VLBA AIPS ans CASA Guide

The Guide walks through the calibration of the data set used in the [1][VLBA Basic Phase-referencing Calibration and Imaging] tutorial. It presents the steps that should be taken to calibrate the data using both AIPS and CASA. In several cases the AIPS VLBAUTIL procedures are shown, as well as the AIPS tasks that those procedures call.

Note that some steps are performed in different order depending on whether you are using AIPS or CASA. Also, be aware that not all tasks in AIPS have an equivalent task in CASA (yet).

To follow along with this Guide, users should download the TL016B.idifits data set used in the VLBA Basic Phase-referencing Calibration and Imaging tutorial.

Initial Calibration

This first section shows the steps for the initial calibration (instrumental delay, fringe fitting, bandpass correction).

Loading Data

Both AIPS and CASA need to take the FITS-IDI file and convert it into a format they can use. AIPS uses the UVDATA format, and CASA uses the Measurement Set (MS) format.

VLBAUTIL Procedure	AIPS Task(s)	CASA Task(s)
VLBALOAD datain 'TL016B.idifits outname 'TL016B'	FITLD datain 'TL016B.idifits outname 'TL016B' clint 0.25	importfitsidi fitsidifile='TL016B.idifits' vis='tl018b.ms' constobsiid=True scanreindexgap_s=15

Observation Summary

Printing a summary of the observation is often useful for identifying the fringe finder, bandpass calibrator, phase reference calibrator, and science target. It is also useful for locating a good time range to use for the instrumental delay correction.

VLBAUTIL Procedure	AIPS Task(s)	CASA Task(s)
VLBASUMM inname 'TL016B' inclass 'UVDATA' inseq 1 docrt 132	PRTAN inname 'TL016B' inclass 'UVDATA' inseq 1 docrt 132 dotv 1	listobs vis='tl016b.ms'
	LISTR inname 'TL016B' inclass 'UVDATA' inseq 1 optype 'scan'

Total Electron Content Correction

VLBAUTIL Procedure	AIPS Task(s)	CASA Task(s)
VLBATECR	TECOR	no equivalent CASA task yet

NOTE: Do not use the tec_maps tool with VLBA data. It does not work properly for VLBI observations.

Earth Orientation Parameter Correction

VLBAUTIL Procedure	AIPS Task(s)	CASA Task(s)
VLBAEOPS	CLCOR opcode 'eops'	no equivalent CASA task yet

Sampler Corrections

VLBAUTIL Procedure	AIPS Task(s)	CASA Task(s)
VLBACCOR	ACCOR	accor
	SNSMO	smoothcal

Instrumental Delay Correction

NOTE: Many VLBA users are likely familiar with the VLBAUTIL procedure VLBAPCOR and the associated AIPS task PCCOR, which use the pulse-cal tone information from the VLBA to correct for the instrumental delay. CASA does not currently have a means of working with the pulse-cal tones, so it cannot use this functionality. It is an area of active development for the NRAO and JIVE CASA software engineers and scientists.

VLBAUTIL Procedure	AIPS Task(s)	CASA Task(s)
VLBAMPCL	FRING	fringefit
	CLCAL	applycal

Global Fringe Fitting

The VLBAUTIL package contains 2 methods for doing the global fringe fit:

• VLBAFRNG does a fringe fit on every source in the observation
• VLBAFRGP does a fringe fit on the phase-reference calibrator(s) and transfers those solutions to the science target(s)

VLBAUTIL Procedure	AIPS Task(s)	CASA Task(s)
VLBAFRNG or VLBAFRGP	FRING	fringefit
	CLCAL	applycal

Bandpass Correction

VLBAUTIL Procedure	AIPS Task(s)	CASA Task(s)
VLBABPSS	BPASS	bandpass

Final Amplitude Calibration

VLBAUTIL Procedure	AIPS Task(s)	CASA Task(s)
VLBAAMP	ACSCL	accor
	SNSMO	smoothcal
	CLCAL	applycal
	APCAL	gencal
	CLCAL	applycal

Parallactic Angle Correction

VLBAUTIL Procedure	AIPS Task(s)	CASA Task(s)
VLBAPANG	CLCOR opcode 'pang'	set parang=True in applycal, fringefit, and bandpass

Additional Tasks

AIPS Task	CASA Task
SPLIT	split
IMAGR	tclean
CALIB	gaincal
POSSM	plotms
SNPLT	plotms
UVPLT	plotms
UVFLG	flagadata
FITTP	exportuvfits or exportfits