Correlation of mm - VLBI Observations. Roger Cappallo ~ Mike Titus MIT Haystack Observatory

Transcription

1 Correlation of mm - VLBI Observations Roger Cappallo ~ Mike Titus MIT Haystack Observatory

2 Talk Flow Practical experiences at the correlator Preparation Hardware Correlation Post-processing Software Correlation & Future Plans

3 Recent mm-vlbi Experiments processed at Haystack April 2007 & 2009 SgrA* - CARMA/SMTO/JCMT at 230 GHz - size measurement (and by implication position!) of emitting region March 2008 SgrA* - VLBA at 86 GHz - parallax measurement

4 Pre-experiment recording checks sually do fringe test of all stations via evlbi (ftp downloads) Must gather and correct a priori information prior to attempting this! Results fed back to the stations and any needed fixes applied, repeat until ready

5 Experimentʼs Passage through the Correlator gather logs and schedules inspect incoming media (disks) - put into library compile logs and schedule in correlator (vex) format pick scans to find fringes; select and run pre-pass scans construct processing & fourfit control files schedule, then perform production processing analyze results/evaluate stations -- clean-up as necessary export results release disks

6 mm setups atypical Schedules often have incorrect a priori info, which needs to be discovered and corrected prior to processing: - source positions - station positions - frequency sequences - LO offsets - record mode (e.g. channel layout)

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8 Collect and insert station info in corr. control files: Need clock information; offset from GPS, drift rate and any jumps Gather station logs, summarize and check station comments -- used to edit schedule Note which IFs are which bands, configure as SB or LSB as appropriate

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10 Pre-pass and production phase pre-passes are a sampling of scans throughout schedule to make sure setup is correct and stable (e.g. clock model) production run -- after pre-passes check OK production can take up to 4X observe time, due to 1 Gb/s playback limit of Mk4 (2 IFs recorded at 2 Gb/s played back at ½ speed)

11 Mark 4 correlator characteristics 16 stations 16 channels / station 1 gb/s per station (actual playback rate) 2 gb/s record rate processed 1/2 speed fringe rate window width typically 4 Hz

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13 Demanding nature of mm VLBI high frequencies high fringe rates - e.g. 260 KHz on SMTO-JCMT baseline upper edge of rotator envelope - different phase increment mode - errors uncovered in digital generator of acceleration model

14 After correlation... Output needs to be examined even more carefully than usual: - Make sure clock/rate OK and LO stable - Was accumulate period adequate (residual fringe rates within window)? - Any scans missed or need re-correlation? - Are manual phase cals needed? Segment in the HOPS post-processing package over coherence time Possibly import into AIPS for further analysis

15 Wideband Postprocessing per pass limit: 16 ch x 32 MHz/ch = 512 MHz multiple passes glued together via fourmer post-processing software fourfit modified to do fringe fitting over 64 ch coherently combining separate IFʼs can be challenging...

16 Mk4 Fringe Plot uponwn, , JC JCMT - CARMA, fgroup, pol LL Fringe quality 9 Amp. and Phase vs. time for each freq., 14 segs, 4 APs / seg (2.00 sec / seg.), time ticks 2 sec SNR 12.0 PFD 3.5e-26 Intg.time Amp Phase 61.3 Sbdelay (us) Mbdelay (us) Fr. rate (Hz) Ref freq (MHz) AP (sec) Exp. Exper # Yr:day Start Stop FRT mm : Corr. date: 2009:112: Fourfit date: 2010:025: Position (J2000) 08h54m s " Validity L Parity Bias Level L L J (S 10) C (S 7) J C J C

17 Mk4 Fringe Plot uponwn, , JC JCMT - CARMA, fgroup, pol LL Fringe quality 9 Amp. and Phase vs. time for each freq., 14 segs, 4 APs / seg (2.00 sec / seg.), time ticks 2 sec SNR 12.4 PFD 2.3e-28 Intg.time Amp Phase Sbdelay (us) Mbdelay (us) Fr. rate (Hz) Ref freq (MHz) AP (sec) Exp. Exper # Yr:day Start Stop FRT mm : Corr. date: 2009:112: Fourfit date: 2010:025: Position (J2000) 08h54m s " Validity L Parity Bias L J (S 10) C (S 7) J C

18 Mk4 Fringe Plot uponwn, , JC JCMT - CARMA, fgroup, pol LL Fringe quality 9 Amp. and Phase vs. time for each freq., 7 segs, 9 APs / seg (4.50 sec / seg.), time ticks 5 sec SNR 15.2 PFD 4.2e-45 Intg.time Amp Phase Sbdelay (us) Mbdelay (us) Fr. rate (Hz) Ref freq (MHz) AP (sec) Exp. Exper # Yr:day Start Stop FRT mm : Corr. date: 2009:112: Fourfit date: 2010:025: Position (J2000) 08h54m s " Validity L Parity Bias L J (S 10) C (S 7) J C

19 Mk4 Fringe Plot uponwn, , JC JCMT - CARMA, fgroup, pol LL Fringe quality 7 Amp. and Phase vs. time for each freq., 13 segs, 49 APs / seg (24.50 sec / seg.), time ticks 30 sec SNR PFD e Intg.time Amp Phase Sbdelay (us) Mbdelay (us) Fr. rate (Hz) Ref freq (MHz) AP (sec) Exp. Exper # Yr:day Start Stop FRT mm : Corr. date: 2009:112: Fourfit date: 2010:025: Position (J2000) 08h54m s " Validity L Parity Bias Level L L J (S 10) C (S 7) J C J C

20 Mk4 Fringe Plot uponwn, , JC JCMT - CARMA, fgroup, pol LL Fringe quality 0 Amp. and Phase vs. time for each freq., 13 segs, 49 APs / seg (24.50 sec / seg.), time ticks 30 sec SNR PFD e Intg.time Amp Phase Sbdelay (us) Mbdelay (us) Fr. rate (Hz) Ref freq (MHz) AP (sec) Exp. Exper # Yr:day Start Stop FRT mm : Corr. date: 2009:112: Fourfit date: 2010:025: Position (J2000) 08h54m s " Validity L Parity Bias Level L J (S 10) C (S 7) J C J C

21 Future Plans Software correlation using DiFX - mm VLBI well-suited for software correlation, due to small N, linear run-time dependence on BW

22 Computers outpacing VLBI data rates Deller & Brisken 2009

23 Deller & Brisken 2009

24 Future Plans Software correlation using DiFX - mm VLBI well-suited for software correlation, due to small N, linear run-time dependence on BW Currently working on interface from DiFX into Mk4 / HOPS post-processing suite Considering a complete integration of DiFX into Mk4 processing system