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
WMAP polarized
brightness:
23 GHz, 4° beam
• 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!
BPOL workshop
27th October 2006
C-BASS motivation
•
•
•
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.
BPOL workshop
27th October 2006
C-BASS
B
90 GHz
Synch.
Synchrotron spectral are smooth!
• Power law is just an
approximation…
• …but a good one
• The best-measured
synchrotron sources
are well fit by a 2ndorder 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 freefree, 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
Pinning down the Galactic
synchrotron spectrum
– need extra info to fix
spectrum.
• WMAP takes us down only
to 23 GHz
– weak lever arm for
extrapolation
Frequency Coverage of All-Sky Surveys
1000000
Synchrotron Emission
100000
Thermal
Dust
Existing Ground-based
WMAP
10000
Planck
1000
Relative Intensity
• Dust polarization well
measured by Planck
• Synchrotron dominates, at
best, only in lowest Planck
channels
Anomalous
Dust
100
10
1
0.1
0.01
Faraday Rotation
0.001
0.0001
100
1000
10000
100000
Frequency / MHz
• Gap between 2.4 and
23 GHz
Ground-based surveys needed to fix synchrotron emission
BPOL workshop
27th October 2006
1000000
Pinning down the Galactic
synchrotron spectrum
– need extra info to fix
spectrum.
• WMAP takes us down only
to 23 GHz
– weak lever arm for
extrapolation
Frequency Coverage of All-Sky Surveys
1000000
Synchrotron Emission
100000
Thermal
Dust
Existing Ground-based
WMAP
10000
Planck
1000
Relative Intensity
• Dust polarization well
measured by Planck
• Synchrotron dominates, at
best, only in lowest Planck
channels
Anomalous
Dust
100
10
1
0.1
0.01
Faraday Rotation
0.001
0.0001
100
1000
10000
Frequency / MHz
• Gap between 2.4 and
23 GHz
C-BASS fills the gap!
BPOL workshop
27th October 2006
100000
1000000
The Survey
•
•
Novel purpose-built single-feed
polarization and total power receiver
(Manchester/Oxford)
Northern survey from OVRO 5.5 m
dish (California)
–
–
•
Southern survey from 7.6 m at
Karoo (KAT) site, South Africa
–
•
sub-reflector tripod designed for low
spillover
high accuracy surface (mm-λ
telescope)
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
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:
• Equipment
–
–
–
–
–
– £104k
0.8 FTE Academic
3 FTE PDRA
1.2 FTE Engineer
2 FTE Technician
Direct costs £215k
• 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…).
BPOL workshop
27th October 2006
C-BASS Workpackage
Breakdown
WP 1
Project
Management
TJP/JPL/MEJ/JLJ
WP 2
Rx Design
WP 3
Optics Design
WP 4
Survey Design
WP 5
OVRO RFI
Characterisation
Tim Pearson
WP 6
Karoo RFI
Characterisation
Justin Jonas
Richard Davis
Mike Jones
Paddy Leahy
WP 7
Rx Construction
WP 8
Rx Integration
WP 9
Rx Testing (UK)
WP 10
Software
WP 11
Prepare 5 m
Telescope
Tim Pearson
WP 12
Rx Shipping &
Installation/OVRO
Mike Jones
Mike Jones
Mike Jones
Paddy Leahy
Tim Pearson
WP 13
OVRO
Commissioning
Tim Pearson
WP 14
Write technical
Papers
PDRA
WP 15
Northern Survey
Operations
Tim Pearson
WP 16
Northern
Data Analysis
PDRA
WP 17
PR & Outreach
Erik Leitch
WP 18
Prepare 7.6 m
Telescope
Justin Jonas
WP 19
Rx Shipping &
Installation/Karoo
Tim Pearson
WP 20
Karoo
Commissioning
Justin Jonas
WP 21
Southern Survey
Operations
Justin Jonas
WP 22
Southern
Data Analysis
PDRA
WP 23
Combine
Surveys
PDRA
WP 24
Foreground
Analysis
Clive Dickinson
C-BASS WP Breakdown
WP 2
Rx Design
R. J. Davis
WP 2.1
Specify
Mechanical I/F
WP 2.2
Specify
JBO/Oxford I/F
WP 2.3
Specify
Oxford/CCB I/F
WP 2.4
Design Rx Cryo
Components
WP 2.6
Design Cold
Load
WP 2.7
Design
Cryostat
WP 2.8
Design
Polarimeter
WP 2.9
Adapt CCB
design
WP 2.5
Design Rx
Backend
WP 3
Optics Design
M. E. Jones
WP 3.1
OVRO
Ground Screen
WP 3.2
5m
Subreflector
WP 3.3
5m
Feedhorn
WP 3.4
Karoo
Ground Screen
WP 3.5
7.6 m
Subreflector
WP 3.6
7.6 m
Feedhorn
C-BASS WP Breakdown
WP 7
Rx Construction
M. E. Jones
WP 7.1
RF cryo
components
WP 7.2
Backend amps
& filters
WP 7.3
Cold Load
WP 7.6
Detectors
WP 7.7
Feedhorn
WP 7.8
CCB
WP 7.4
Cryostat
WP 7.5
Phase switch
system
WP 9.5
Polarization
purity
WP 9
Rx Testing
J. P. Leahy
WP 9.1
White Noise
optimization
WP 9.2
Bandpass
measurement
WP 9.3
1/f noise
optimisation
WP 9.4
Noise diode
WP 9.6
Phase stability
& zero point
WP 9.7
Cold Load
Stability
WP 9.8
Feed radiation
pattern
WP 9.9
Backend
modes
C-BASS WP Breakdown
WP 10
Software
Tim Pearson
WP 10.1
Data logging
WP 10.2
Quick-Look
Software
WP 10.3
Calibration
Software
WP 10.4
Mapping
Software
WP 10.5
Foreground
Analysis S/W
WP 15.4
Main Beam
Mapping
WP 15.5
Cryo
Maintenance
WP 15
Northern Ops
Tim Pearson
WP 15.1
Night-time
Scheduling
WP 15.2
Preventative
Maintenance
WP 15.3
Far-sidelobe
Mapping
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 Emodes 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
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
BPOL workshop
27th October 2006