BPOL workshop 27th October 2006
C-Band All Sky Survey(C-BASS)
J. P. Leahy (PI, Manchester), M. E. Jones (PI, Oxford)
Clive Dickinson (JPL)
AIMS: • Definitive survey of Galactic synchrotron radiation and its
polarization• Anchor for synchrotron emission in future CMB polarimetry
experiments up to CMBPOL. • Prototype for possible ground-based surveys at frequencies up
to CMB band: 10, 15, 30… GHz• New window on Galactic magnetic field and cosmic rays
BPOL workshop 27th October 2006
Galactic foregrounds
• Sky is full of polarized interstellar synchrotron emission– 91% of pixels detected at this resolution
• All components have significant spectral variations
We must have more measurements than parameters!
WMAP polarized brightness: 23 GHz, 4° beam
BPOL workshop 27th October 2006
C-BASS motivation
Lens
ing CLO
VER N
oise
B 90 GHzSynch.
• B-POL probably has primary frequencies at ≥ 90 GHz
• Satellite → nearly all sky survey: not just regions of minimum foreground
• Even at 90 GHz, extrapolation of 22 GHz WMAP polarization outside P06 mask (73% of sky) is larger than r=0.1 B-mode signal
– For r=0.002, signal is 7 times weaker
• We must correct for synchrotron emission to get even close to B-POL sensitivity requirements, even for > 90 GHz.
C-BASS
BPOL workshop 27th October 2006
Synchrotron spectral are smooth!
• Power law is just an approximation…
• …but a good one• The best-measured
synchrotron sources are well fit by a 2nd-order log-log polynomial over 2 decades of frequency
BPOL workshop 27th October 2006
The Penticton Survey
• Wollaben, Landecker, Reich & Wielebinski (2006)
• survey of northern sky polarization at λ21 cm with Pentiction 25-m dish
• Comparison with WMAP: • Spectral index β:
– T(ν) = T0 (ν/ν0)β • Faraday rotation RM:
– χ(ν) = χ0 + RM λ2
• Depolarization:– Unresolved RM structure
BPOL workshop 27th October 2006
Spectral Index 21:1.3 cm
BPOL workshop 27th October 2006
Spectral Index 21:1.3 cm
• Affected by depolarization @ λ21 cm, especially near Galactic plane– Tail of relatively flat
apparent spectral indices• Relatively well-defined peak
at βP = −3.2– Seems unaffected by depol.
• C.f. usual assumptions:– (− 2.7 ≥ β ≥ −3)
• Polarized emission steeper than total?
• Less contaminated by free-free, spinning dust?
BPOL workshop 27th October 2006
Spectral Index: 1.3:3 mm
• Low sensitivity in WMAP data at λ < 1.3 cm gives limited sky coverage
• Note flat spectrum for Crab nebula
• Mean βP ≈ −3.0– Slightly flatter than at
lower frequencies. (−3.1 in same regions)
BPOL workshop 27th October 2006
• Dust polarization well measured by Planck
• Synchrotron dominates, at best, only in lowest Planck channels – need extra info to fix
spectrum.
• WMAP takes us down only to 23 GHz– weak lever arm for
extrapolation• Gap between 2.4 and
23 GHzGround-based surveys needed to fix synchrotron emission
ThermalDust
Faraday Rotation
AnomalousDust
Pinning down the Galactic synchrotron spectrum
BPOL workshop 27th October 2006
• Dust polarization well measured by Planck
• Synchrotron dominates, at best, only in lowest Planck channels – need extra info to fix
spectrum.
