Description

task importvla description

Imports an arbitrary number of VLA archive-format data sets into a CASA MeasurementSet.

The task importvla reads in VLA data in archive format, as downloaded from the VLA data archive. It will handle archival VLA data in both old style (before July 2007) and new style (after July 2007). It can apply the system temperature (Tsys) to the data and to the weights. If more than one band is present, each band will be put in the same MeasurementSet but in a separate spectral window.

NOTE: importvla will import the on-line flags (from the VLA system) along with the data. Shadowed antennas will also be flagged. The flags will be put in the MAIN table and thus available to subsequent tasks and tools. If you wish to revert to unflagged data, use flagmanager to save the flags (if you wish), and then use flagdata with mode=’manualflag’ and unflag=True to toggle off the flags.

 

Parameter descriptions

archivefiles

The parameter archivefiles is used to specify the input VLA Archive format file names, as can be found in the NRAO Archive. Note that archivefiles takes a string or list of strings, as there are often multiple files for a project in the archive. If the data are located in a different directory on disk, then use the full path name to specify each archive file. The scaling of VLA data both before and after the June 2007 Modcomp-turnoff is fully supported, based on the value of applytsys.

vis

Name of output visibility file.

bandname

The importvla task allows selection on the frequency band. The bandname indicates the VLA Frequency band(s) to load, using the traditional bandname codes. These are:

  • ’4’ = 48-96 MHz
  • ’P’ = 298-345 MHz
  • ’L’ = 1.15-1.75 GHz
  • ’C’ = 4.2-5.1 GHz
  • ’X’ = 6.8-9.6 GHz
  • ’U’ = 13.5-16.3 GHz
  • ’K’ = 20.8-25.8 GHz
  • ’Q’ = 38-51 GHz
  • ’’ = all bands (default)

NOTE: After the transition from the VLA to JVLA, the actual frequency ranges covered by the bands have changed, and additional bands have been added (namely ’S’ from 1-2 GHz and ’Ka’ from 26.4-40 GHz). See the frequency ranges of the JVLA bands for details.

frequencytol

The frequencytol parameter specifies the frequency separation tolerated when assigning data to spectral windows. The default is frequencytol='150000.0Hz'. For Doppler tracked data, where the sky frequency changes with time, a frequencytol < 10000 Hz may produce too many unnecessary spectral windows.

project

You can specify a specific project name to import from archive files. The default ’’ will import data from all projects in file(s) archivefiles. For example for VLA Project AL519:

  • project = 'AL519'    # this will work
  • project = 'al519'    # this will also work

NOTE: project=’AL0519’ will NOT work (even though that is what queries to the VLA Archive will print it as).

starttime and stoptime

You can specify start and stop times for the data, e.g., starttime='1970/1/31/00:00:00' and stoptime='2199/1/31/23:59:59'. The blank defaults will load all data fitting other criteria.

applytsys

The applytsys parameter controls whether the nominal sensitivity scaling (based on the measured TSYS, with the weights scaled accordingly using the integration time) is applied to the visibility amplitudes or not. If True, then it will be scaled so as to be the same as AIPS FILLM (i.e., approximately in deciJanskys). Note that post-Modcomp data is in raw correlation coefficient and will be scaled using the TSYS values, while Modcomp-era data had this applied online. In all cases, importvla will do the correct thing to data and weights based on an internal flag in the VLA Archive file, either scaling it or unscaling based on your choice for applytsys.

NOTE: If applytsys=True and you see strange behavior in data amplitudes, it may be due to erroneous TSYS values from the online system. You might want to then fill with applytsys=False and look at the correlation coefficients to see if the behavior is as expected.

autocorr

Autocorrelations are written to the MeasurementSet if autocorr=True. Generally for the VLA, autocorrelation data is not useful, and furthermore the imaging routine will try to image the autocorrelation data (it assumes it is single dish data) which will swamp any real signal. Thus, if you do fill the autocorrelations, you will have to flag them before imaging.

antnamescheme

The antnamescheme parameter controls whether importvla will try to use a naming scheme where JVLA antennas are prefixed with EA (e.g., ’EA16’) and old VLA antennas have names prefixed with VA (e.g., ’VA11’).

keepblanks

Turns on or off whether importvla fills the scans with blank (empty) source names (e.g., tipping scans).

evlabands

The evlabands=True option is provided to allow users to access JVLA frequencies outside the standard VLA tunings (e.g., the extended C-band above 6 GHz).

WARNING: Use of this option for standard VLA data will cause unexpected associations, such as X-band data below 8 GHz being extracted to C-band (as the JVLA C-band is 4–8 GHz). Use with care.