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

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


Interferometer Observation

• With interferometers, we measure– Position & distribution of emission– Flux– Spectra


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.


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


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…


Observations of Calibrators


time

fluxPrimary Cal.

Gain Cal.

target

Channel/frequency

flux



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)


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)



setjy

bandpass

gaincal

fluxscale

applycal

Import data to CASA

Imaging with CASA

flagdata







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.


setjy


In the LOG window,

Flux of 1331+305: 14.76 Jy

Primary calibrator



setjy

bandpass

gaincal

fluxscale

applycal

Import data to CASA

Imaging with CASA

flagdata







bandpass

• Calibrate the frequency-dependent response• Bandpass calibrator should have a flat spectrum in

certain (desired) frequency range.• Bandpass calibration table is generated.


bandpass


Field 0=1331+305

Average the whole integration

All spectrum window



setjy

bandpass

gaincal

fluxscale

applycal

Import data to CASA

Imaging with CASA

flagdata







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.


gaincal


Field 0,1Exclude edge channels

Use bandpass results



setjy

bandpass

gaincal

fluxscale

applycal

Import data to CASA

Imaging with CASA

flagdata







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”).




setjy

bandpass

gaincal

fluxscale

applycal

Import data to CASA

Imaging with CASA

flagdata






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.


Use bandpass/gaincal results

Bandpass result


Gaincal result




setjy

bandpass

gaincal

fluxscale

applycal

Import data to CASA

Imaging with CASA

flagdata









setjy

bandpass

gaincal

fluxscale

applycal

Import data to CASA

Imaging with CASA

flagdata







If you want to know more..

• More detailed information about parameters is available in– CASA Help (> help taskname)– CASA cookbook (500pages..)


Documents

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