bernardis (sapienza university of rome) lspe collaboration ... · p. de bernardis (sapienza...

P. de Bernardis (Sapienza University of Rome) for the LSPE CollaborationSCAR AAA Meeting

26/07/2013 Certosa di Pontignano (Siena, Italy)

The LSPE collaboration

The LSPE collaboration: G. Amicoa, P. Battagliab, E. Battistellia, A. Baùc, P. de Bernardisa, M. Bersanellib, A. Boscalerid, F. Cavaliereb, A. Coppolecchiaa, A. Cruciania, F. Cuttaiae, A. D’ Addabboa,

G. D'Alessandroa, S. De Gregoria, F. Del Tortoc, M. De Petrisa, L. Fiorineschif, C. Franceschetb, E. Franceschif, M. Gervasic, D. Goldieg, A. Gregorioh,p, V. Haynesi, N. Krachmalnicoffc , L. Lamagnaa, B. Maffeii,

D. Mainoc, S. Masia, A. Mennellac, Ng Ming Wahi, G. Morgantee, F. Natia, L. Paganoa, A. Passerinic, O. Peverinil, F. Piacentinia, L. Piccirilloi, G. Pisanoi, S. Ricciardie, P. Rissonef, G. Romeom, M. Salatinoa, M. Sandrie, A.Schillacia, L. Stringhettin, A. Tartaric, R. Tasconel, L. Terenzie, M. Tomasin, E. Tommasio,

F. Villae, G. Vironel, S. Withingtong, A. Zaccheip, M. Zannonic

a Dipartimento di Fisica, Sapienza Università di Roma, P.le A. Moro 2, 00185 Roma, Italyb Dipartimento di Fisica, Università di Milano, Via Caloria 16. 20133 Milano, Italyc Dipartimento di Fisica, Università di Milano Bicocca, Piazza della Scienza 3, 20126 Milano, Italyd IFAC‐CNR Via Madonna del Piano, 10 50019 Sesto Fiorentino (FI), Italye IASF‐INAF Via Gobetti 101, 40129 Bologna, Italyf Dip. Meccanica e Tecnologie Industriali, Univ. di Firenze Via S. Marta, 3, 50139 Firenze, ItalygCavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, UKh Physics Department, University of Trieste via A. Valerio 2, 34127 Trieste, Italyi Jodrell Bank Centre for Astrophysics, University of Manchester, Macclesfield, SK11 9DL, UK l IEIIT‐CNR, Corso Duca degli Abruzzi 24, 10129, Torino, ItalymIstituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata, 605, 00143 Roma, Italyn IASF‐INAF, Via Bassini 15, 20133 Milano, Italyo Agenzia Spaziale Italiana, Viale Liegi 26, 00198 Roma, Italyp OAT‐INAF, Via G.B. Tiepolo 11, 34143 Trieste, Italy

• The Large‐Scale Polarization Explorer is– a spinning stratospheric balloon payload

– flying long‐duration, in the polar night

– aiming at CMB polarization at large angular scales

– using polarization modulators to achieve high stability

• Frequency coverage: 40 – 250 GHz (5 channels)

• Two instruments: SWIPE (here) and STRIP (hear Mario Zannoni)

• Angular resolution: 1.5 – 2.3 deg FWHM

• Sky coverage: 20‐25% of the sky per flight

• Combined sensitivity: 10 μK arcmin per flight

In a nutshell :

Scientific Target• Cosmic Microwave Background photons undergo repeated

Thomson scatterings in the early universe, until the universecools down enough to become neutral.

• Small inhomogeneities of the primeval plasma at the epoch of the last scattering (13.7Gy ago, 380ky after the big bang) induce a small degree of linear polarization in the CMB (E‐modes).

• Gravitational waves produced during inflation, a split‐secondafter the big‐bang, also induce linear polarization in the CMB (both E‐modes and B‐modes)

• Measuring CMB polarization with high precision one can studythe very early universe and the inflation process, happening at energies (>1016 GeV) which cannot be reproduced in the laboratory.

