calibration with casa masaaki hiramatsu 平松正顕 (asiaa/nthu) alma user workshop, feb 8-10, 2010
TRANSCRIPT
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
ALMA User Workshop, Feb 8-10, 2010
time
fluxPrimary Cal.
Gain Cal.
target
Channel/frequency
flux
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
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
Sample observations
• You can get the information about the data withCASA> listobs()
Primary Cal.Gain Cal.Target
Primary Cal.(1331+305)
Gain Cal.(1445+099)
Target (NGC5921)
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
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
setjy
ALMA User Workshop, Feb 8-10, 2010
In the LOG window,
Flux of 1331+305: 14.76 Jy
Primary calibrator
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
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
ALMA User Workshop, Feb 8-10, 2010
Field 0=1331+305
Average the whole integration
All spectrum window
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
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
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
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
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
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
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
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