The Atacama B-mode Search: A TES-Based CMB Polarization Instrument CMBpol Workshop, Chicago July 2, 2009 Joe Fowler Princeton University

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

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

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

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

Receiver  Pulse tube cryocooler plus 4 He+ 3 He sorption  cm cold mirrors  Cryostat delivery: June 2009 Window Up ↑

Direct-machined aluminum feed  48 corrugations  1” long, ½” packing  16° FWHM beam  Glen Atkinson (PU shop)  Fabricate ~15/week

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

Focal plane  24 “pods” of 10 feeds  Polarization TDB (mix of X,+)  Full set: cm across MUX package on rigid + flex

150 GHz CMB Polarimeter (NIST) One TES Bandpass filter in microstrip Bond pads “Dark TES” OMT More: Mike Niemack (ACTpol)

Atacama site: 5200 meters  ACT infrastructure  23° south latitude

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

Scan approach Requirements for B-mode sensitivity:  Smallest (deepest) possible area  Uniform coverage  Many crossing angles ~2.3 Fλ detector spacing

Short scan for deep observations

Sky regions  Co-ordinate with QUIET and POLARBEAR  2 main regions at 50° South

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

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 )

Atacama Cosmology Telescope Site