We present the conceptual design of the modular detector and readout system for the Cosmic Microwave Background – Stage four (CMB-S4) ground-based survey experiment. CMB-S4 will map the cosmic microwave background (CMB) and the millimeter-wave sky to unprecedented sensitivity, using 500,000 superconducting detectors observing from Chile and Antarctica to map over 60% of the sky. The fundamental building block of the detector and readout system is a detector module package operated at 100 mK, which is connected to a readout and amplification chain that carries signals out to room temperature. It uses arrays of feedhorn-coupled orthomode transducers (OMT) that collect optical power from the sky onto dc-voltage-biased transition-edge sensor (TES) bolometers. The resulting current signal in the TESs is then amplified by a two-stage cryogenic Superconducting Quantum Interference Device (SQUID) system with a time-division multiplexer to reduce wire count, and matching room-temperature electronics to condition and transmit signals to the data acquisition system. Sensitivity and systematics requirements are being developed for the detector and readout system over wide range of observing bands (20 to 300 GHz) and optical powers to accomplish CMB-S4’s science goals. While the design incorporates the successes of previous generations of CMB instruments, CMB-S4 requires an order of magnitude more detectors than any prior experiment. This requires fabrication of complex superconducting circuits on over 10 m2 of silicon, as well as significant amounts of precision wiring, assembly and cryogenic testing
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 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.
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 Dark Energy Survey Collaboration has completed construction of the Dark Energy Camera (DECam), a 3 square
degree, 570 Megapixel CCD camera which will be mounted on the Blanco 4-meter telescope at CTIO. DECam will be
used to perform the 5000 sq. deg. Dark Energy Survey with 30% of the telescope time over a 5 year period. During the
remainder of the time, and after the survey, DECam will be available as a community instrument. All components of
DECam have been shipped to Chile and post-shipping checkout finished in Jan. 2012. Installation is in progress. A
summary of lessons learned and an update of the performance of DECam and the status of the DECam installation and
commissioning will be presented.
The Dark Energy Survey Camera (DECam) will be comprised of a mosaic of 74 charge-coupled devices (CCDs). The
Dark Energy Survey (DES) science goals set stringent technical requirements for the CCDs. The CCDs are provided by
LBNL with valuable cold probe data at 233 K, providing an indication of which CCDs are more likely to pass. After
comprehensive testing at 173 K, about half of these qualify as science grade. Testing this large number of CCDs to
determine which best meet the DES requirements is a very time-consuming task. We have developed a multistage
testing program to automatically collect and analyze CCD test data. The test results are reviewed to select those CCDs
that best meet the technical specifications for charge transfer efficiency, linearity, full well capacity, quantum efficiency,
noise, dark current, cross talk, diffusion, and cosmetics.
Large mosaic multiCCD camera is the key instrument for modern digital sky survey. DECam is an extremely
red sensitive 520 Megapixel camera designed for the incoming Dark Energy Survey (DES). It is consist of sixty
two 4k2k and twelve 2k2k 250-micron thick fully-depleted CCDs, with a focal plane of 44 cm in diameter and
a eld of view of 2.2 square degree. It will be attached to the Blanco 4-meter telescope at CTIO. The DES will
cover 5000 square-degrees of the southern galactic cap in 5 color bands (g, r, i, z, Y) in 5 years starting from
2011.
To achieve the science goal of constraining the Dark Energy evolution, stringent requirements are laid down
for the design of DECam. Among them, the
atness of the focal plane needs to be controlled within a 60-micron
envelope in order to achieve the specied PSF variation limit. It is very challenging to measure the
atness of
the focal plane to such precision when it is placed in a high vacuum dewar at 173 K. We developed two image
based techniques to measure the
atness of the focal plane. By imaging a regular grid of dots on the focal plane,
the CCD oset along the optical axis is converted to the variation the grid spacings at dierent positions on the
focal plane. After extracting the patterns and comparing the change in spacings, we can measure the
atness
to high precision. In method 1, the regular dots are kept in high sub micron precision and cover the whole focal
plane. In method 2, no high precision for the grid is required. Instead, we use a precise XY stage moves the
pattern across the whole focal plane and comparing the variations of the spacing when it is imaged by dierent
CCDs. Simulation and real measurements show that the two methods work very well for our purpose, and are
in good agreement with the direct optical measurements.