• The signal from primordial B‐modes is extremely small, < 0.1 μKrms and is mainly at large angular scales.

primevalfireball

structures inthe making

with CMB datawe can study

all these phases

B-mod

epo

lariza

tion,

n s

Prim

ary

Anis

otro

py,

E-m

ode

SZ

effe

ct,

lens

ing

Scientific Target• Power spectra (variance versus angular frequency) for the CMB

Anisotropy:Measured very well

Gradient polatization:Measured

Curl polarization:Lensing partrecently detectedby SPT

Primordial part stillto be detectedBB

Scientific Target• Gradient (E‐mode) polarization vs Curl (B‐mode) polarization

E‐modes as detected by Planck B‐modes

Scientific TargetFigure 1. The bottom red line represents the contribution from each multipole to the total mean square fluctuation of the tensor component of CMB polarization (B‐modes, assuming a tensor to scalar ratio r = 1). The bump at small multipoles is due to photons last scattered during reionization, while the second bump is due to photons from z=1100. The thin blue line is the cumulative of the B‐modes, i.e. the variance measured by an experiment sensitive from multipole 2 to a given multipole l. The top blue thick line represents the beam function B2l for an experiment with a 1.5o FWHM Gaussian beam: despite of the coarse angular resolution such an experiment collects most of the polarization signal from B‐modes.

1.5° beam

Primordial B‐modes

cumulative

Instrument configuration• Small signal at large angular scales requires :

– 1.5° : Small telescope aperture sufficient (λ/D) – Wide frequency coverage (foregrounds) : two instruments

– Large sky coverage (cosmic variance) : spinning payload – LDB night flight

– Polarization modulator (to beat systematic effects) : HWP

– Large mapping speed (to reach target survey senstivity)Two ways:• Traditional (EBEX, SPIDER …) : Large number of single‐mode detectors

• SWIPE‐LSPE : Reasonable number of multi‐moded detectors

SWIPESTRIP

Batteries + Electronics(430 Kg)

Star Sensor(40 Kg)

STRIPPolarimeter

(350 Kg)

Al frame(500 Kg)

Azimuth Pivot(100 Kg)

Sketch of the LSPE experiment. The height of the gondola for this configuration is about 4.5m.

STRIPPolarimeter

(350 Kg)Spin3 rpm

Attitude Control & Reconstruction

Fast Star SensorNati et al. Rev.Sci.Inst.74, 9, 2003

Actuators: Azimuth pivot with torque motors

and flywheels;Linear elevation actuators

Processor: PC104 with ADC in / PWM out

H-bridges for motors

Attitude sensors: Star sensor

Laser GyroscopesElevation Encoders

TMTC

• Attitude Control System (ACS): composed of a set of actuators, a processor and a set of attitude sensors.

• Rotates the payload at 3 rpm and controls the elevation of the instrument, to scan the CMB sky and the calibration sources

• Acquires attitude data precise enough to allow sub‐arcmin reconstruction of the pointing for data analysis.

• Is derived directly by systems we have developed for previous balloon missions, like BOOMERanG and Archeops.

• The experiment is flown (2015) as a stratospheric balloon payload during the polar night, in a long duration flight launched from Longyearbyen (Svalbard). See fig.3.

• In this way it can access most of the northern sky in a single flight,

– without contamination from the sun in the sidelobes

– within a cold and very stableenvironment

– Accumulating more than 10 days of integration at float (38 km altitude).

The mission

Figure 3. Bottom: Ground path of a small pathfinder test flight performed in January 2011, in the middle of the polar night. The eastward trajectory is evident.. Top: Launch of a heavy-lift balloon from the Longyearbyen airport (SvalbardIslands, latitude 78oN).

