Searching for evidence of inflation by measuring B-modes in the cosmic microwave background (CMB) polarization at degree angular scales remains one of the most compelling experimental challenges in cosmology. BICEP2 and the Keck Array are part of a program of experiments at the South Pole whose main goal is to achieve the sensitivity and systematic control necessary for measurements of the tensor-to-scalar ratio at σ(r) ~0:01. Beam imperfections that are not sufficiently accounted for are a potential source of spurious polarization that could interfere with that goal. The strategy of BICEP2 and the Keck Array is to completely characterize their telescopes' polarized beam response with a combination of in-lab, pre-deployment, and on-site calibrations. We Sereport the status of these experiments, focusing on continued improved understanding of their beams. Far-field measurements of the BICEP2 beam with a chopped thermal source, combined with analysis improvements, show that the level of residual beam-induced systematic errors is acceptable for the goal of σ(r) ~ 0:01 measurements. Beam measurements of the Keck Array side lobes helped identify a way to reduce optical loading with interior cold baffles, which we installed in late 2013. These baffles reduced total optical loading, leading to a ~ 10% increase in mapping speed for the 2014 observing season. The sensitivity of the Keck Array continues to improve: for the 2013 season it was 9:5 μK _/s noise equivalent temperature (NET). In 2014 we converted two of the 150-GHz cameras to 100 GHz for foreground separation capability. We have shown that the BICEP2 and the Keck Array telescope technology is sufficient for the goal of σ(r) ~ 0:01 measurements. Furthermore, the program is continuing with BICEP3, a 100-GHz telescope with 2560 detectors.
The inflationary paradigm of the early universe predicts a stochastic background of gravitational waves which would generate a B-mode polarization pattern in the cosmic microwave background (CMB) at degree angular scales. Precise measurement of B-modes is one of the most compelling observational goals in modern cosmology. Since 2011, the Keck Array has deployed over 2500 transition edge sensor (TES) bolometer detectors at 100 and 150 GHz to the South Pole in pursuit of degree-scale B-modes, and Bicep3 will follow in 2015 with 2500 more at 100 GHz. Characterizing the spectral response of these detectors is important for controlling systematic effects that could lead to leakage from the temperature to polarization signal, and for understanding potential coupling to atmospheric and astrophysical emission lines. We present complete spectral characterization of the Keck Array detectors, made with a Martin-Puplett Fourier Transform Spectrometer at the South Pole, and preliminary spectra of Bicep3 detectors taken in lab. We show band centers and effective bandwidths for both Keck Array bands, and use models of the atmosphere at the South Pole to cross check our absolute calibration. Our procedure for obtaining interferograms in the field with automated 4-axis coupling to the focal plane represents an important step towards efficient and complete spectral characterization of next-generation instruments more than 10000 detectors.
The Bicep2 and Keck Array experiments are designed to measure the polarization of the cosmic microwave background (CMB) on angular scales of 2-4 degrees (ℓ = 50–100). This is the region in which the <i>B</i>-mode signal, a signature prediction of cosmic inflation, is expected to peak. Bicep2 was deployed to the South Pole at the end of 2009 and is in the middle of its third year of observing with 500 polarization-sensitive detectors at 150 GHz. The Keck Array was deployed to the South Pole at the end of 2010, initially with three receivers—each similar to Bicep2. An additional two receivers have been added during the 2011-12 summer. We give an overview of the two experiments, report on substantial gains in the sensitivity of the two experiments after post-deployment optimization, and show preliminary maps of CMB polarization from Bicep2.
The Keck Array (SPUD) began observing the cosmic microwave background's polarization in the winter of 2011 at the South Pole. The Keck Array follows the success of the predecessor experiments BICEP and BICEP2,<sup> 1</sup> using five on-axis refracting telescopes. These have a combined imaging array of 2500 antenna-coupled TES bolometers read with a SQUID- based time domain multiplexing system. We will discuss the detector noise and the optimization of the readout. The achieved sensitivity of the Keck Array is 11.5 <i>μK<sub>CMB</sub>√s</i> in the 2012 configuration.
Between the BICEP2 and Keck Array experiments, we have deployed over 1500 dual polarized antenna coupled bolometers
to map the Cosmic Microwave Background’s polarization. We have been able to rapidly deploy these detectors because
they are completely planar with an integrated phased-array antenna. Through our experience in these experiments, we
have learned of several challenges with this technology- specifically the beam synthesis in the antenna- and in this paper
we report on how we have modified our designs to mitigate these challenges. In particular, we discus differential steering
errors between the polarization pairs’ beam centroids due to microstrip cross talk and gradients of penetration depth in the
niobium thin films of our millimeter wave circuits. We also discuss how we have suppressed side lobe response with a
Gaussian taper of our antenna illumination pattern. These improvements will be used in Spider, Polar-1, and this season’s
retrofit of Keck Array.
