AIPS Info

Welcome to the GMRT data reduction help page of NCRA!

Astronomical Image Processing System, developed by the National Radio Astronomy Observatory(NRAO), USA, is one of the various software packages available for reduction of radio interferometric data like that of the GMRT.  AIPS is the standard data reduction package used at NCRA. Here you'll find a brief overview of  AIPS setup at GMRT and NCRA, and also some help on getting started with it along with remedies to some of the commonly faced problems and recent issues specific to the GMRT data format.  Its curve-of-growth to a large extent depends on your comments which may be e-mailed to  aips@ncra.tifr.res.in.  Before getting into AIPS please make sure that you have used the latest version of listscan and gvfits to generate the GMRT data FITS files.  The latest versions of these and various other utilities are available on GMRT and NCRA machines at /astro/bin/Linux and can be sourced using 'source /astro/RC' or 'source /astro/RC.csh'.  For the last few years, we have been following the practice of installing the latest frozen version AIPS on the general users' and visitors' machines at GMRT and NCRA.

 

Starting AIPS

Before starting AIPS it is required to define AIPS' data areas and also the environment variable pointing to the data area where user wants to read/write into/from AIPS.  These can be done as following:

  • Defining AIPS' data areas

This can be done by running this shell script and choosing the data areas interactively.  This will create a file .dadevs in the home area of user say 'visitor' .  Or else one may simply create the .dadevs contaning lines similar to

- /analysis/visitor/ASTRO5_1/
-  /analysis/visitor/ASTRO5_2/
-  /analysis/visitor/ASTRO5_3/

This will create three AIPS' disks ASTRO5_1, ASTRO5_2 and ASTRO5_3 in the area /analysis/visitor/ on the machine astro5.  

However, if you have predefined AIPS data areas during installation, the above step can be bypassed. For example, if you have installed AIPS in /home/aips/ the data areas are defined in the file /home/aips/DA00/DADEVS.LIST

 

  • Now start AIPS as following:

  • For bash:

    >source /usr/aips/LOGIN.SH
    >export MYDIR=/home/visitor/
    >aips

 

  • For csh:

    >source /usr/aips/LOGIN.CSH
    >setenv MYDIR/home/visitor/
    >aips

If the data areas have been created using the script (which is true for all the gtac and visitor accounts at GMRT) then one does not need to source LOGIN.SH or LOGIN.CSH as appropriate line has been added to .bashrc or .cshrc, respectively.

 

Getting started with AIPS

The description of various AIPS tasks and detailed instructions can be found in the AIPS COOKBOOK.  Users unfamiliar with the AIPS environment may find it useful to first go through the easy guide to GMRT data analysis.

 

GVFITS and LISTSCAN

  • GVFITS and LISTSCAN are the utilities used to convert lta and ltb data files produced by GMRT into FITS format. Development of these, apart from fixing minor bugs, has largely been driven by efforts to incorporate new features/observing modes at GMRT and also to keep the FITS files compatible with any changes in AIPS. It is therefore important to use the latest versions of these. Here is some information on the recent gvfits' releases. There is no version called 1.15. The versions earlier than 1.17 are no longer supported, but are listed here for historical completeness only. The latest version, however, encapsulates all the previous changes.

gvfits 1.17 and listscan-1.17 : Merges the GSB and old correlator versions into one. This version of gvfits can therefore convert lta/ltb files from both GSB and the hardware correlator. The corresponding listscan is available too at astro0:/astro/Linux/bin at GMRT.

gvfits 1.16 and listscan-1.16 : Updated for leap-second correction on 31 Dec 2008. Internal header is now consistent with changes made in DAS-2.05 to improve fringe tracking.

gvfits 1.14 and listscan-1.14 : Corrected bugs in the determination of the start date for the FLAG table. Also added option for honouring flags embeded in the lta data.

gvfits 1.13 and listscan-1.13 : Fixes a bug in which the uvw co-ordinates were not computed at the centre of the integration. Also added some more information in the history file.

gvfits 1.12 : Fixes a bug which caused 31DECO5 version of AIPS to get confused when the frequency decreased with channel no.

gvfits 1.11 : Allows user to label the polarisations as XX/YY or RR/LL (By default they are labelled RR/LL). Also generates the correct FQ table when more than one IF is present.

gvfits 1.10 : Modifications to support the polar and other new modes of the GMRT correlator.