Specific technical problems for a winter polar flight:

– Thermal management. The temperature of the stratosphere during the polar night is around ‐80oC, and there is no solar radiation available to warm‐up the payload. • Electronic systems must be thermally insulated from the environment to achieve self‐heating conditions

• Vacuum seals must be manufactured in indium or in special elastometers• Mechanical actuators play and lubrication must be specially designed for low temperature and low pressure

– Power supply for the experiment and the telemetry: 700 W for 15 days.• Electrical energy storage close to 1 GJ : lithium batteries• Located inside the same insulated box containing the powered electronics, to reach a temperature above 0oC and maintain most of their nominal capacity.

– Data management: LSPE will produce a raw data rate of about 400kbps • Entirely stored on‐board on solid state disks. • Selected data and essential housekeeping / flight info transmitted though the Iridium network

• Line‐of‐sight telemetry at full rate available during the first day of the flight• Data‐dumps when the system flies over selected locations hosting dedicated receiving stations.

• Spin rate : 3 rpm• Latitude: +78° N• Longitude : variable• Elevation range : 30°‐40°• 23% of sky outside WMAP mask

The STRIP Instrument

0.62.2Improvement factor wrt Planck-LFI

6.681.783.773.953.27

Delta Q(U) per 1.5° pixel (micro-K)

104331531731471-s sensitivity (μK×s1/2)

1.1131.8221.3821.8042.068Tsky (antenna) (K)

87< 1Telesc. / window (K)

412033.217.011.3Tnoise (K)

749632N_horn

16.27.712.04.14.5Bandwidth (GHz)

0.4670.467303030Obs Time (months)

1818100100100Sky coverage (%)

0.51.00.220.470.55Resolution (deg)

9043704430Frequency (GHz)

STRIPPLANCK LFI

• An array of coherent polarimeters for accurate measurement of the low‐frequency polarized emission, dominated by Galactic synchrotron. • Its design is described in detail in a companion poster by Bersanelli et al. • In the Table the performance of STRIP is compared to Planck‐LFI.

Hear Mario ZANNONI in a while

The SWIPE Instrument

4.93.22.7Improvement factor wrt Planck-HFI

0.170.130.10--2.60.80.40.3Delta Q(U) (uK)on SWIPE beams

225415660--10152033Integration/beam (s)

1.91.81.9////140453125Channel NET (uK s^1/2)

1108680008888N_det (polarized)

252525333333333333Bandwidth (%)

0.4670.4670.467303030303030Obs Time (months)

202020100100100100100100Sky coverage (%)

7489110555679FWHM Resolution (arcmin)

24514595857545353217143100Frequency (GHz)

SWIPE PLANCK – HFI

• An array of high-throughput bolometric polarimeters to measure accurately the linear polarization of the CMB and of the galactic dust foreground.• Design described in detail in a companion poster by de Bernardis et al. • In the Table the performance of SWIPE is compared to Planck-HFI.

foam window

HWP

HDP Lens

Array 1

Array 2

3He Fridge4He

Instrument SketchBoresight

elevation 20 o-60 o

Foam window

Thermal filters

HWP

UHDPE lens f/1.88, 470 mm Ø+ 440 mm Ø cold stop

L 4He tank (250 liters)

Wire grid polarizer

Reflectedarray

Transmittedarray

3He sorption refrigerator

145 GHz 90 GHz90 GHz

FocalPlaneMap

12o

20o

scan

245 GHz

In second cryostatscan

Compensation on scansNon linearities

TES Correlated thermal drift

AC bias / T stabilization1/f noise

Calibration on anisotropyGain stability

Calibration on anisotropyGain uncertaintyDetectors

Not polarized / orthogonal detectorsPendulation and atmospheric emission

ACS SensorsPointing errorPointing

Large shields, cold stopSidelobes pickup of Earth and Balloon

Large shields, cold stopSidelobes pickup of sky signal

Reduced in multimode system; lab. and flight calibrationMain beam ellipticity

Laboratory calibration / observation of planetsMain beam uncertaintyOptics

Scan with HWP steady, thermal link HWP – cryogenThermal fluctuations of HWP

Lab Calibration / Moon / CrabAbsolute polar angles calibration

Lab. CalibrationCross polar leakage

Scan with HWP steadySlant incidence of rays on HWP

Scan with HWP steady, Spectral bandwidth optimizationDifferential phase shift by HWP

