C-BASS C-Band All-Sky Survey Tim Pearson (Caltech) 2009 July 2 - - PowerPoint PPT Presentation

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C-BASS C-Band All-Sky Survey Tim Pearson (Caltech) 2009 July 2 - - PowerPoint PPT Presentation

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C-BASS C-Band All-Sky Survey Tim Pearson (Caltech) 2009 July 2 Summary Image the whole sky at 5 GHz (C band). In both brightness and polarization . Broad-band (1 GHz) correlation polarimeter and correlation radiometer. Two

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C-BASS C-Band All-Sky Survey

Tim Pearson (Caltech)

2009 July 2

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Tim Pearson 2009 Jul 2

Summary

  • Image the whole sky at 5 GHz (“C band”).
  • In both brightness and polarization.
  • Broad-band (1 GHz) correlation polarimeter and

correlation radiometer.

  • Two telescopes: one in California, and another in

South Africa.

  • FWHM 0.85° – similar to Haslam-408, WMAP.
  • rms noise < 0.1 mK in I, Q, U
  • Completion in 2011(?) to support Planck analysis.
  • Northern survey: start 2009
  • Southern survey: start 2010

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Tim Pearson 2009 Jul 2

Motivation

  • CMB Task Force recommendations (2005):
  • “A systematic program to characterize astrophysical

foregrounds, especially from the galaxy, over a wide range of frequencies.”

  • “Continued support for ground-based efforts to produce

3–15 GHz large-scale maps of the polarized Galactic foreground.”

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Tim Pearson 2009 Jul 2

Science Goals

  • Survey of diffuse Galactic emission at a frequency low

enough to be dominated by synchrotron radiation but high enough to be uncorrupted by Faraday rotation effects.

  • Enable accurate subtraction of foreground contaminating

signals from higher-frequency CMB polarization sky surveys, including WMAP and Planck.

  • Major resource for studying the interstellar medium and

magnetic field of the Galaxy.

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Tim Pearson 2009 Jul 2

CMB Foregrounds

  • Low-frequency foregrounds:
  • Synchrotron
  • Free-free
  • “Spinning dust”
  • C-BASS at 5 GHz is dominated by synchrotron
  • A polarized synchrotron template? – input for

foreground modeling (e.g., spatial variation of spectral index and curvature).

  • 5 GHz is lowest frequency where Faraday rotation is

negligible, < 1° except in Galactic plane

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Tim Pearson 2009 Jul 2

What does C-BASS add?

  • Compare Planck alone to Planck + C-BASS
  • Single pixel analysis (1000 realizations), simple Galactic model
  • Clive Dickinson FGFIT (MCMC parametric fitting)
  • Typical high-latitude pixel (2° beam):
  • Spectral index bias reduced:
  • Stokes I: −0.14 → 0.015
  • Stokes Q,U: −0.16 → 0.03
  • 70 GHz synchrotron amplitude error reduced:
  • 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. amplitude bias reduced:
  • Stokes I: 0.9 μK → 0.15 μK
  • Stokes Q,U: 0.015 μK →0.003 μK
  • 5–7 times reduction in synchrotron residuals in the CMB

band!

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WMAP5 23 GHz map of polarized intensity (color) and direction (vectors) from WMAP (Hinshaw et al. 2008). DRAO 1.4 GHz Polarized intensity (Wolleben et al. 2006 A&A 448, 411)

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Tim Pearson 2009 Jul 2

Predictions

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I Q U

(Planck Sky Model)

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Tim Pearson 2009 Jul 2

Survey Requirements

  • Sensitivity: In order to subtract the polarized Galactic

foregrounds to below the sensitivity levels of Planck requires an rms noise level of < 100 µK per pixel. Our goal is to produce a substantially lower noise level and reduce systematic errors to well below 5% level.

  • Resolution: To detect the B-mode peak at l ~ 90 we need

measurements up to l ≈ 150, which fixes the resolution of the survey to about 1°.

  • Frequency: High enough to avoid Faraday rotation, low

enough to maximize sensitivity to synchrotron.

  • Bandwidth: Limited by manmade interference (RFI).

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Tim Pearson 2009 Jul 2

Collaboration

  • Caltech/JPL/OVRO: Dayton Jones, Russ Keeney, Charles

Lawrence, Erik Leitch, Stephen Muchovej, Tim Pearson, Tony Readhead, Graça Rocha, Matthew Stevenson.

