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The Atacama B-mode Search: A TES-Based CMB Polarization Instrument CMBpol Workshop, Chicago July 2, 2009 Joe Fowler Princeton University
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Science Goal: Primordial Polarization B-modes Target inflation B-mode range: 30 < ℓ < 300 Development for an ACT-POL Institutions: Princeton UBC NIST Image: NASA/WMAP science team ABS ℓ range
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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 4 h and 20 h, 55 ° S (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 s 1/2 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 Sapphire HWP Focal plane
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Receiver Pulse tube cryocooler plus 4 He+ 3 He sorption 50-60 cm cold mirrors Cryostat delivery: June 2009 Window Up ↑
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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) One TES Bandpass filter in microstrip Bond pads “Dark TES” OMT 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 Fλ 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: ΔT E+B (differential beam shape) ΔT E+B (differential pointing) Atmosphere HWP rate limits scan rate Some ABS-specific: Ground shield Stop and HWP don’t coincide Can we spin HWP? Fast enough? Control of mirror temperature Foregrounds Sapphire HWP Focal plane
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Half-wave plates at oblique incidence Ideal HWP modulation, spin frequency ω: P H/V = I ± Q [cos 4ωt] ± U [sin 4ωt] Fields at ψ=12° off axis pass through HWP at angle ψ. 1. Apparent ω is not constant (puts 0.1% of power into 2ω and 6ω) 2. Rays not orthogonal to e-axis see n’ ≠ n e. n’ = [ (cos ψ/n e ) 2 + (sin ψ/n o ) 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) P H/V = I ± Q [cos 4ωt (1-ε) + ε] ± U [sin 4ωt (1- ε)] ± V [-sin 2ωt ] (2ε ½ )
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Atacama Cosmology Telescope Site
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