• WMAP takes us down only to 23 GHz – weak lever arm for
extrapolation• Gap between 2.4 and
23 GHzC-BASS fills the gap!
ThermalDust
Faraday Rotation
AnomalousDust
Pinning down the Galactic synchrotron spectrum
BPOL workshop 27th October 2006
The Survey
• Novel purpose-built single-feed polarization and total power receiver (Manchester/Oxford)
• Northern survey from OVRO 5.5 m dish (California)
– sub-reflector tripod designed for low spillover
– high accuracy surface (mm-λ telescope)
• Southern survey from 7.6 m at Karoo (KAT) site, South Africa
– high quality communication antenna
• Exquisite control of spillover– new, large sub-reflectors– ground screens & baffles– simulations & measurements
OVRO 5.5 m
BPOL workshop 27th October 2006
Receiver: combining technologies
• Novel architecture: analogue correlation radiometer + polarimeter • Unique ultra-stable cold load (collaboration with RAL)• Draws on current technology (e-MERLIN, Clover, Planck)
– e-MERLIN amplifiers: broad-band, low-noise– correlation receiver prototyped under Oxford Experimental Cosmology
grant
BPOL workshop 27th October 2006
Survey Parameters
• FWHM resolution 52 arcmin– Same as 408 MHz survey– Smooth to 1º for high-latitude
analysis, to reduce pixel noise
• Sensitivity: < 0.1 mK / beam rms.
– Extrapolated map at 60 GHz has SNR > 2 for 90% of pixels even at high latitudes (outside WMAP polarization mask ‘P06’)
• Timescale: Complete by end 2010
– Northern survey released 2009
7.6 m Telescope
BPOL workshop 27th October 2006
Survey Strategy
• Based on Effelsberg experience• Long, fast sweeps
– small dish can be scanned rapidly!
• Full coverage of one quadrant of the sky after ~ 1 week.
• Many observations per pixel– spread over many months– several different parallactic
angles• Gives redundancy and
robustness of polarization solution
• Bonus: transients! Example 1-night coverage
High sensitivity allows identification & control of systematics
BPOL workshop 27th October 2006
Project Partners
• Manchester: – front end systems and backend amps & filters– low-level and calibration software
• Oxford: – cryostat, cold load, polarimeter and detectors, sub-reflector, optical design– mapping software
• Caltech: – 5.5 m telescope, ground screen/baffles, digital backend, control, site
support• Rhodes/HartRAO:
– 7.6 m telescope, ground screen/baffles, site support
All partners contribute to observations, analysis & interpretation
FUNDEDFUNDED
BPOL workshop 27th October 2006
Impact of C-BASS
• Planck alone → Planck + C-BASS• Typical high-latitude pixel (2° beam):
– Spectral index bias • Stokes I: −0.14 → 0.015• Stokes Q,U: −0.16 → 0.03
– 70 GHz synchrotron amplitude error (assuming straight spectrum)
• Stokes I σ: 0.9 μK → 0.3 μK (SNR: 3.5 → 12)• Stokes Q,U σ: 0.3 μK → 0.045 μK (SNR: 1 → 7)
– 70 GHz synch. Amp. Bias• Stokes I: 0.9 μK → 0.15 μK • Stokes Q,U: 0.015 μK → 0.003 μK
5-7 times reduction in systematic synchrotron residuals in the CMB Band!
BPOL workshop 27th October 2006
C-BASS: Summary
• C-BASS provides anchor for polarized synchrotron spectrum– c.f. also Parkes 2.3 GHz survey (Caretti et al.)
• Requires at least one more frequency close to primary CMB frequencies to fix synchrotron spectral index (70-90 GHz)
• We probably need 1 or 2 more intermediate frequencies, e.g. 10-15 GHz; 30-40 GHz– Fix spectral curvature– Check for polarized emission from anomalous dust, free-free– Can be obtained from ground/ VLDF balloon (especially if
we can calibrate very large scales from space).