The Keck Array (SPUD) is a set of microwave polarimeters that observes from the South Pole at degree angular scales in search of a signature of Inflation imprinted as B-mode polarization in the Cosmic Microwave Background (CMB). The first three Keck Array receivers were deployed during the 2010-2011 Austral summer, followed by two new receivers in the 2011-2012 summer season, completing the full five-receiver array. All five receivers are currently observing at 150 GHz. The Keck Array employs the field-proven BICEP/ BICEP2 strategy of using small, cold, on-axis refractive optics, providing excellent control of systematics while maintaining a large field of view. This design allows for full characterization of far-field optical performance using microwave sources on the ground. We describe our efforts to characterize the main beam shape and beam shape mismatch between co-located orthogonally-polarized detector pairs, and discuss the implications of measured differential beam parameters on temperature to polarization leakage in CMB analysis.
We report on the preliminary detector performance of the Bicep2 mm-wave polarimeter, deployed in 2009 to
the South Pole. Bicep2 is currently imaging the polarization of the cosmic microwave background at 150 GHz
using an array of 512 antenna-coupled superconducting bolometers. The antennas, band-defining filters and
transition edge sensor (TES) bolometers are photolithographically fabricated on 4 silicon tiles. Each tile consists
of an 8×8 grid of ~7 mm spatial pixels, for a total of 256 detector pairs. A spatial pixel contains 2 sets of
orthogonal antenna slots summed in-phase, with each set coupled to a TES by a filtered microstrip. The detectors
are read out using time-domain multiplexed SQUIDs. The detector pair of each spatial pixel is differenced to
measure polarization. We report on the performance of the Bicep2 detectors in the field, including the focal
plane yield, detector and multiplexer optimization, detector noise and stability, and a preliminary estimate of
the improvement in mapping speed compared to Bicep1.
The Keck Array is a cosmic microwave background (CMB) polarimeter that will begin observing from the South
Pole in late 2010. The initial deployment will consist of three telescopes similar to BICEP2 housed in ultracompact,
pulse tube cooled cryostats. Two more receivers will be added the following year. In these proceedings
we report on the design and performance of the Keck cryostat. We also report some initial results on the
performance of antenna-coupled TES detectors operating in the presence of a pulse tube. We find that the
performance of the detectors is not seriously impacted by the replacement of BICEP2's liquid helium cryostat
with a pulse tube cooled cryostat.
We describe SPIDER, a balloon-borne instrument to map the polarization of the millimeter-wave sky with degree
angular resolution. Spider consists of six monochromatic refracting telescopes, each illuminating a focal plane
of large-format antenna-coupled bolometer arrays. A total of 2,624 superconducting transition-edge sensors are
distributed among three observing bands centered at 90, 150, and 280 GHz. A cold half-wave plate at the
aperture of each telescope modulates the polarization of incoming light to control systematics. SPIDER's first
flight will be a 20-30-day Antarctic balloon campaign in December 2011. This flight will map ~8% of the sky to
achieve unprecedented sensitivity to the polarization signature of the gravitational wave background predicted
by inflationary cosmology. The SPIDER mission will also serve as a proving ground for these detector technologies
in preparation for a future satellite mission.
Bicep2 deployed to the South Pole during the 2009-2010 austral summer, and is now mapping the polarization
of the cosmic microwave background (CMB), searching for evidence of inflationary cosmology. Bicep2 belongs
to a new class of telescopes including Keck (ground-based) and Spider (balloon-borne) that follow on Bicep's
strategy of employing small, cold, on-axis refracting optics. This common design provides key advantages ideal
for targeting the polarization signature from inflation, including: (i) A large field of view, allowing substantial
light collecting power despite the small aperture, while still resolving the degree-scale polarization of the CMB;
(ii) liquid helium-cooled optics and cold stop, allowing for low, stable instrument loading; (iii) the ability to
rotate the entire telescope about the boresight; (iv) a baffled primary aperture, reducing sidelobe pickup; and
(v) the ability to characterize the far field optical performance of the telescope using ground-based sources. We
describe the last of these advantages in detail, including our efforts to measure the main beam shape, beammatch
between orthogonally-polarized pairs, polarization efficiency and response angle, sidelobe pickup, and
ghost imaging. We do so with ground-based polarized microwave sources mounted in the far field as well as
with astronomical calibrators. Ultimately, Bicep2's sensitivity to CMB polarization from inflation will rely on
precise calibration of these beam features.
