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Calibration with CASA

Calibration with CASA. Masaaki Hiramatsu 平松正顕 (ASIAA/NTHU). Outline. Calibration: Why and How? Visibility and Gain Calibrators Step-by-step instructions With a sample data taken with VLA. Interferometer Observation. With interferometers, we measure Position & distribution of emission

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Calibration with CASA

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  1. Calibration with CASA Masaaki Hiramatsu 平松正顕(ASIAA/NTHU) ALMA User Workshop, Feb 8-10, 2010

  2. Outline • Calibration: Why and How? • Visibility and Gain • Calibrators • Step-by-step instructions • With a sample data taken with VLA ALMA User Workshop, Feb 8-10, 2010

  3. Interferometer Observation • With interferometers, we measure • Position & distribution of emission • Flux • Spectra ALMA User Workshop, Feb 8-10, 2010

  4. Why do we need calibration? • Visibility: • In ideal case, • But, in reality,Need another term “G” (gain) to compensate the atmospheric/instrumental effects. • “Calibration” is a process to derive this “G” and recover the true visibility. ALMA User Workshop, Feb 8-10, 2010

  5. Why do we need calibration? • Atmosphere is not perfectly transparent. • Absorption (mostly by H2O/O2 ) • Time variation of water vapor: phase variation • Instruments are not always perfect. • Telescope pointing error: time-dependent amplitude variation • System does not have a flat frequency response. frequency-dependent response ALMA User Workshop, Feb 8-10, 2010

  6. Observations of Calibrators • Primary calibrator: • Bright point source(or known structure) with known flux • Planets, satellites, quasars (in cm-wave)… • Bandpass calibrator: • Bright source with flat spectrum in desired frequency range • Quasar, planets, satellites, etc… • Gain calibrator: • Bright point sourcenear your target source • Flux should not change during your observations. • Quasar, etc… ALMA User Workshop, Feb 8-10, 2010

  7. Observations of Calibrators flux Primary Cal. Gain Cal. target time flux Channel/frequency ALMA User Workshop, Feb 8-10, 2010

  8. Calibration with CASA Import data to CASA flagdata Flag the bad data points setjy Set the flux of the primary calibrator bandpass Calibrate frequency-dependent response of the system gaincal Calibrate the time-dependent antenna gain fluxscale Calibrate the flux of gain cal. based on primary cal. applycal Apply the calibration to the (target) data Imaging with CASA ALMA User Workshop, Feb 8-10, 2010

  9. Sample data • A galaxy NGC5921 observed with VLA • HI line (1.4 GHz) • 1 spectrum window (24.1 kHz resolution, 63channels) • Primary/Bandpass calibrator • J1331+305 (quasar) • Gain calibrator • J1445+099 (quasar) Adam Block/NOAO/AURA/NSF ALMA User Workshop, Feb 8-10, 2010

  10. Sample observations • You can get the information about the data withCASA> listobs() Primary Cal. Gain Cal. Target Gain Cal.(1445+099) Primary Cal. (1331+305) Target (NGC5921)

  11. Calibration with CASA Import data to CASA flagdata Flag the bad data points setjy Set the flux of the primary calibrator bandpass Calibrate frequency-dependent response of the system gaincal Calibrate the time-dependent antenna gain fluxscale Calibrate the flux of gain cal. based on primary cal. applycal Apply the calibration to the (target) data Imaging with CASA ALMA User Workshop, Feb 8-10, 2010

  12. setjy • Set the flux of the primary flux calibrator(s) • When the calibrator is a point source, input the flux value, if known. • CASA has a list of the flux of some calibrators. CASA automatically setsthe flux for these sources. • If the calibrators are not listed butthe flux is known, then input the flux value. • When the calibrator is a resolved source, input a source model. ALMA User Workshop, Feb 8-10, 2010

  13. setjy Primary calibrator In the LOG window, Flux of 1331+305: 14.76 Jy ALMA User Workshop, Feb 8-10, 2010

