cosmo bicep2 systematicscosmo2014.uchicago.edu/depot/talk-sheehy-christopher.pdf · Fridge mounting...

Christopher Sheehy for the BICEP2 collaboration University of Chicago

Cosmo 2014, Chicago, IL August 25, 2014

BICEP2 Instrumental Systematics

Christopher Sheehy for the Bicep2 Collaboration

Instrumental Systematics

All systematic effects that we could imagine were investigated! !We find with high confidence that the apparent signal cannot be explained by instrumental systematics!

0 50 100 150 200 250 30010−7

10−6

10−5

10−4

10−3

10−2

10−1

100

Multipole

l(l+1

)ClBB

/2π

[µK2 ]

lensed−ΛCDM+r=0.2BICEP2 stat. uncer.extended beamthermalsys pol error

rand pol errorghosted beamstransfer functionscrosstalk


The BICEP2 Telescope

Telescope as compact as possible while still having the angular resolution to observe degree-scale features. !On-axis, refractive optics allow the entire telescope to rotate around boresight for polarization modulation. !Liquid helium cools the optical elements to 4.2 K. !A 3-stage helium sorption refrigerator further cools the detectors to 0.27 K.

Lens

Nylon filterLens

Nb magnetic shield

Focal plane assemblyPassive thermal filter

Flexible heat straps

Refrigerator

Fridge mounting bracket

Camera plate

Cam

era

tube

Opt

ics

tube

1.2 m


BICEP2 on the Sky

Projection of the BICEP2 focal plane on the sky! !(Background is the CMB temperature map as measured with BICEP2)


BICEP2 on the Sky

AB

Each focal plane pixel is really two detectors — a horizontally polarized one and a vertically polarized one. !Measure CMB T by summing the signal from orthogonally polarized detector pairs. !Measure CMB polarization by differencing the signal. !!


Raw Data

Telescope Movement

time (~1 hour)


Raw Data

Telescope Movement

Sum of detector pairs

time (~1 hour)


Raw Data

Telescope Movement

Sum of detector pairs

Difference of detector pairs

time (~1 hour)


Beam Systematics➢ Each focal plane pixel is really two orthogonally polarized detectors ➢ Call these detectors “A” and “B”. They nominally point to the same

location on the sky and have identical response to the CMB, i.e. have the same “beam”.

➢ If the CMB is unpolarized, the “pair difference” should be zero ➢ If the response to the CMB is different for A and B, then even if the

CMB is unpolarized, the pair difference is non-zero. This is contaminates polarization!

Each focal plane pixel is really two detectors — a horizontally polarized one and a vertically polarized one.

AB

Beam systematics

“A” and “B” beams


example: pointing center mismatch

AB

Beam systematics


A-B difference beam



AB

Beam systematics


A-B difference beam



AB

A and B’s path along the sky as the telescope scansPlanck T map

Systematics removal: deprojection


A-B difference beam



AB

Planck dT/dDec. map

Use the Planck 143 GHz map to form templates of leakage from mismatched elliptical Gaussian beams.

Fit these templates to the data and subtract.


Systematics removal: deprojectionDifferences of elliptical Gaussians


Far field beam mapping: microwave source mounted here

Detailed description in companion Instrument Paper

We know our beam shapes


High fidelity beam maps of individual detectors



beam ellipticity


differential ellipticity

−5 0 5

−5

0

5

degrees

degr

ees

differential beam centroids (x20)

−5

0

5

degrees

degre

es

!

p2 +c2 = 5%

−5 0 5

−5

0

5

degrees

!

δp2 +δc2 = 2%

Use the Planck 143 GHz map to form templates of leakage from mismatched elliptical Gaussian beams.Fit these templates to the data and subtract.






Measured from beam maps

Dep

roje

ctio

n fit

coe

ffici

ents

δg

−0.05 0 0.05−0.05

0

0.05

δx (deg)

−0.02 0 0.02 0.04−0.02

0

0.02

0.04

δy (deg)

measured

depr

oj. c

oeff.

−0.02 0 0.02 0.04−0.02

0

0.02

0.04

δσ (deg)

−0.01 0 0.01−0.01

−0.005

0

0.005

0.01

δp

−0.05 0 0.05−0.05

0

0.05

δc

−0.05 0 0.05−0.05

0

0.05

gain width

pointing x

pointing y

ellip. p

ellip. c


We know our beam shapesBecause contamination from beam shape mismatch is entirely deterministic, we can both remove it (deprojection) and predict it in simulation using calibration data as input.

Calibration data for each detector



Simulation (explicit convolution with Planck T map)

Because contamination from beam shape mismatch is entirely deterministic, we can both remove it (deprojection) and predict it in simulation using calibration data as input.


*



Simulation (explicit convolution with Planck T map)

Predictions of contamination

Because contamination from beam shape mismatch is entirely deterministic, we can both remove it (deprojection) and predict it in simulation using calibration data as input.


*




0 100 200 300

10−4

10−3

10−2

10−1

100

Multipole

l(l+1

) Cl/2π

[µK2 ]

BB signal

(1) no deprojection(2) dp(3) dp+rg(4) dp+rg+ellip.lensing + r=0.2

0 100 200 3000

0.05

0.1

∆BB signal

(1) − (2)(2) − (3)(3) − (4)

0 100 200 3000

0.02

0.04

∆BB FP inner/outer jack

0 100 200 300−0.2

−0.1

0

0.1∆TE tile inner/outer jack



Without ellipticity deprojection, predicted contamination in BB is

negligible compared to noise level

example: slightly elliptical beams

r=0.2 + lensing



beam ellipticity

differential ellipticity

−5

0

5

degrees

degre

es

!

p2 +c2 = 5%

−5 0 5

−5

0

5

degrees

!

δp2 +δc2 = 2%






r=0.2 + lensing






r=0.2 + lensing

…but the focal plane inner/outer jackknife is still sensitive to it

real data


We know our beam shapesexample: slightly elliptical beams

…and the jackknife cleans up exactly as simulations predict

real data

With ellipticity deprojection, predicted contamination in BB is

still negligible

r=0.2 + lensing


Instrumental Systematics

All systematic effects that we could imagine were investigated! !We find with high confidence that the apparent signal cannot be explained by instrumental systematics!

0 50 100 150 200 250 30010−7

10−6

10−5

10−4

10−3

10−2

10−1

100

Multipole

l(l+1

)ClBB

/2π

[µK2 ]

lensed−ΛCDM+r=0.2BICEP2 stat. uncer.extended beamthermalsys pol error

rand pol errorghosted beamstransfer functionscrosstalk


Thanks

cosmo bicep2 systematicscosmo2014.uchicago.edu/depot/talk-sheehy-christopher.pdf · Fridge mounting...

Documents

Transcript of cosmo bicep2 systematicscosmo2014.uchicago.edu/depot/talk-sheehy-christopher.pdf · Fridge mounting...