VIEWS: 4 PAGES: 25 POSTED ON: 7/12/2011
Astrometry with VSOP RIOJA Maria (OAN, Spain) PORCAS Richard (MPIfR, Germany) DODSON Richard (OAN, Spain) ASAKI Yoshiharu (ISAS, Japan) Outline Phase Referencing in Space: study team Astrometry observations with VSOP 1038+528A/B, 33” away 1308+326/8, 14‟ away New phase referencing technique for mm- VLBI - Demonstration experiment Strategies for Astrometry with VSOP-2 Phase referencing VLBI Phase-reference mode observing has proved to be a useful technique for astronomical VLBI. Both for mapping weak sources and for high precision relative astrometry. Prerequisites for success are: Sufficiently strong, compact nearby reference source Instrumental phase coherence between the sites Accurate correlator model for the path delay between antennas (e.g. accurate orbit determination) …extended to Space VLBI In principle the phase-referencing technique can be extended to Space VLBI. In practice there may be special problems: Reference sources more likely to be resolved out, Relatively poor sensitivity, Rapid source changes required, Instrumental, coherence limited, space-ground link, And uncertainties in spacecraft orbit. Astrometry with VSOP VSOP was not designed with phase-referencing in mind! Phase reference observations are thus limited to close source pairs where both target and reference sources are simultaneously within HALCA primary beam. While such observations are feasible, it was not known whether the orbit errors would prevent transfering the phase between sources on GRT- HALCA baselines. Why need accurate orbit? Errors in orbit determination can prevent successful phase referencing. Errors in orbit propagate into relative phase, diluted by the source separation measured in radians. How accurate? HALCA position error (m) - Orbit error target-reference source separation (rad) error in transferred relative phase function Given that: For successful phase-referencing, rad Largest source separation for “in beam” phase-ref is given by, roughly D (where D is antenna diameter) Thus the condition for phase referencing becomes D / (2 Since D is 8 m, orbit errors no more than 1 to 2 m are needed to permit phase ref anywhere within the HALCA primary beam. 1308+326/1308+328 (Exp.code:v049a; PI: Rioja) Two strong flat spectrum sources, 14.3’ away, about half of the HALCA primary beam FWHM at 5 GHz (dilution factor 4x10 -3). Ground array telescopes: VLBA and Effelsberg, switching cycle 7 mins. Visibility amplitude vs (u,v) distance 1308+326 1308+328 GRT-only maps 1308+326 (target) 1308+328 (ref.) Phase-reference analysis (1) VSOP hybrid maps of reference source 1308+328 All baselines (peak 171 mJy) Only HALCA (peak 105 mJy) Beam 0.35x0.35mas Porcas & Rioja, 2000 Phase-reference analysis (2) VSOP phase-referenced maps of target source 1308+326 All baselines (peak 579 mJy) Only Halca (peak 372 mJy) Beam 0.35x0.35mas Phase of 1308+326 visibility function, after applying instrumental phase corrections determined using 1308+328. Astrometric “core” shifts 1308+326: VSOP phase-referenced map . 8.4 GHz, EVN, 1995 + 1.6 GHz, VSOP, 1998 . 86 GHz, VLBA, 2001 . .+ Rioja & Porcas, 1996 Porcas & Rioja, 2002 Orbit error determination Hybrid maps of 1308+326 All baselines (peak 641 mJy) Only HALCA (peak 578 mJy) Halca position accuracy 2-5 m Residual relative phase errors (up to 60 o) HALCA Orbit Accuracy JPL Navigation team (T. You etal. „98) found typical values of 3-6m. Our values from relative astrometry: 2-5m (Guirado etal. „01 obtained a similar result from similar analysis). GEDEX program (Frey etal. „00) used residual delays, found “consistent values”. Phase referencing possible. 1038+528 A & B S-band beam (Exp.code: v046a1; PI: Rioja) Quasar pair 33” away - both quasars in all antenna beam simultaneously (dilution factor 10 -4) Long term astrometric studies: multi-epoch (17 years) & multi-frequency (S/X - Interpretation) observations. HALCA+VLBA+DSN Robledo&Goldstone @1.6GHz 1038+52A, X band Phase reference analysis (1) - Maps 1038+52B, GRT+HALCA Phase-referenced map All baselines 1038+52A, GRT+HALCA Hybrid map Porcas & Rioja, 2000 Astrometry with VSOP S-band beam L-band beam VSOP 1.6 GHz 2.3 GHz 8.4 GHz 15 GHz 22 GHz 1038+52A, X band New phase referencing technique for mm- VLBI: `Source/Frequency Phase Transfer’ Enhancement of `Fast-Frequency Switching’, to implement astrometric capability: Observe at lower band (e.g. 21.5 GHz) Apply to higher band (e.g. 43 GHz) ‘Frequency-phase Transfer’ VLBI demonstration: Middelberg, Roy, Walker, Falcke `05 VERA+KVN Group (priv. comm) Improved phase coherence - but No astrometry due to residual dispersive terms New phase referencing technique for mm-VLBI: Source/Frequency Phase Transfer Dispersive terms can be handled by a further cycle of calibration: ‘Source and frequency switching’ ‘Source/Frequency Phase Transfer’: Our VLBI Demonstration (BD119) 43 GHz to 86 GHz Target Basics of new method: SOURCE/FREQ. phase referencing fast slo slow w A,GEO + A,TRO + ST A,STR+ nA X A,GEO + + A,TRO + ST n RR,GEOA,TRO + ST nA A) R A,TRO - R * A,TRO = 0 A,ANT - R * A,ANT = 0 - R* = (R-1/R) … slow slow RSTR + 2c (D . A,shift) + ST Frequency-referenced Visibility phase Basics of new method: SOURCE/FREQ. phase referencing R2c (D . B,shift) + ION NST Source/freq. referenced Visibility phase: A,STR + 2c D (A,shift - B,shift) Source/Frequency Phase Transfer Demonstration (Exp.code: BD119, PI: Richard Dodson) Observe at lower band (43 GHz) Apply to higher band (86 GHz) Two pairs (1308+326/8 and 3C273/4) One close (14‟), one (very) distant (10o) 1 minute frequency switching 5-10 minutes source switching Source/Frequency referenced maps at 86 GHz 1308+328, 14’ 3C274, 10o Dodson etal, 2007 Strategies for Astrometry with VSOP-2 Conventional Phase Referencing: fast 43 43 GHz Increase coherence AND astrometry Asaki et al .“ARIS simulation tool”: PASJ 59 (2007), & next talk 43 GHz fast Same Frequency Phase Transfer: source 22 Increase coherence at 43 GHz fast Different 22 43 GHz source Source/Frequency Phase Transfer: 43GHzslow 43 GHz fast Increase coherence AND astrometry 22 22 Summary Phase Referencing with HALCA -- Orbit errors 2-5 metres Astrometric results achieved New calibrations strategies for mm-VLBI -- Astrometric results achieved Calibrations strategies for VSOP-2 Astrometric results are achievable!
Pages to are hidden for
"Astrometry with VSOP"Please download to view full document