4.1 Copying data into  multi-source disk files

There are several ways to write VLA data to  multi-source uv data sets on disk. They include:

1. For data from the historic VLA, download the files from the archive and run FILLM to load the data. See §O.1 for the details.
2. For data from the new EVLA, download the files from the archive and run BDF2AIPS to load the data. See §4.1.1 below for the details.
3. For an  multi-source data set written to a FITS tape or disk file during an earlier  session, use UVLOD or FITLD to read the tape.
4. Single-source data sets that are already on disk may be combined with multiple executions of UV2MS or, if the files are dissimilar, with multiple executions of MULTI followed by DBCON.
5. Data from the Australia Telescope may be loaded from disk files into  using the task ATLOD which is now included with .

Data from other telescopes can be read into  only if they are written in -like FITS files already or if you have a special format-translation program for that telescope. The VLBA correlator produces a format which is translated by the standard  task FITLD; see §4.1.2. A translation task for the Westerbork Synthesis Telescope (WSLOD) is available from the Dutch, but is not distributed by the NRAO with the normal  system.

4.1.1 Reading EVLA archive files into

The NRAO Archive makes available, among other things, data from the EVLA, which began observing in January 2010. Your EVLA data are stored as an “Science Data Model” (SDM) format file in “SDMBDF” (Science Data Model Binary Data Format) in the NRAO archive. They may be read out of the archive in that format or a CASA measurement set format. Go to the web page

You may now run AIPS and load the data via the verbs BDFLIST and BDF2AIPS. These verbs run programs in the OBIT software package to load your data directly into  including flag (FG), index (NX), calibration (CL), over-the-top (OT), SysPower (SY), CalDevice (CD) and Planet (PO) tables which you will not get from CASA. Note that these verbs require that OBIT be installed on your computer — as it is in Socorro — and that ObitTalk be in your $PATH. OBIT is relatively easy to install and may be obtained from www.cv.nrao.edu/cotton/Obit.html

SDMBDF files may be read into  using BDFLIST to learn what is in your data set and then BDF2AIPS to translate the data. Thus

There are other adverbs — NCHAN, NIF, BAND, CALCODE, and DOSTOKES — available if needed to limit which data are read. CONFIG is frequently all that is needed to select data, but these others may be needed if more complicated modes of observing were used. If you running Python 3 rather than 2, set OBITVERS to 3.

If BDF2AIPS is executing correctly, your message terminal will report information about the data being loaded including the data selection, the data scans loaded, and information about some of the tables written. Once BDF2AIPS has completed, you can find the database on disk using:

This should produce a listing such as:

You might then examine the header information for the disk data set by:

This should produce a listing like:

This header identifies the file as a multi-source data set (Image=MULTI) with 876000 floating-point visibilities in time-baseline (TB) order. There are 128 spectral channels in each of 31 entries on the IF axis. These correspond to “spectral windows” in the correlator and have, at best, only a loose correspondence to the separate electronic channels. The description of the frequency (FREQ) axis shows that the first IF is at 4039 MHz and has 128 MHz total bandwidth. The parameters of the other IFs are determined from the data in the FQ table file and cannot be read directly from this header; these values are shown in the ’SCAN’ listing from LISTR. In 31DEC23, task LISUN may be used as an alternative to LISTR SCAN operation. It shows the angular distance to the Sun of each scan in addition to all of the standard LISTR output. The header shown above indicates that the data are in compressed format since the number of pixels on the COMPLEX axis is 1 and the WEIGHT and SCALE random parameters are present. Uncompressed data does not use these random parameters and has 3 pixels on the COMPLEX axis.

Complex VLA observations may have the spectral windows in a strange frequency order. BDF2AIPS with ORDER=1 can rearrange them. If the groups of windows overlap, it may be best to rearrange them after calibration (particularly FRING which can use groups of adjacent windows) using the new task FIXFR.

If your experiment contains data from several bands BDF2AIPS will place the data from each band in separate data sets. Also, if you observed with several sets of frequencies or bandwidths in a given observing run these will be assigned different FQ numbers by BDF2AIPS. You can determine which frequencies correspond to which FQ numbers from the ’SCAN’ listing provided by LISTR or LISUN. If you find multiple FQ numbers in your data set, we strongly advise you to run UVCOP to separate them into different files. This will greatly simplify your data reduction.

4.1.2 Reading data from FITS files with FITLD

FITLD is used to read FITS-format disk files (and tapes) into . It recognizes images, single- and multi-source uv data sets, and the special FITS uv-data tables produced by the VLBA and DiFX correlators (“FITS-IDI” format). In particular, VLA data sets that have been read into  previously with FILLM and then saved to tape (or pseudo-tape disk) files with FITTP and FITAB can be recovered for further processing with task FITLD. (The older task UVLOD will also work with uv data sets in FITS format, but it cannot handle image or FITS-IDI format files.)

A multi-source data file with all of its tables can be read from a FITS FILE by:

The data-selection adverbs SOURCES, QUAL, CALCODE, and TIMERANG and the table-control adverbs CLINT and FQTOL are used for VLBA-format data only.. See Chapter 9 for more specific information.