The Dark Energy Camera is an wide field imager currently
under construction for the Dark Energy Survey.
This instrument will use fully depleted 250 μm thick
CCD detectors selected for their higher quantum efficiency
in the near infrared with respect to thinner devices.
The detectors were developed by LBNL using
high resistivity Si substrate. The full set of scientific
detectors needed for DECam has now been fabricated,
packaged and tested. We present here the results of
the testing and characterization for these devices and
compare these results with the technical requirements
for the Dark Energy Survey.
The Dark Energy Survey Collaboration is building the Dark Energy Camera (DECam), a 3 square degree, 520
Megapixel CCD camera which will be mounted on the Blanco 4-meter telescope at CTIO. DECam will be used to
perform the 5000 sq. deg. Dark Energy Survey with 30% of the telescope time over a 5 year period. During the
remainder of the time, and after the survey, DECam will be available as a community instrument. Construction of
DECam is well underway. Integration and testing of the major system components has already begun at Fermilab and
the collaborating institutions.
We describe the results obtained cleaning the surface of DECam CCD detectors with a new electrostatic dissipative
formulation of First ContactTM polymer from Photonic Cleaning Technologies. We demonstrate that
cleaning with this new product is possible without ESD damage to the sensors and without degradation of the
antireflective coating used to optimize the optical performance of the detector. We show that First ContactTM
is more effective for cleaning a CCD than the commonly used acetone swab.
DECam is a 520 Mpix, 3 square-deg FOV imager being built for the Blanco 4m Telescope at CTIO. This facility
instrument will be used for the "Dark Energy Survey" of the southern galactic cap. DECam has chosen 250 μm thick
CCDs, developed at LBNL, with good QE in the near IR for the focal plane. In this work we present the characterization
of these detectors done by the DES team, and compare it to the DECam technical requirements. The results demonstrate
that the detectors satisfy the needs for instrument.
KEYWORDS: Charge-coupled devices, Clocks, Electronics, CCD cameras, Cameras, Stars, Silicon, Field programmable gate arrays, Energy efficiency, Control systems
The Dark Energy Camera will be comprised of 74 CCDs with high efficiency out to a wavelength of 1 micron.
The CCDs will be read out by a Monsoon-based system consisting of three boards: Master Control, CCD
Acquisition, and Clock boards. The charge transfer efficiency (CTE) is closely related to the clock waveforms
provided by the Clock Board (CB). The CB has been redesigned to meet the stringent requirements of the Dark
Energy Survey. The number of signals provided by the clock board has been extended from 32 (the number
required for 2 CCDs) up to 135 signals (the number required for 9 CCDs). This modification is required to fit
the electronics into the limited space available on the imager vessel. In addition, the drivers have been changed
to provide more current. The first test result with the new clock board shows a clear improvement in the CTE
response when reading out at the higher frequencies required for the guide CCDs.
We describe the Dark Energy Camera (DECam), which will be the primary instrument used in the Dark Energy Survey.
DECam will be a 3 sq. deg. mosaic camera mounted at the prime focus of the Blanco 4m telescope at the Cerro-Tololo
International Observatory (CTIO). DECam includes a large mosaic CCD focal plane, a five element optical corrector,
five filters (g,r,i,z,Y), and the associated infrastructure for operation in the prime focus cage. The focal plane consists of
62 2K x 4K CCD modules (0.27"/pixel) arranged in a hexagon inscribed within the roughly 2.2 degree diameter field of
view. The CCDs will be 250 micron thick fully-depleted CCDs that have been developed at the Lawrence Berkeley
National Laboratory (LBNL). Production of the CCDs and fabrication of the optics, mechanical structure, mechanisms,
and control system for DECam are underway; delivery of the instrument to CTIO is scheduled for 2010.
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