  14. Calibration with CASA Import data to CASA flagdata Flag the bad data points setjy Set the flux of the primary calibrator bandpass Calibrate frequency-dependent response of the system gaincal Calibrate the time-dependent antenna gain fluxscale Calibrate the flux of gain cal. based on primary cal. applycal Apply the calibration to the (target) data Imaging with CASA ALMA User Workshop, Feb 8-10, 2010

  15. bandpass • Calibrate the frequency-dependent response • Bandpass calibrator should have a flat spectrum in certain (desired) frequency range. • Bandpass calibration table is generated. ALMA User Workshop, Feb 8-10, 2010

  16. bandpass Field 0=1331+305 All spectrum window Average the whole integration ALMA User Workshop, Feb 8-10, 2010

  17. Calibration with CASA Import data to CASA flagdata Flag the bad data points setjy Set the flux of the primary calibrator bandpass Calibrate frequency-dependent response of the system gaincal Calibrate the time-dependent antenna gain fluxscale Calibrate the flux of gain cal. based on primary cal. applycal Apply the calibration to the (target) data Imaging with CASA ALMA User Workshop, Feb 8-10, 2010

  18. gaincal • Calibrate the time-dependent amplitude/phase • Gain calibrator should be observed periodically (depends on the weather & frequency) during the observation run. • Gain calibration table is generated. ALMA User Workshop, Feb 8-10, 2010

  19. gaincal Field 0,1 Exclude edge channels Use bandpass results ALMA User Workshop, Feb 8-10, 2010

  20. Calibration with CASA Import data to CASA flagdata Flag the bad data points setjy Set the flux of the primary calibrator bandpass Calibrate frequency-dependent response of the system gaincal Calibrate the time-dependent antenna gain fluxscale Calibrate the flux of gain cal. based on primary cal. applycal Apply the calibration to the (target) data Imaging with CASA ALMA User Workshop, Feb 8-10, 2010

  21. fluxscale • Since the gain calibrator has unknown flux, CASA assumed the flux is 1Jy. • “fluxscale” rescales the gain solutions and derives flux density of gain calibrator, based on the flux of the primary calibrator (remember “setjy”). ALMA User Workshop, Feb 8-10, 2010

  22. Calibration with CASA Import data to CASA flagdata Flag the bad data points setjy Set the flux of the primary calibrator bandpass Calibrate frequency-dependent response of the system gaincal Calibrate the time-dependent antenna gain fluxscale Calibrate the flux of gain cal. based on primary cal. applycal Apply the calibration results to the target Imaging with CASA ALMA User Workshop, Feb 8-10, 2010

  23. applycal • Apply the calibration (gain/bandpass) tables to data. • Transfer between fields, interpolation • CORRECTED_DATA column will be created. The original DATA column is untouched. Use bandpass/gaincal results ALMA User Workshop, Feb 8-10, 2010

  24. Bandpassresult ALMA User Workshop, Feb 8-10, 2010

  25. Gaincal result ALMA User Workshop, Feb 8-10, 2010

  26. Calibration with CASA Import data to CASA flagdata Flag the bad data points setjy Set the flux of the primary calibrator bandpass Calibrate frequency-dependent response of the system gaincal Calibrate the time-dependent antenna gain fluxscale Calibrate the flux of gain cal. based on primary cal. applycal Apply the calibration to the (target) data Imaging with CASA ALMA User Workshop, Feb 8-10, 2010

  27. Calibration with CASA Import data to CASA flagdata Flag the bad data points setjy Set the flux of the primary calibrator bandpass Calibrate frequency-dependent response of the system gaincal Calibrate the time-dependent antenna gain fluxscale Calibrate the flux of gain cal. based on primary cal. applycal Apply the calibration to the (target) data Imaging with CASA ALMA User Workshop, Feb 8-10, 2010

  28. If you want to know more.. • More detailed information about parameters is available in • CASA Help (> help taskname) • CASA cookbook (500pages..) ALMA User Workshop, Feb 8-10, 2010

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