The Atacama B-mode Search: A TES-Based CMB Polarization Instrument

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The Atacama B-mode SearchA TES-Based CMB
Polarization Instrument

• CMBpol Workshop, Chicago

Joe Fowler Princeton University
July 2, 2009
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Science Goal Primordial Polarization B-modes

• Target inflation B-mode range 30 lt l lt 300
• Development for an ACT-POL
• Institutions
• Princeton
• UBC
• NIST

ABS l range
Image NASA/WMAP science team
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Overview of ABS Instrument

• Focal plane array of NIST polarization-sensitive
  TES bolometers
• 240 machined aluminum feeds
• Cryogenic crossed Dragone mirrors
• Feeds and on-chip filters define a band around
  150 GHz
• Rotating half-wave plate outside cryostat
• 0.6º (FWHM) beams, 20º overall field of view
• Compact telescope, easy to deploy

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ABS Experiment Summary
Frequencies 145 only GHz
Angular resolutions 35 arcmin at each freq
Field centers and sizes 4h and 20h, 55S (not fixed) 300 sq deg each Ra/Dec/Sq-Deg
Telescope type Gregorian Refractor, Gregorian, Compact-range etc
Polarization Modulations HWP, Sky rotation, Scan Waveplate, boresight rot., sky rot., scan etc. list all that apply
Detector type Bolometers Bolometer, HEMT etc.
Location Atacama (Cerro Toco)
Instrument NEQ/U 20 Q, 20 U ?K s1/2 for both Q and U
Observation start date 2010
Planned observing time 600 d elapsed, 150 d science data Elapsed/effective days
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ABS reflecting optics

• 60 cm mirrors
• Crossed-Dragone design
• Cold stop inside window (25 to 30 cm diameter)
• Rotating HWP outside window.
• 240 feeds in focal plane

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Receiver

• Pulse tube cryocooler plus 4He3He sorption
• 50-60 cm cold mirrors
• Cryostat delivery June 2009

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Direct-machined aluminum feed

• 48 corrugations
• 1 long, ½ packing
• 16 FWHM beam
• Glen Atkinson (PU shop)
• Fabricate 15/week

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Feed beam patterns

• Beam measured warm to 30 dB at 147 GHz
• Cold measurements with NIST TES soon.

200 Hz chopped source (Gunn)
Test feed on rotary stage
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Focal plane

• 24 pods of 10 feeds
• Polarization TDB (mix of X,)
• Full set 25-30 cm across

MUX package on rigid flex
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150 GHz CMB Polarimeter (NIST)
More Mike Niemack (ACTpol)
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Atacama site 5200 meters

• ACT infrastructure
• 23 south latitude

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Telescope and platform

• Portable ABS Container
• Shippable container with a hole in the roof
• Can raise load without motors
• Electronics and computers next to lift
• Legacy telescope (2000) az/alt mount
• Adding a linear actuator for scanning

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Scan approach

• Requirements for B-mode sensitivity
• Smallest (deepest) possible area
• Uniform coverage
• Many crossing angles

2.3 F? detector spacing
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Short scan for deep observations
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Sky regions

• Co-ordinate with QUIET and POLARBEAR
• 2 main regions at 50 South

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Systematics

• Similar concerns to other low-l experiments
• ?T ? EB (differential beam shape)
• ?T ? EB (differential pointing)
• Atmosphere
• HWP rate limits scan rate
• Some ABS-specific
• Ground shield
• Stop and HWP dont coincide
• Can we spin HWP? Fast enough?
• Control of mirror temperature
• Foregrounds

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Half-wave plates at oblique incidence

• Ideal HWP modulation, spin frequency ?
• PH/V I Q cos 4?t U sin 4?t
• Fields at ?12 off axis pass through HWP at
  angle ?.
• Apparent ? is not constant
• (puts 0.1 of power into 2? and 6?)
• Rays not orthogonal to e-axis see n ? ne.
• n (cos ?/ne)2 (sin ?/no)2 -½
• For ABS, a 1 shift in phase lag.
• This, plus band center shift and finite width
• (a 1 suppression of Q, U averaged over band)
• PH/V I Q cos 4?t (1-e) e U sin 4?t
  (1- e) V -sin 2?t (2e½)

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