 

gvfits and listscan are special utilities that are specific to the LTA data format of the GMRT. It is recommended that GTAC users external to NCRA convert their LTA/LTB files into FITS files before leaving the observatory or NCRA, using these utilities.

 

If you are logged in to one of the GTAC machines at GMRT, follow these steps in sequence

  • After logging into one of gtac1/2/3/4 or astro1/2/3/4, at the prompt run source /astro/RC or source /astro/RC.csh depending on whether you are working with bash or tcsh shell respectively.

  • Type listscan and gvfits to see their usage; by default these are the latest versions.

  • Run listscan on your lta/ltb files to generate log and plan files.

  • Edit the log files appropriately.

  • Run gvfits on the edited log files to obtain FITS files.

  • If you want combined sideband data, then combine the USB and LSB data using the utility ltamerge before running listscan and gvfits.

 

 

NEW AIPS TASKS

This section intends to list the tasks that are not part of the official AIPS package but can be installed and used as normal AIPS tasks.

UVFXT : Task to fix timestamps on GMRT uv-data files within AIPS.

UVFLP : Task to to re-order channels in GMRT LSB uv-data files within AIPS.

Both these tasks have been written by Dave Green of Mullard Radio Astronomy Observatory, Cavendish Laboratory, Cambridge. Task UVFXT is useful if you have GMRT data FITS files generated using listscan and gvfits version 1.12 or earlier and want to correct for the error that uvw co-ordinates were not computed at the centre of the integration. UVFLP (along with AIPS tasks BLOAT and DBCON) can be used to combine lta and ltb (i.e. the two IFs) data inside AIPS. Although the program `ltamerge' can be used to do the same with lta and ltb files it may be more convenient to merge data right at the end, i.e. just before imaging. This does make sense since the data editing as well as calibration for the two IFs are anyway done independently. Detailed help and guidelines to install and use these new tasks by Dave Green are available at the above link. However, for the convenience of users at GMRT we have made available both these tasks on all the general user machines.

 

LTA Utilities

Right click on any of the links and "Save Target as" to download the LTA utilities tarball. These are binaries should work on bash/tcsh. The LTA utilities are now called "lute" - it has now reached lute-1.17. This version merges the source codes of the GSB and hardware correlator versions into one common utility for data originating from either. The changes and updates to lute follow those to gvfits. Identically numbered versions of lute and gvfits are tied to each other. All the lute programs are available on astro0:/astro/Linux/bin at GMRT. The latest version of the most important LTA utilities are:

ltahdr : Utility to list the scans in the LTA/LTB file

ltaedit : Utility to edit parameters in the LTA/LTB file

ltamerge : Utility to merge the LTA and LTB files. LTA and LTB are two different IFs in default observing modes, and correspond to two polarizations in the HIGHRES mode.

ltacomb : Utility to combine/concatenate LTA files. LTA1:LTA2:LTA3:.... or LTB1:LTB2:LTB3...
Files can be combined in time order also after merging LTA/LTB files.

 

FAQ

How do I run aips on machine astro1 from astro2?
This can be done as following:

>ssh -Y astro1 (from astro2)
>source /usr/aips/LOGIN.SH
>export DATA=/temp1/visitor/
>aips tv=astro2:astro1

 

How do I use the tasks UVFLP and UVFXT at GMRT machines?

For this type `setenv MYAIPS /usr/aips/MORETASKS/' for cshell or `export MYAIPS=/usr/aips/MORETASKS/' for bash before starting AIPS. Then inside AIPS say: version 'MYAIPS'. Saying version 'CDTST' inside AIPS will make these unavailable i.e. restore to default AIPS as obtained from NRAO site.

 

Last revised: 28 Mar 2014
aips@ncra.tifr.res.in

 

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