Scan with HWP steady, antireflection coating on HWP Differential reflection of HWP

Scan with HWP steadyDifferential transmission of HWP

Low temperature WG, antireflection coating on HWPWire grid emission, reflected by HWP

Scan with HWP steady, Low temperature HWPHWP emission Polarization

MitigationSystematic effect

High mapping speed

• Two ways:• Traditional (EBEX, SPIDER …) : Large number of single‐mode detectors


• Example: Nm = 25 modes. With respect to a single mode detector: – Increase by a factor 5 the SNR.

– 70 uK sqrt(s) ‐> 12 uK sqrt(s) per detector

– Decrease by a factor 5 the angular resolution

– 15’ FWHM becomes 1.3° FWHM

mNW ∝

mNNET ∝ mN∝θmNNS ∝/2λΩ

≈ANm

SWIPE• The Short Wavelength Instrument for the Polarization Explorer • Uses overmoded bolometers, trading angular resolution for sensitivity• Sensitivity of photon-noise limited bolometers vs # of modes:

Number of modes actually coupling to the bolometer absorber

Light collectors and detectors

Figure 5. Far field calculation of the angular efficiency of a SWIPE Winston horn coupling 17 modes at 145GHz. The lower solid and dashed lines represent two orthogonal cross‐sections for linearly polarized photons. For reference, the solid top line is the beam for unpolarized radiation. In the inset, 2D map of the same beam.

• Baseline detectors: spider‐web bolometers with Transition Edge Sensors (TESs) thermistors:– extreme electro‐thermal feedback, reasonable time

constants, despite of the large absorber area necessary to couple to a multi‐mode beam.

– Rejection of primary cosmic rays present in the stratosphere and potentially very dangerous for CMB measurements.

• Baseline light collector: parabolic Winston horns– very high coupling efficiency in the main beam – high suppression of stray radiation, through the

incoherent superposition of the propagated modes at large angles.

– HFSS + custom‐developed polarized far field calculation code: results in Fig. 5

– Beam ellipticity is very small, a highly desirable feature for this instrument, and will be mitigated even more coupling the horn to the telescope and its Lyot‐stop.

– SWIPE will feature 80 horns at 95 GHz (each coupled to 23 modes), 86 horns at 145 GHz (35 modes), 110 horns at 245 GHz (64 modes). This setup will provide a performance comparable to that of competing instruments based on thousands of single‐mode detectors.

Polarization Modulator• The 50 cm Ø rotating HWP modulator

is the first optical element encounteredby the incoming radiation. This allowsus to relax the specifications for the polarization purity of the telescope and of the radiation collectors.

• The HWP is made of anisotropic metallic grids behaving differently along two orthogonal axes. Cascading many grids with specific geometries it is possible to produce a flat differential phase‐shift of 180° across wide bandwidths. Across a 30% bandwidth the measured cross‐polarisationresulted to be below ‐25dB5.

• The HWP is kept at <4K to limitsystematic effects related tounbalanced emission. The cryogenic rotator is similar to the one we haveproduced for the PILOT experiment6.

20 cm diameter prototype of a mesh‐HWP for the W‐band (left), and a close‐up of the grids (right).

The cryogenic polarization modulator of the PILOT experiment. It works with a power dissipation of a few mW.

LSPE HWP

• SWIPE is a sky‐scanning Stokes polarimeter, using a rotating half‐wave plate (HWP) and a steady polarizer in front of the detectors to modulate the linear polarization component of the incoming brightness.

• The HWP is rotated by a given angular step (say 22.5°) after a number of full revolutions of the gondola (say every 3 minutes, i.e. after 9 revolutions), and kept steady otherwise.

