AstroGrid Data Access Worksheet

This is a step by step worksheet that demonstrates some ways to retrieve and visualise astronomical images, spectra and catalogue data using AstroGrid.

To access AstroGrid services, launch the Workbench (NB: the Workbench is launched using Java webstart, therefore you will need the Java Runtime Environment installed on your computer).

Answer 'Yes' or 'Start' at security warnings.

AstroScope

Acquiring data

AstroScope is an AstroGrid tool that allows you to query archives of images, spectra and catalogue data around a given position or SIMBAD object, visualise the results and download data files either to your local machine or to your storage space on the Astrogrid system, called MySpace.

In the Workbench main panel, click on AstroScope. Full instructions are given by AstroScope Help. For example, enter the symbiotic star HM Sge as the Object and a Region (search radius, decimal degrees) of 0.1. Select some data by clicking to open up an entry and then double-clicking - e.g. a couple of HST images, a radio catalogue and one each IUE and ISO spectra Click on the Save icon, and log in to MySpace. Then select or create a suitable subdirectory (see MySpace Help for details) and click OK.

AstroScopeSave.png

You can check when the data have been downloaded by launching the MySpace browser from the workbench.

Visualising data

You can download data to your desktop or copy URLs to any suitable viewer. AstroGrid also provides direct links to data visualisation and manipulation tools. At present (April 2006) these can be accessed directly via the workbench but the instructions below refer in some cases to recent versions which have not yet been fully integrated. You need to run the Workbench and the relevant application(s) at the same time for full interoperability in visualisation.

Images

A prototype version of Aladin can see MySpace directly. Use the Aladin V3.6 link and download or webstart the Prototype. Click on PLASTIC in the Aladin window and Register with hub (this takes a few seconds). The Aladin logo will then appear in AstroScope. If you click on the icon any selected images will be automatically loaded. You can also use the Aladin LOAD window to get data already in MySpace.

Aladin2.png

Spectra

Click on the VOSpec icon in AstroScope and the spectra will be displayed. You can do some basic manipulations such as change units, fit a black body, and save the spectra to your local disc.

hmsge.png

IUE and ISO spectra displayed in VOSpec ovelaid with images created in Aladin after using the MERLIN Imager.

Tables

Virtual observatories and many other applications exchange tabular data in VOTable format. This can include lists of images with their URLs or simple spectra as well as lists of e.g. sources and properties. Aladin and VOSpec, described above, use VOTables and TopCat is a dedicated tool which lets you browse and plot tables, add new columns (including coordinate conversions) and exchange data with Aladin and any other tool which can see MySpace. The latest version can be downloaded and started using
java -jar topcat-full.jar
and automatically registers itself with Plastic if you have the Workbench running

You can then load tables from MySpace by clicking on 

File 
  Load Table
    Filestore Browser
      Location MySpace
        Log in
Warning: at present, it is slow to open and navigate in MySpace but this does not affect loading the data once it has been located.

Topcat1.png

If you view the same table in Aladin and TopCat, a source selected in one application will be highlighted in the other.

Data provider applications

Using the Application Launcher

Multiple selection criteria

The Astronomers' Data Query Language (ADQL) lets you select the columns you want within chosen ranges. If you select the Application Launcher from the workbench, you can search for a data set, e.g. "FIRST and object and catalogue" (Faint Images if the Radio Sky at Twentyone cm, see Registry Browser help). If you get back a service which supports ADQL, you can build a query constraining any parameters in the catalogue. Click the tick-box on the left and a Query Builder box will appear. You can use the GUI Tree or enter SQL directly under the Adql/s tab. The following is an example: 

 
SELECT * FROM catalogue AS T1 WHERE 
 T1.POS_EQ_RA > 186 AND T1.POS_EQ_RA < 188 
 AND T1.POS_EQ_DEC > 1 AND T1.POS_EQ_DEC < 3  
 AND T1.EXTENSION_FWHM_MAJ > 0.0
which is then expanded into the Tree and also translated into Adql/x for sending to the data centre.

Qbuilder.png

Once you have finished the query using the Parameter button to select the file you want to save the results in in MySpace. Finally, click the green button to launch the query. You should be able to monitor progress in the Lookout and view the results e.g. in TopCat or Aladin.

Cone Search and Simple Image/Spectral Access

You may see one of these services offered for your chosen dataset. The simplest type of query takes a position and a search radius and returns all the catalogue objects within that radius, or the images or spectra with reference positions inside the radius. This is the way that AstroScope works. Some Simple Image Access Protocol (SIAP) services also recognise any overlap with the field of view and other constraints. If you select a data set offering Cone or SIAP/SSAP access, a box will pop up asking for the position and radius (J2000, decimal degrees). You also need to use the Parameter button (see above) if you want to save the results to MySpace. SIAP will return a VOTable of the URLs of images and a brief description.

On-the-Fly data extraction

Some data centres offer custom-made data products. For example, the MERLIN Imager produces an image to your specifications (or the nearest possible values of size and resolution) from radio interferometry data. Search for "MERLIN and Imager" in the Application Launcher. Fill in either the position or the object name and, optionally, other criteria, and the destination of the Output. At present only MERLIN names are recognised but you can check what sources have been observed by browsing the MERLIN Processed Data archive

MERLINImager.png Aladin1.png

If there are data anywhere within the field of view of an observation which match your criteria, the five best images will be made and supplied via a SIAP-like VOTable which can be loaded into Aladin or browsed with TopCat. The service uses ParselTongue, a python-based scripting language developed by RadioNet. It will be developed to link with SIMBAD, and other data products (e.g. spectra, time series) could be extracted in a simlar fashion from visibility data, X-ray event series etc.

Using the Workflow Builder

Queries available through the Application Launcher can also be inserted in a workflow using the Workflow Builder launched from the Workbench. If you insert a Step into a workflow, the Task Editor will then let you select a queryable dataset and build a query in the same way.

Using the Colour Cutter

This Parameterised workflow is launched from the workbench. The Colour Cutter allows selection by position and magnitude difference from one IR and one optical catalogue and cross-matches the sources in common.

Author: A M S Richards, amsr@jb.man.ac.uk
Last revised : 2 April 2006

MERLINImager.png Aladin1.png Qbuilder.png