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Thoughts on Ground-based lensing measurements. Chao-Lin Kuo Stanford/SLAC KIPAC. The primordial, Gaussian E-polarization. Large Scale Structure. B-polarization. J Tolan. Lensing B -polarization is a LSS experiment. Hu , Huterer and Smith, 2006.

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thoughts on ground based lensing measurements

Thoughts on Ground-based lensing measurements

Chao-Lin Kuo



lensing b polarization is a lss experiment
LensingB-polarization is a LSS experiment

Hu , Huterer and Smith, 2006

  • Deep polarization measurements (4 mK rms) on 1.5% of the sky can significantly improve Planck+SNAP constraints on

{w0, wa, Wk, ∑mn},



  • There is a strong theoretical preference:

w0= -1, wa=0, Wk <10-4 → lensing B provides a constraint on ∑mn

the status of b polarization measurements 07 2009
The Status of B-Polarization Measurements (07/2009)

Lensing B-mode

Gravity wave


Chiang et al.

  • QUaD/BICEP (50~100 detectors) still miss the (lensing) B-polarization by ~ 2 orders of magnitude.
  • The current round of experiments (~1000 detectors) can only hope for a statistical detection.
  • To perform high S/N imaging of lensing B-polarization, one must increase the survey speed by 102.
  • The ground based platforms (DASI “drum”, SPT) will be maxed out already in the current round of exp.
the simple strategy to get 10 000 detectors on the sky
The simple strategyto get ~10,000 detectors on the sky…
  • Use an optical design that has the largest possible focal plane area
  • Choose an aperture size that optimizes throughput/dollar
optics comparison
Optics Comparison


Crossed Dragone

  • The advantage of a crossed-Dragone system
  • (1). > 4X more FOV area than a Gregorian
  • (2). Flat/telecentric focal plane, no re-imaging
  • Optics
  • Good polarization properties verified in numerous
  • studies
  • The required primary aperture for lensing
  • B-mode is ~2 meters – for 10m class telescopes
  • the measurements will be sensitivity (throughput)
  • limited, not resolution limited.

Strehl Ratio

H. Tran et al., CMBPOL Technology Workshop, 2008

the simple strategy to get 10 000 detectors on the sky1
The simple strategyto get ~10,000 detectors on the sky…
  • Use an optical design that has the largest possible focal plane area
  • Choose an aperture size that optimizes throughput/dollar
  • The Proposed Experiment:

An array of 5-10 crossed-Dragone multifrequency telescopes, each with ~2-meter primary aperture and ~2,000+ detectors

a pilot project one 1 5 2m telescope
A Pilot Project: one 1.5-2m telescope
  • Serving as the prototype for two experiments
    • Pol-Len: Polarimeter array for Lensing
    • EPIC-IM (in collaboration w/ JPL)
  • The telescope will be integrated with
    • Room temperature sources/detectors
    • A BICEP-2 style 512-detector bolometric receiver
    • A larger format camera – see the next page
  • Many issues can be characterized in full details with this pilot projet:
    • Near and far sidelobe responses, baffling
    • Infrared filtering
    • Magnetic field shielding
    • Detector loading
    • Mitigation of polarization systematics
the expansion prospects
The expansion prospects

(major technology

dev. required)

1 Telescope +

8,000 bolometers

5-10 Telcps., each

w/8,000 detectors

(minor technology

dev. required)

LDRD funds

1 Telescope

1 Telescope +

512* bolometers

1 Telescope +

2,000 bolometers

5 Telescopes, each

w/ 2,000 bolometers

Deployment for field observations

*# of detectors projected for 150 GHz

EPIC-IM mission




The “major” technology development

  • By reducing the size of the feeds
  • we can pack more detectors (~4x) onto the
  • focal plane (~1.5 fl)
  • The price to pay is increased spillover
  • – which must be intercepted at 4K
  • 8,000 detectors to read per dewar

warm baffle

cold stop




IR filter

Teflon (50 k)





cold stop

also a great gravity wave b mode experiment for r 0 1
Also a great gravity wave B-mode experiment (for r<0.1)

Compared to degree beam experiments (BICEP/Keck, ABS), a 2 m class telescope offers:

  • Smaller maps → lower noise
  • Smaller maps → potentially less foreground
  • Small beams → de-lensing possible
  • Small beams → less Beam systematics

(Polarized Dust, 5%)

3.6 deg

7.2 deg

14.4 deg

28.8 deg

the trade offs
The Trade-offs
  • No Half-Wave-Plate modulators.
  • No full - rotation.
  • Modulation relies on scanning – QUaD/BICEP style.
  • For the same , 2 possible angles can serve as a systematic check.
funding fielding prospects
Funding/fielding prospects
  • SLAC “LDRD” under review (1 telescope, warm tests)
  • An NSF proposal will go in this August (“Pol-Len1”, one telescope +mount+ receiver development)
  • BICEP/Keck collaboration supportive of the deployment of the telescope to the South Pole DSL site in 2011. Pending approval from NSF-OPP (office of polar programs).
  • We have not thought about how to fund the full array…Let me

know if you have $ or are interested.