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Data Access Work Sheet

This worksheet demonstrates some ways to retrieve and visualise astronomical images, spectra and catalogue data using AstroGrid.

To access AstroGrid services, launch the VO Desktop

All-VO Scope

Launch the VO Desktop, click on Start and select the All-VO Scope, an AstroGrid tool that allows you to find catalogues, images, spectra or time series which cover a given position or SIMBAD object, with the option of selecting by time period. It provides easy links to visualise the results and download data files either to your local machine or to your VO Workspace on the AstroGrid system (VOSpace).

For example, enter the symbiotic star HM Sge as the Object, and use the default of all except Timed data and a Search Radius of 0.01 deg.

Saving data

Select some data by double-clicking (e.g. the Catalogues 'Two Micron All Sky Survey' and 'INT-WFS DQC'). Actions on the left side now offers Download which will open up a destination browser for your local machine.

If you have an AstroGrid account you can also select VO Workspace and save data to your VOSpace.

Tip: You are advised to start by selecting just one or a few items, and ensure that you have used the Clear selection button before later saving further data.

VO Explorer

VO Explorer provides pre-selection of data, increasingly necessary now that tens of thousands of separate data collections and catalogues are published to VOs. VOExplorer Help explains how to use this to full advantage, but at the simplest, just click on + New Smart List and enter the name of a data set or a likely keyword, e.g. radio images or Symbiotic Star, and then Create. You can then use the right-hand top down arrows to provide options for filtering the results, e.g. set Resource Type to Image Access (see VOExplorer Help for full details).

You can now select an individual data set for detailed data selection (see below) or select many data sets (shift-click on first and last items in the list) and click on Query on the left. This will open up a new All-VO Scope which searches only the selected archives.

Tip: This is not a positional search - that is the next stage - don't enter the name of a single object like HM SGe unless you only want catalogues etc. which have the source name in the heading, column names or other metadata - usually more restrictive than you want.

Visualising data

PLASTIC directly links AstroGrid and data visualisation and manipulation tools such as TopCat, Aladin, SPLAT, GAIA, VOSpec and VisiVO. The PLASTIC page provides web links which allow you to use webstart and/or download and run the compatible versions of each helper application.

Start the visualistion tool(s) you want to use. After you have selected some data in All VO Scope, the icons for suitable tools will appear on the left - click to send the data to the chosen visualiser. You can also use any of these tools to load data directly from !VOSpace if you have the VO Desktop running.

Tip: There are ways to avoid out-of-memory errors when manipulating bulky data sets.

Images

FITS or JPEG images (including FITS cubes) can be dispalyed in GAIA or Aladin (as long as you have these running).

Tip: If you chose to 'send as table' to Aladin, this will provide an easily-readable summary of the available image properties (size, wavelength etc.) so that you can make a more refined selection and avoid downloading many bulky images uneccesarily.

Spectra

SPLAT and VOSpec are available. For example, after starting VOSpec and selecting a few spectra, click on the 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.

ISO, IUE and FUSE (IR - EUV) spectra of Markarian 273, redshifted to 0.05. VOSpec also contains a line identification tool.

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 !VOSpace.

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

Detailed data selection

Database queries

The Astronomers' Data Query Language (ADQL) lets you select the columns you want within chosen ranges. Some of the accessible catalogues are listed in the VOExplorer Examples: Queryable database examples. When you select a suitable catalogue, e.g. choose the FIRST object catalogue (Faint Images of the Radio Sky at Twentyone cm), the Build ADQL button appears on the left. This opens up a Task Runner configured as the Query Builder. You can use the GUI Tree or enter SQL directly. The following is an example query to FIRST:

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

Click on Validate Edit and if the tree view is satisfactory, choose a file in VOSpace for the Result (top centre) and then Execute (top right). If you have not chosen an output file, the result will be returned into memory. Tip: This is only suitable for small files; save large area searches to VOSpace. You can then view the output in TopCat or any suitable application.

Advanced data acquisition

On-the-Fly data extraction

Some data centres offer custom-made data products. For example, the MERLINImager produces an image to your specifications (or the nearest possible values of size and resolution) from radio interferometry data. Search for "MERLIN" in VOExplorer. 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.

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.

Scripting

AstroGrid's services can be run from scripts in python or other common languages, see Scripting. This can be used to construct complex workflows such as ColourCutter.