Analysis of WSRT data

 Analysis of WSRT data

 

WSRT data can be calibrated and analyzed using different data reduction packages. Depending on the type of experiment/data, certain packages can be preferred. The most commonly used packages are listed here with a few lines of comments on the pros and cons and possible problems encountered in each of the packages with regard to WSRT data.

 

Getting the data

This is of course the first step in any data analysis. WSRT data are archived in our online repository and generally only distributed through the internet. Users are provided with DVD ROMs only in exceptional cases. There are basically two ways to retrieve WSRT data. Users who have been involved in a WSRT observing project since 2004 are requested to use our Management of Measurements (MoM) system for downloading data:

i)
use a browser to go to http://wsrt.astron.nl/mom3.
ii) log in using the same username and password as for the Northstar proposal tool.
iii)
go to your project folders by clicking on the project name.
iv)
click on "Add" either next to a specific observation or next to the parent project (on the top of the list). A window pops up.
v)
in the window, for "type" choose "Export" and, if you like, give the export job a name and a description.
vi)
in the window, click "OK".
vii)
in the window, choose the data format you want.
viii)
in the window, click "OK". In the project window, the download job appears in the project list.
ix)
click on "Execute" in the project list. A window pops up.
x)
in the window, choose the status of projects that you want to have data from. In most cases, the default "successful" is a good choice, but "finished" is another option you might want to check. If you have downloaded the data before, check "Include previously exported data".
xi)
select the observations you want to retrieve from the list.
xii)
click on "Ok". The project status will be set to "scheduled", then to "running". Eventually the export is finished and you will be sent an email with instructions how to download the data.

 

Users who want to search the WSRT archive and who want to retrieve data from the WSRT archive follow the instructions on this web page.

Spectral line experiments

Spectral line observations can be reduced with Miriad or classic Aips. Cookbooks for reduction of WSRT line data are available for Miriad and Aips (see below). Note that Newstar is not well suited to calibrate WSRT line data. For large mosaic observations, Miriad is recommended. The Karma visualization software works very well in combination with Miriad and is a helpful tool for line observations.

Note that the standard practice of calibrating a WSRT line observation is different from that used at the VLA or the ATCA. During a WSRT observation, no secondary calibrator is observed. Instead, self-calibration on the line-free channels is used to track the changes in the gains during the observations.

Continuum observations

If one is not interested in polarization, calibration and reduction of WSRT continuum observations can be done in both Miriad and Aips. Note that WSRT continuum observations are basically line observations, so the calibration is not different from spectral line observations. Miriad has a few programs that can take the broad observing band of the WSRT into account, while also mosaic observations of large continuum fields can be calibrated and reduced very well in Miriad.

If, however, one wants to use the full polarization information, the recommended packages are classic Aips or CASA. In that notion, it is important to know the specific definition of the WSRT reference system for polarzation measurements. NEWSTAR is a software package optimised for the analysis of WSRT data, and as such perfectly suited for analyses of full polarisation data.

Polarization measurements

For WSRT, the polarization reference system is defined by the X-direction going from East to West, and the Y-direction going from South to North (for historical reasons; this is not conforming to the IAU convention, which would define X going from South to North, Y going from West to East). Hence, the measured polarisation products for linear dipoles (all receivers except for the 13cm receiver) are

XX = I - Q      XY = -U + iV      YX = -U - iV      YY = I + Q    

with the inverse operations

I = (XX + YY)      Q = (YY - XX)      U = -(XY + YX)      iV = (XY - YX)     .

For the 13cm band the polarisation products are given by

RR = I - V      RL = Q + iU      LR = Q - iU      LL = I + V

with the inverses

I = (RR + LL)      Q = (RL + LR)      iU = (RL - LR)      V = (LL - RR)       .

In these relations, the following definitions are used:

XX - Complex interferometer response between both X receivers
XY - Complex interferometer response between X and Y receivers
YX - Complex interferometer response between Y and X receivers
YY - Complex interferometer response between both Y receivers

RR - Complex interferometer response between both Right circular feeds.
RL - Complex interferometer response between Right and Left circular feeds.
LR - Complex interferometer response between Left and Right circular feeds.
LL - Complex interferometer response between both Left circular feeds.

I - Complex value describing Stokes I
Q - Complex value describing Stokes Q
U - Complex value describing Stokes U
V - Complex value describing Stokes V
i - SQRT(-1)

 

Classic AIPS
  A CookBook for WSRT data reduction using classic
(cookbook written by R. Braun).
MIRIAD
 

Miriad is a radio interferometry data reduction package that can be used for the reduction of continuum and line experiments. Miriad has been specially designed to support a number of applications (specially polarization and mosaicing) of interest for the users in particular of the ATCA (Australia Telescope Compact Array). Reduction of WSRT data with Miriad is possible and straightforward for non-polarisation experiments (i.e. if no cross-polarisation data are analysed). Polarisation experiments can be reduced with Miriad only at a later stage, after an initial calibration has been done using other software packages and the prepared data have been imported to Miriad. Dual polarisation data in UVFITS format (xx, yy, rr, ll) can be easily imported.

If you intend to use Miriad for reducing WSRT data please have a look at the notes Miriad and the WSRT or contact oosterloo [at] astron [dot] nl (Tom Oosterloo ).

Find more information about Miriad and the Visualization Software based on the Karma Library.

NEWSTAR
 

NEWSTAR is a package specifically developed to reduce WSRT data (whether archival data from the 1970'ies or the latest AIPS++ Measurement Sets). Its functionality allows one to exploit the strong points of an east-west array like the WSRT: wide field imaging (including mosaicing), high dynamic range, polarimetry and multi-frequency synthesis.

Unfortunately the documentation about Newstar is sparse and often out-of-date. For a general introduction about all programs, please contact ger [at] astron [dot] nl (Ger de Bruyn).

Design: Kuenst.    Development: Dripl.    © 2014 ASTRON