Data Access

by Stephan W. Witz last modified Aug 04, 2015 by Lorant Sjouwerman

The online archive contains all VLA data since observing started in 1976, and its contents are available via the Archive Access Tool (AAT) . This interface provides a basic data retrieval tool if you know the program code of your observations. It also provides an advanced query tool which enables searches based on a large number of user-specified criteria. Data can be downloaded via standard ftp protocols and via shipment of physical harddisks, subject to the conditions of our disk shipment policy. With the exception of some rapid response and large proposals, VLA data associated with a given proposal normally are restricted to proprietary use by the proposing team for a period of 12 months from the date of the last observation in a proposal (Note: data taken more than 12 months previously may still be proprietary, if additional data for the same proposal have been taken within the last 12 months). Proprietary data may be downloaded by any member of the observing team by logging in to the AAT with the user account or by making use of the project key (see The Observations and Remote Observing).

VLA data taken after January, 2010, when the transition to the WIDAR correlator took place, are stored in the Science Data Model (SDM) format that is used by both the VLA and ALMA. VLA data are available through the AAT in the following formats:

  • In the native Science Data Model (SDM) format.
  • As a CASA Measurement Set (MS) created from the SDM, which may be averaged in frequency and/or time, to be used with CASA.

Since the start of D configuration in 2013 the archive also contains pipeline calibrated MS.  Though eventually these data will be made accessible through the AAT as well, for the time being they have to be requested through our helpdesk

Some archival data have known problems which, together with the possible fix, are listed at the archive issues page.  Delivery of data in the UVFITS format has been discontinued in June 2013. Instructions to create UVFITS from the SDM or CASA format are given below.

Creating UVFITS or loading data into AIPS using the SDM format

Observers who have installed Obit can convert SDM data directly into UVFITS or load SDM data directly into AIPS.  Note that Obit cannot convert complex correlator modes into a single UVFITS/AIPS file, but plain continuum or simple line data is not a problem.

The latest version of Obit can be found by scrolling to the bottom of the Obit distribution page. Be aware that Obit is a bit harder to install on Macs than on Linux architectures.

To convert SDM data into UVFITS use these steps:

  1. download the SDM format from the archive
  2. using Obit run the task "ASDMList" to obtain a correlator configuration listing for selection
  3. using Obit run the task "BDFIn" to specify the output UVFITS file name

Alternatively use the AIPS path below and write out the file using the AIPS task "FITAB" (or FITTP)

To load SDM data into AIPS use these steps:

  1. download the SDM format from the archive
  2. using AIPS run the task "BDFLIST" to obtain a correlator configuration listing for selection
  3. using AIPS run the task "BDF2AIPS" to specify the output AIPS file name and disk number

Creating UVFITS formatted data using CASA

To produce UVFITS formated data using CASA take the following steps:

  1. download the data from the archive either in SDM or in MS format
  2. Start CASA
  3. Unless you downloaded MS data, convert the SDM formatted data to MS using the CASA task importvla
  4. Apply the requantizer gains as described in the section on 3-bit data observing in the VLA Observing Guide
  5. Convert this MS file to UVFITS format using the CASA task exportuvfits