Astrometry with VSOP by wulinqing

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									  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
RR,GEOA,TRO + ST nA A)
R 
       A,TRO - R * A,TRO = 0
       A,ANT - R * A,ANT = 0
             - R*       = (R-1/R) 
         …




                                                 slow   slow
RSTR + 2c (D . A,shift) + ST

             Frequency-referenced Visibility phase
     Basics of new method:
 SOURCE/FREQ. phase referencing

    R2c (D . B,shift) + ION   NST


Source/freq. referenced Visibility phase:

      A,STR + 2c  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!

								
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