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Polarization at IRAM Status and Plans

Polarization at IRAM Status and Plans. S.Guilloteau Laboratoire d’Astrophysique de Bordeaux. 30-m: EMIR and XPOL. Credits : Thum , Wiesemeyer , Paubert , Navarro & Morris IRAM receiver group IRAM backend group EMIR ( Eight Mixer Receiver ) 4 frequencies

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Polarization at IRAM Status and Plans

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  1. Polarizationat IRAMStatus and Plans S.Guilloteau Laboratoire d’Astrophysique de Bordeaux

  2. 30-m: EMIR and XPOL • Credits: • Thum, Wiesemeyer, Paubert, Navarro & Morris • IRAM receiver group • IRAM backend group • EMIR (Eight Mixer Receiver) • 4 frequencies • on-axis atNasmyth focus • Dual linearpolarization • SidebandSeparating Mixers, 2 x 8 GHz instantaneousbandwidtheach.

  3. 30-m XPOL • A correlationpolarimeter • I measuredfromsum and Q fromdifference of linearpolarizedintensities • U and V fromcorrelations • Uses VESPA VErSatilePolarimeter and Spectral Analyzer correlator to compute all cross and parallelproducts. • Plus a switchablegridinto the calibration system • To derive the relative phase between the twolinearfeeds

  4. VESPA capabilities • Several modes • 40 kHz channelspacing, 120 MHz bandpass • 80 kHz, 240 MHz • 625 kHz, 480 MHz optimal for continuum • 2.5 MHz, two 480 MHz bands atsame IF • Mixed modes and split windows possible within limitations

  5. XPOL tricks • Matching of IF cablelengthsbetween the two mixers (0.05 m over > 100 m length…) • A new calibration stepwith Hot / Cold loadwith the wiregridat 54° to producestronglinearpolarization gives relative delay • Decorrelationloss due to phase noise in the LO measured on calibration device 14 %, consistent withmeasurement on sky on strongpolarized sources • U and V signdetermination… • Carefullalignment of the two mixers • Proper orientation of the G3 grid

  6. Performances • Q 1.5 % • U 0.6 % perhapsbetter (contamination by calibratorpolarization) • V 0.3 % • No elevationdependency for the polarized gains to a level of 1.5 % • SeeThum et al 2008 PASP • And IRAM web site for updates

  7. Interferometer • Credit • S.Trippe, V.Pietu, Y.W.Tang, S.Guilloteau, D.Broguière • IRAM receiver group • IRAM backend group • Dual Frequencyreceivers on Plateau de Bure • Correlator(s) onlyproducingparallel hand products HH and VV

  8. Interferometer • First simple step: • use the parallactic angle modulation • So called « rotation » polarizationmethod • Trippe et al 2010: a survey of 86 quasars • Position angle to 1.2° • Polarization fraction to 0.1 % • Multi frequency : 86 to 270 GHz • Labtestedreceiverpolarization < 0.3 % • Routinelydone in standard calibration • Otherwise quasar polarization affects amplitude gain calibration

  9. Interferometer • Towards full Stokes measurement… • Trick • A commutable switch in the IF system can insert either H or V receiverindependently for eachantenna in the two « parallel » correlators • With Walsh functions, all products HH, VV, HV and VH canbemeasuredafter one cycle • Cycle lengthdepends on number of antennas. 8 steps for 6 antennas

  10. InterferometerStatus • Deviceinserted in IF path (a cheap retrofit…) • 1 k€, plus one man week • On-line software controlsit • Currently on « long » timescales (scan length, 45 sec) • Software development time dominates the overallcost and schedule… • Data calibration performedoff-line • Good bandpassstability shouldbe OK ! • Relative H and V delaymeasurement • Simplification of observingprocedure possible • Ultimate performance stillunknown • Stabilityiskey to the accuracy • « rotation » polarizationmethodresultssuggeststabilityis good • Although « rotation » performances obtainedafterfilteringoutliers

  11. Interferometer • Off-line data processing • CLIC modified to automate all the requiredsteps • Minor (but subtle) data format modifications • GILDAS data format modified to handle full polarization information • Mappingimaging program modified to manage the polarization information • Nextsteps • Real time control software to operate the switchevery second • New tests foreseen for thissummer

  12. NOEMANorthernExtendedMillimeterArray • Currentlyfunded for 10 antennas • Partners for 2 more and longer baselineswelcome… • Polarization not a major design goal • Polarizationrequires a lot of sensitivity • ALMA bettersuited for this (2 types of antennas, polarization in design since the beginning) • However, not designed out • will use same trick with IF switch • Switch willbelocatedatdifferent place (in antennasratherthan in backend room)

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