BICEP2/Keck and SPIDER are cosmic microwave background (CMB) polarimeters targeting the B-mode polarization
induced by primordial gravitational waves from inflation. They will be using planar arrays of polarization
sensitive antenna-coupled TES bolometers, operating at frequencies between 90 GHz and 220 GHz. At 150 GHz
each array consists of 64 polarimeters and four of these arrays are assembled together to make a focal plane, for a
total of 256 dual-polarization elements (512 TES sensors). The detector arrays are integrated with a time-domain
SQUID multiplexer developed at NIST and read out using the multi-channel electronics (MCE) developed at
the University of British Columbia. Following our progress in improving detector parameters uniformity across
the arrays and fabrication yield, our main effort has focused on improving detector arrays optical and noise
performances, in order to produce science grade focal planes achieving target sensitivities. We report on changes
in detector design implemented to optimize such performances and following focal plane arrays characterization.
BICEP2 has deployed a first 150 GHz science grade focal plane to the South Pole in December 2009.
The Bicep2 telescope is designed to measure the polarization of the cosmic microwave background on angular
scales near 2-4 degrees, near the expected peak of the B-mode polarization signal induced by primordial gravitational
waves from inflation. Bicep2 follows the success of Bicep, which has set the most sensitive current limits
on B-modes on 2-4 degree scales. The experiment adopts a new detector design in which beam-defining slot antennas
are coupled to TES detectors photolithographically patterned in the same silicon wafer, with multiplexing
SQUID readout. Bicep2 takes advantage of this design's higher focal-plane packing density, ease of fabrication,
and multiplexing readout to field more detectors than Bicep1, improving mapping speed by nearly a factor of
10. Bicep2 was deployed to the South Pole in November 2009 with 500 polarization-sensitive detectors at 150
GHz, and is funded for two seasons of observation. The first months' data demonstrate the performance of the
Caltech/JPL antenna-coupled TES arrays, and two years of observation with Bicep2 will achieve unprecedented
sensitivity to B-modes on degree angular scales.
Spider is a balloon-borne array of six telescopes that will observe the Cosmic Microwave Background. The 2624
antenna-coupled bolometers in the instrument will make a polarization map of the CMB with approximately
one-half degree resolution at 145 GHz. Polarization modulation is achieved via a cryogenic sapphire half-wave
plate (HWP) skyward of the primary optic. We have measured millimeter-wave transmission spectra of the
sapphire at room and cryogenic temperatures. The spectra are consistent with our physical optics model, and
the data gives excellent measurements of the indices of A-cut sapphire. We have also taken preliminary spectra of
the integrated HWP, optical system, and detectors in the prototype Spider receiver. We calculate the variation
in response of the HWP between observing the CMB and foreground spectra, and estimate that it should not
limit the Spider constraints on inflation.
We describe the cryogenic system for SPIDER, a balloon-borne microwave polarimeter that will map 8% of the
sky with degree-scale angular resolution. The system consists of a 1284 L liquid helium cryostat and a 16 L
capillary-filled superfluid helium tank, which provide base operating temperatures of 4 K and 1.5 K, respectively.
Closed-cycle <sup>3</sup>He adsorption refrigerators supply sub-Kelvin cooling power to multiple focal planes, which are
housed in monochromatic telescope inserts. The main helium tank is suspended inside the vacuum vessel with
thermally insulating fiberglass flexures, and shielded from thermal radiation by a combination of two vapor
cooled shields and multi-layer insulation. This system allows for an extremely low instrumental background and
a hold time in excess of 25 days. The total mass of the cryogenic system, including cryogens, is approximately
1000 kg. This enables conventional long duration balloon flights. We will discuss the design, thermal analysis,
and qualification of the cryogenic system.
Here we describe the design and performance of the SPIDER instrument. SPIDER is a balloon-borne cosmic
microwave background polarization imager that will map part of the sky at 90, 145, and 280 GHz with subdegree
resolution and high sensitivity. This paper discusses the general design principles of the instrument inserts,
mechanical structures, optics, focal plane architecture, thermal architecture, and magnetic shielding of the TES
sensors and SQUID multiplexer. We also describe the optical, noise, and magnetic shielding performance of the
145 GHz prototype instrument insert.
We describe the design and performance of polarization selective antenna-coupled TES arrays that will be used
in several upcoming Cosmic Microwave Background (CMB) experiments: SPIDER, BICEP-2/SPUD. The fully
lithographic polarimeter arrays utilize planar phased-antennas for collimation (F/4 beam) and microstrip filters
for band definition (25% bandwidth). These devices demonstrate high optical efficiency, excellent beam shapes,
and well-defined spectral bands. The dual-polarization antennas provide well-matched beams and low cross
polarization response, both important for high-fidelity polarization measurements. These devices have so far
been developed for the 100 GHz and 150 GHz bands, two premier millimeter-wave atmospheric windows for
CMB observations. In the near future, the flexible microstrip-coupled architecture can provide photon noise-limited
detection for the entire frequency range of the CMBPOL mission. This paper is a summary of the
progress we have made since the 2006 SPIE meeting in Orlando, FL.