  • Northern survey, OVRO antenna, backend and data
  • acquisition. Supported by NSF.
  • Oxford University: Christian Holler, Jaya John John, Mike Jones,

Oliver King, Angela Taylor.

  • Feed optics, receiver and polarimeter, cold loads.
  • Manchester University: Rod Davies, Richard Davis, Clive

Dickinson,Tess Jaffe, Paddy Leahy, Stuart Lowe, Neil Roddis, Althea Wilkinson, Peter Wilkinson.

  • Low-noise amplifiers.
  • Rhodes University / HartRAO: Roy Booth, Charles

Copley, Justin Jonas.

  • Southern survey.

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Tim Pearson 2009 Jul 2

Antennas

6.1m antenna at OVRO, California (donated by JPL) 7.1m antenna in South Africa (moved to a site in the Karoo)

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Tim Pearson 2009 Jul 2

  • Rapid scanning of the telescope in AZ to reduce 1/f

systematics.

  • Highly-redundant coverage at a variety of scan crossing

angles.

  • Optical layout and feed-horn have optimized for minimal

sidelobes and cross-polarization. No subreflector support legs.

  • Absorbing tunnels reduce the sidelobes to more than 40

dB below the main beam, while contributing ~ 0.8 K to the system temperature.

  • Ground screens to shield the receiver from polarized

ground-reflected radiation and RFI.

Systematics

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Tim Pearson 2009 Jul 2

Scanning Strategy

  • Constant elevation scanning: constant ground and

atmosphere loading

  • Many scan crossing angles at each pixel to reduce

systematics

  • Scan speed ~ 6°/sec
  • Many orientations of the polarimeter at each pixel
  • 50% of each night through the pole (baseline reference)
  • 50% through pole + 22°

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Tim Pearson 2009 Jul 2

Scanning Strategy

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Tim Pearson 2009 Jul 2

Antenas and Optics

No feed support legs, absorbing tunnels

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Tim Pearson 2009 Jul 2

Calculated beam patterns

  • ---E-field
  • ---Cross polar
  • --- with absorber
  • --- without absorber

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Credit: C.M. Holler (Oxford)

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Tim Pearson 2009 Jul 2

Correlation Receiver

∆T Tsys = ∆G G ∆T Tsys = ∆G G TA − Tref Tsys

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Tim Pearson 2009 Jul 2

Receiver layout: I, Q, U

Tsys < 20 K 4.5 to 5.5 GHz σQ,U < 0.1 mK

Q ∝ ELER U ∝ ELEReiπ/2

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Tim Pearson 2009 Jul 2

OMT

Cross-pol < -58 dB over 40% BW Return loss ~ -20 dB Very compact, easy to cool to 4 K (Grimes et al. 2007)

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Tim Pearson 2009 Jul 2

Polarimeter Components

Phase switch 26 dB amplifier 180° hybrid Bandpass filter + eMerlin C-band LNAs

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Oliver King (Oxford)

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Tim Pearson 2009 Jul 2

Digital Readout

  • Based on particle physics readout boards

developed at Oxford

  • On-board FPGAs perform subtraction

demodulation, integration

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Tim Pearson 2009 Jul 2

RFI 4.5 – 5.5 GHz

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Passive Microwave Imagery at 6.9 GHz from AMSR-E on the NASA EOS Aqua platform.

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Tim Pearson 2009 Jul 2

RFI Monitor

  • Heterodyne design based around the CASPER iADC and iBOB boards.
  • Entire 1GHz band is Nyquist sampled, with all DSP occurring on the

FPGA.

  • 512 element spectrometer (2MHz resolution) is implemented.
  • Can detect horizontal RFI below the C-BASS detection threshold,

allowing for unambiguous flagging of corrupted measurements.

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Tim Pearson 2009 Jul 2

Schedule

  • OVRO telescope commissioning (Apr-Jul 2009)
  • Receiver installation and commissioning (Jul-Sep)
  • Northern survey (1 year)
  • nights only, allowing 50% efficiency
  • Build second receiver (multichannel?)
  • in collaboration with King Abdulaziz City for Science and Technology

(KACST), Saudi Arabia; Yaser Hafez

  • Southern survey (1 year, starting Feb 2010)
  • Data release in 2011-12?

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