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 H 2 O/O 2 ) – 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 source near your target source Flux should not change during your observations. Quasar, etc… ALMA User Workshop, Feb 8-10, 2010

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

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

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) ALMA User Workshop, Feb 8-10, 2010 Adam Block/NOAO/AURA/NSF

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

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

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 sets the flux for these sources. – If the calibrators are not listed but the 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 ALMA User Workshop, Feb 8-10, 2010 In the LOG window, Flux of 1331+305: 14.76 Jy Primary calibrator

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

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

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

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

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

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

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. ALMA User Workshop, Feb 8-10, 2010 Use bandpass/gaincal results

24. Bandpass result ALMA User Workshop, Feb 8-10, 2010

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

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

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

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