Measurement Method

Figure 1. Top: Noiseless simulation of the measurement of microwave emission from the CMB, the instrument, and the stratosphere, at 145 GHz (in CMB temperature units) during 4 revolutions of the payload. In this simulation, at the end of each revolution the HWP is stepped by 22o.5. Non‐idealities of the HWP and its thermal emission produce large steps in the detected power. Bottom:the same data once the averages over each revolution and the dipole have been removed. CMB anisotropy is evident. Inset: detail of the signal detected from the same sample sky region for 4 HWP orientations. The small differences are due to the small degree of polarization of the CMB.

Observations and Calibration Plan• For SWIPE, the telescope bore‐sight will be

at a nominal elevation of 40o and can be moved in the range from 20o to 60o , mainly to observe planets for beam calibration and investigate the effect of ground pickup.

• Elevation changes every several hours in a limited range (from 30o to 40o). This, combined with the azimuthal spin, and with Earth rotation, results in a 35% coverage of the sky, of which 25% unmasked. This is suitable for estimation low‐ell multipoles.

• In addition, the rotation of the waveplateresults in a very uniform coverage of the polarimeter angles for all observed pixels.

Sky coverage in a single flight

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WWS αα

Distribution of polarization anglesnon‐uniformity indicator

• SWIPE in flight calibration is based on: (i) absolute calibration of each detector on Planck CMB anisotropies; (ii) Point Spread Function (beam) and time response estimation based on scans over planets, with different scan speed; (iii) polarimeters calibration (efficiency and polarization angle), by observation of the Moon

• The in‐flight polarimeter calibration is used to confirm a more complete ground‐based calibration, obtained illuminating the telescope by means of a polarized source in the far field.

• Scanning strategy: payload spin in azimuth, at 3 rpm (18°/s)

• Coverage of the same sky area by the two instruments

• Elevation changes once a day, at the same time for both instruments

• Specific calibration observations of– Jupiter (to map the main beam, see

figure below, samples = white dots)

– the Crab nebula and the Moon Limb (to calibrate the main axis of the polarimeters)

– the Moon can be used to map sidelobes

Observations and Calibration Plan

24%33%STRIP 30

20%27%STRIP 45

26%35%SWIPE [30‐50]

18%22%SWIPE 45

19%24%SWIPE 35

20%27%SWIPE [40‐50]

23%31%SWIPE [30‐40]

UnmaskedCoverageElevationLSPE coverage for different sets of elevation changes. The first column reports the boresight elevation range in degrees for the two instruments. Second column, the full coverage. Third column, the coverage after masking the galaxy with the WMAP polarization mask.

2.50.80.30.240.0516Uranus

7024971.4-6Saturn

850275100801527Jupiter

185.621.60.300Mars

282322182034Crab

200000070000020000002000003750030Moon

S/N per sampleat 245 GHz

S/N persampleat 145 GHz




Culmination (deg)

Source

Sources culmination angle, and sensitivity for a launch on Jan 1st, 2015. Sampling rate is set at 60 Hz. We assume full Moon, as it is when it is observable by LSPE. The Crab flux is based on the free‐free spectrum reported in Macías‐Pérez, et al. Ap. J., 711, 417 (2010)

Constraining Cosmological Parameters

Constraining Cosmological Parameters

Left: Likelihood for the optical depth to reionization as estimated from Spider3 (dashed line) and SWIPE (continuous line). Right: Same for the tensor to scalar ratio. The two experiments have similar expected performance, but quite different measurement techniques. The estimates have been obtained using the publicly available Markov Chain Monte Carlo package cosmomc as usual in this field. This is based on the direct evaluation of the likelihood in the pixel space, without any power spectrum estimation step, assuming the CMB sky to be gaussian distributed.

Conclusions and Acknowledgments

We gratefully acknowledge support from the Italian Space Agency through

contract I‐022‐11‐0 “LSPE”

PRISM white paperastro-ph/1306.2259

www.prism-mission.org

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