BPOL workshop 27th October 2006
UK Costings (PRD grant)
• Staff:– 0.8 FTE Academic– 3 FTE PDRA– 1.2 FTE Engineer– 2 FTE Technician– Direct costs £215k
• Equipment– £104k
• T & S: – £22k
• Estate & indirect– £158.6k
FEC Total: £500k (pre-FEC: £416k)
BPOL workshop 27th October 2006
UK Phasing
• As suggested by PPARC secretariat:• C-BASS PRD Bid:
– Receiver design & construction– Commissioning
• C-BASS Exploitation Grant – submitted June 2007– Observation, analysis, publication
• Future Project bid– Submission 2009 if justified by C-BASS, CLOVER et al.– 10 GHz survey exploiting C-BASS technology
BPOL workshop 27th October 2006
C-BASS as a PRD scheme
• Exploitation of PPARC technology infrastructure?– World-class Expertise and equipment at Jodrell Bank and Oxford
• High-Priority Science? – Internationally identified as such (e.g. Dark Energy Task Force report)
• Novel technology?– New receiver architecture; stabilised cold load
• Paves the way for UK intellectual leadership in international projects?– Provides leadership of international C-BASS project, and likely successor at
10 GHz• Paves the way for UK industrial return?
– A 10 GHz multi-feed system would involve industrial contracts for receiver components (~ £1M) and possibly for custom telescopes (~£1M)
• Pre-construction phase?– Exploratory research for a major instrument at 10 GHz, as well as versatile
working 5 GHz instrument
BPOL workshop 27th October 2006
Timeliness
• Planck proprietary period ends Q1 2011• We must start now to complete C-BASS
(North & South) in time to incorporate in official Planck analysis.
• Similar time-line for ground-based and balloon B-mode experiments (Clover, BICEP, QUIET, EBEX, SPIDER…).
C-BASS Workpackage Breakdown
WP 1Project
ManagementTJP/JPL/MEJ/JLJ
WP 2Rx Design
Richard Davis
WP 3Optics Design
Mike Jones
WP 4Survey Design
Paddy Leahy
WP 7Rx Construction
Mike Jones
WP 8Rx Integration
Mike Jones
WP 9Rx Testing (UK)
Paddy Leahy
WP 5OVRO RFI
CharacterisationTim Pearson
WP 6Karoo RFI
CharacterisationJustin Jonas
WP 11Prepare 5 m
TelescopeTim Pearson
WP 10Software
Tim Pearson
WP 12Rx Shipping &
Installation/OVROMike Jones
WP 13OVRO
CommissioningTim Pearson
WP 14Write technical
PapersPDRA
WP 16Northern
Data AnalysisPDRA
WP 18Prepare 7.6 m
TelescopeJustin Jonas
WP 19Rx Shipping &
Installation/KarooTim Pearson
WP 20Karoo
CommissioningJustin Jonas
WP 21Southern Survey
OperationsJustin Jonas
WP 22Southern
Data AnalysisPDRA
WP 23CombineSurveysPDRA
WP 24Foreground
AnalysisClive Dickinson
WP 15Northern Survey
OperationsTim Pearson
WP 17PR & Outreach
Erik Leitch
WP 2Rx DesignR. J. Davis
WP 2.2Specify
JBO/Oxford I/F
WP 2.1Specify
Mechanical I/F
WP 2.4Design Rx Cryo
Components
WP 2.5Design Rx Backend
WP 2.6Design Cold
Load
WP 2.7Design
Cryostat
WP 2.8Design
Polarimeter
WP 2.9Adapt CCB
design
WP 3Optics DesignM. E. Jones
WP 3.25 m
Subreflector
WP 3.1OVRO
Ground Screen
WP 3.35 m
Feedhorn
WP 3.4Karoo
Ground Screen
WP 3.57.6 m
Subreflector
WP 3.67.6 m
Feedhorn
WP 2.3Specify
Oxford/CCB I/F
C-BASS WP Breakdown
C-BASS WP BreakdownWP 7
Rx ConstructionM. E. Jones
WP 7.2Backend amps
& filters
WP 7.1RF cryo
components
WP 7.4Cryostat
WP 7.5Phase switch
system
WP 7.6Detectors
