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Calibration with CASA Masaaki Hiramatsu 平松正顕 (ASIAA/NTHU) ALMA User Workshop, Feb 8-10, 2010 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 Interferometer Observation • With interferometers, we measure – Position & distribution of emission – Flux – Spectra ALMA User Workshop, Feb 8-10, 2010 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 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 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 Observations of Calibrators flux Primary Cal. Gain Cal. target time flux Channel/frequency ALMA User Workshop, Feb 8-10, 2010 Calibration with CASA Import data to CASA flagdata setjy bandpass gaincal Flag the bad data points Set the flux of the primary calibrator Calibrate frequency-dependent response of the system 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 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 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) Calibration with CASA Import data to CASA flagdata setjy bandpass gaincal Flag the bad data points Set the flux of the primary calibrator Calibrate frequency-dependent response of the system 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 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 setjy Primary calibrator In the LOG window, Flux of 1331+305: 14.76 Jy ALMA User Workshop, Feb 8-10, 2010 Calibration with CASA Import data to CASA flagdata setjy bandpass gaincal Flag the bad data points Set the flux of the primary calibrator Calibrate frequency-dependent response of the system 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 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 bandpass Field 0 =1331+305 All spectrum window Average the whole integration ALMA User Workshop, Feb 8-10, 2010 Calibration with CASA Import data to CASA flagdata setjy bandpass gaincal Flag the bad data points Set the flux of the primary calibrator Calibrate frequency-dependent response of the system 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 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 gaincal Field 0,1 Exclude edge channels Use bandpass results ALMA User Workshop, Feb 8-10, 2010 Calibration with CASA Import data to CASA flagdata setjy bandpass gaincal Flag the bad data points Set the flux of the primary calibrator Calibrate frequency-dependent response of the system 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 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 Calibration with CASA Import data to CASA flagdata setjy bandpass gaincal Flag the bad data points Set the flux of the primary calibrator Calibrate frequency-dependent response of the system 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 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 Bandpass result ALMA User Workshop, Feb 8-10, 2010 Gaincal result ALMA User Workshop, Feb 8-10, 2010 Calibration with CASA Import data to CASA flagdata setjy bandpass gaincal Flag the bad data points Set the flux of the primary calibrator Calibrate frequency-dependent response of the system 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 Calibration with CASA Import data to CASA flagdata setjy bandpass gaincal Flag the bad data points Set the flux of the primary calibrator Calibrate frequency-dependent response of the system 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 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