Download Calibration with CASA - ASIAA

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