiments, while single-pixel polarimeters are to be deployed in the Atacama B-mode
Search (ABS) experiment.
We discuss the development, at Argonne National Laboratory, of a four-pixel camera suitable for photometry of distant
dusty galaxies located by Spitzer and SCUBA, and for study of other millimeter-wave sources such as ultra-luminous
infrared galaxies, the Sunyaev-Zeldovich (SZ) effect in clusters, and galactic dust. Utilizing Frequency Selective
Bolometers (FSBs) with superconducting Transition-Edge Sensors (TESs), each of the camera's four pixels is sensitive
to four colors, with frequency bands centered approximately at 150, 220, 270, and 360 GHz.
The current generation of these devices utilizes proximity effect superconducting bilayers of Mo/Au or Ti/Au for TESs,
along with frequency selective circuitry on membranes of silicon nitride 1 cm across and 1 micron thick. The operational
properties of these devices are determined by this circuitry, along with thermal control structures etched into the
membranes. These etched structures do not perforate the membrane, so that the device is both comparatively robust
mechanically and carefully tailored in terms of its thermal transport properties.
In this paper, we report on development of the superconducting bilayer TES technology and characterization of the FSB
stacks. This includes the use of new materials, the design and testing of thermal control structures, the introduction of
desirable thermal properties using buried layers of crystalline silicon underneath the membrane, detector stability control,
and optical and thermal test results. The scientific motivation, FSB design, FSB fabrication, and measurement results are
discussed.
A new 10 meter diameter telescope is being constructed for deployment
at the NSF South Pole research station. The telescope is designed for
conducting large-area millimeter and sub-millimeter wave surveys
of faint, low contrast emission, as required to map primary and secondary anisotropies in the cosmic microwave background. To achieve the required sensitivity and resolution, the telescope design employs an off-axis primary with a 10 meter diameter clear aperture. The full aperture and the associated optics will have a combined surface accuracy of better than 20 microns rms to allow precision operation in the submillimeter atmospheric windows. The telescope will be surrounded with a large reflecting ground screen to reduce sensitivity to thermal emission from the ground and local interference. The optics of the telescope will support a degree field of view at 2mm wavelength and will feed a new 1000-element micro-lithographed planar bolometric array with superconducting transition-edge sensors and frequency-multiplexed readouts. The first key project will be to conduct a survey over &dbigwig;4000 degrees for galaxy clusters using the Sunyaev-Zel'dovich Effect. This survey should find many thousands of clusters with a mass selection criteria that is remarkably uniform with redshift. Armed with redshifts obtained from optical and infrared follow-up observations, it is expected that the survey will enable significant constraints to be placed on the equation of state of the dark energy.
The SPEED camera is being developed to study the spectral energy distributions of high redshift galaxies, Sunyaev-Zel'dovich effect in X-ray clusters and other cold objects in the universe. Its initial runs will be done on the 10 m Heinrich Hertz Submillimeter Telescope (HHSMT), with later runs using the Large Millimeter Telescope (LMT). SPEED requires a 2x2 pixel cryogenic detector array of Frequency Selective Bolometers (FSB). Each of the pixels will have four frequency bands in the ~150-350 GHz range. Here we describe the development of the detector array of these high efficiency FSBs. The FSB design provides the multi-pixel multi-spectral band capability required for SPEED in a compact, light weight, stackable array. The SPEED FSB bolometers will use proximity effect superconducting transition edge sensors (TES) as their temperature-sensing element permitting significantly higher levels of electronic multiplexing in future applications where larger numbers of detectors may be required.
EDGE is a Long Duration Balloon (LDB) borne instrument designed to measure the large-scale anisotropy of the Cosmic Infrared Background (CIB). The goal is to use this signal as a new observational tool to measure the character of the spatial distribution of galaxies at the largest spatial scales. With a 6\arcmin\ beam mapping more than 400 square degrees of sky at 8 frequency bands between 250GHz and 1.5 THz the experiment can determine the variation of galaxy density on
spatial scales ranging from >200h-1 Mpc, where dark matter
variations are determined directly from Cosmic Microwave Background Radiation (CMBR) anisotropy, to <5h-1 Mpc where the distribution of dark matter and galaxies is determined from galaxy redshift surveys and the underlying dynamics of structure growth is non-linear. The instrument consists of a 1-meter class off-axis telescope and a Frequency Selective Bolometer (FSB) array radiometer. The FSB design provides the compact, multi-chromatic, high sensitivity focal plane needed for this measurement.
The TopHat instrument was designed to operate on the top of a high altitude balloon. From this location, the experiment could efficiently observe using a clean beam with extremely low contamination from the far side lobes of the instrument beam. The experiment was designed to scan a large portion of the sky directly above it and to map the anisotropy of the Cosmic Microwave Background (CMB) and thermal emission from galactic dust. The instrument used a one-meter class telescope with a five-band single pixel radiometer spanning the frequency range from 150-600 GHz. The radiometer used bolometric detectors operating at ~250mK. Here, we report on the flight of the TopHat experiment over Antarctica in January, 2001 and describe the scientific goals, the operation, and in-flight performance.
Advances in bolometer device and readout technologies make it possible to build photon-noise limited bolometric cameras for ground-based observations at mm-wave frequencies. However, today's bolometer cameras are limited not by photon-noise of the telescope and atmosphere but by fluctuations in the atmosphere signal. To realize the full potential of bolometer cameras on large aperture ground-based telescopes, one must find a way to defeat this foreground.
The SPEctral Energy Distribution Camera - or SPEED - is a four pixel, four frequency camera planned for eventual use on the Large Millimeter Telescope (LMT). A prototype version of this camera is currently being built for initial operation on the Heinrich Hertz Telescope (HHT). SPEED incorporates Frequency Selective Bolometers to sample the sky with a frequency-independent beam simultaneously at four frequencies (from 150 to 375 GHz) in each pixel. SPEED's ability to separate the temporally varying atmospheric signal from the true sky signal will potentially result in a per-detector sensitivity between 2 and 5 times greater than that achieved with contemporary bolometer cameras. We describe the basic design and motivation for SPEED, the expected sensitivity of the camera on the LMT, and give examples of some of the science programs we will undertake.
The Frequency Selective Bolometer (FSB) is a bolometer with a patterned frequency selective absorber, coupled with a band-reflecting backshort. The resulting unit absorbs in-band radiation, and passes out-of-band radiation. Thus a series of FSBs tuned to different bands packed in series in a light pipe forms a compact multi-band photometer. The compact form factor makes it an attractive detector for a mm-wave array camera.
We have built and characterized prototypes that demonstrate this technology. We are now developing a set of FSBs for SPEED (the SPEctral Energy Distribution camera), an FSB array camera which will observe 4 pixels in 4 mm-wave spectral bands, to be used on the Heinrich Hertz Telescope and the Large Millimeter Telescope. These FSBs are fabricated on a free-standing SiN film with TES thermometers. We will discuss the design and performance of these detectors.
The lower stratosphere in the polar regions offers conditions for observation in the near-infrared comparable to those obtained from space. We describe a concept for a 6-meter, diluted aperture, near-infrared telescope carried by a tethered aerostat flying at 12 km altitude, to serve as a testbed for future space astronomical observatories while producing frontier science.
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