WP 7.7Feedhorn
WP 9Rx TestingJ. P. Leahy
WP 9.4Noise diode
WP 9.5Polarization
purity
WP 9.6Phase stability
& zero point
WP 7.8CCB
WP 7.3Cold Load
WP 9.7Cold LoadStability
WP 9.1White Noise optimization
WP 9.2Bandpass
measurement
WP 9.8Feed radiation
pattern
WP 9.9Backendmodes
WP 9.31/f noise
optimisation
C-BASS WP BreakdownWP 10
SoftwareTim Pearson
WP 10.3CalibrationSoftware
WP 10.2Quick-Look
Software
WP 10.5Foreground
Analysis S/W
WP 15Northern OpsTim Pearson
WP 15.2PreventativeMaintenance
WP 15.1Night-timeScheduling
WP 15.3Far-sidelobe
Mapping
WP 15.4Main Beam
Mapping
WP 15.5Cryo
Maintenance
WP 10.4MappingSoftware
WP 10.1Data logging
BPOL workshop 27th October 2006
Technology in place:
• E-Merlin C-band LNA:
• 1/f knee, with differencing, ~ 1 mHz
• Allows full rotation scan at ~ 1°/sec– Several times faster
in practice
BPOL workshop 27th October 2006
C-BASS Motivation
• Holy Grail for CMB work: – ‘smoking gun’ of inflation: – B-mode polarization from
gravitational waves
• < 3% of small-scale E-modes that are already detected.
• Accurate E/B separation needs contiguous large solid angle.
• If B-modes too weak, masked by gravitational lensing converting E→ B Le
nsin
g
E
B
r = 0.1
BPOL workshop 27th October 2006
5 GHz because…
• Halfway between quasi-reliable surveys at 1.4 GHz (Stockert, Reich & Reich) and 23 GHz (WMAP).
• Expected high-latitude Faraday rotation a few degrees, c.f. ~30° at 2.3 GHz.– Residual correction at high latitude via 1.4 GHz polarization
survey from Penticton/Villa Elisa (Wolleben/Testori et al.)
• Below main emission from anomalous dust, so predominantly synchrotron.
• Signal still strong enough (few mK) to map the sky in a reasonable time (< 1 year) with a single receiver.
BPOL workshop 27th October 2006
Impact of C-BASS
• Planck alone → Planck + C-BASS• Typical high-latitude pixel (2° beam):
– Spectral index bias • Stokes I: −0.14 → 0.015• Stokes Q,U: −0.16 → 0.03
– 70 GHz synchrotron amplitude error (assuming straight spectrum)
• Stokes I σ: 0.9 μK → 0.3 μK (SNR: 3.5 → 12)• Stokes Q,U σ: 0.3 μK → 0.045 μK (SNR: 1 → 7)
– 70 GHz synch. Amp. Bias• Stokes I: 0.9 μK → 0.15 μK • Stokes Q,U: 0.015 μK → 0.003 μK
5-7 times reduction in systematic synchrotron residuals in the CMB Band!
BPOL workshop 27th October 2006
A Proof of Concept
• The SPLASH survey (Abidin et al 2004) used the Effelsberg dish at 1.4 GHz to measure faint synchrotron polarization at high Galactic Latitude.
• Absolute polarization levels recorded to within ± 8 mK, ~10% of mean signal.
– Limited by relatively infrequent (90 min cycle) calibration to counter baseline drifts.
BPOL workshop 27th October 2006
Data Analysis
• Npix ~ 5x105 (cf Planck ~ 5x107)
• Ndata ~ 109 (cf Clover ~ 1013)
• Long-solved problem (e.g. Haslam et al 1981)• Improved techniques for eliminating residual
striping, but all algorithms Ndata
– No higher powers of N
BPOL workshop 27th October 2006
Competition?
• “Galactic Emission Mapping”
• Recently began preparation for 5 GHz polarization survey
• Operational at various frequencies since 1991
• No results to date• Originally intended to
complement COBE• Sensitivity too low to
achieve goals of C-BASS– 10 x noisier
GEM Brazil