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HST Images of DQ Her
This tutorial demonstrates using the "Task Launcher" to search the HST Preview Images service for images of the cataclysmic variable, DQ Her. The results of this search, which are in the form of a Virtual Observatory Table (VOT) containing internet web addresses of the images, are passed into Topcat and filtered to remove the unwanted files. Finally, it is shown how to download these images in Aladin and how to plot catalogue data over the images.
1 The Example Case
DQ Her is a type of semi-detached binary star called a cataclysmic variable (CVs). These binary star systems consist of a late spectral type, low mass main-sequence (red) star and a white dwarf which is the primary star. DQ Her belongs to a particular sub-set of CVs known as Intermediate Polars (IPs) which is are characterised by rapidly rotating, highly magnetic white dwarf whose magnetic field causes partial disruption in the flow of mass from the secondary red star to the primary white dwarf. Images of DQ Her reveal the presence of an expanding shell of gas surrounding the system which is generally agreed to originate from a previous nova outburst.
This example case demonstrates the use of the Task Launcher to query the "Hubble Space Telescope Preview Images" service. This service is a type of Simple Image Access Protocol (SIAP) service and, it returns a Virtual Observatory Table listing the web addresses of the images and further details such as the filter through which the observations were taken. This tutorial demonstrates how to send this table to Topcat and generate a new table containing just those results which are desired - in this demonstration this will involve removing the GIF images from the table to leave just the FITS images, although the method could be used to remove images through unwanted filters, for example. Finally this example case demonstrates how to load these images into Aladin and how to plot the data from a catalogue on top of this image.
2 The Task Launcher
2.1 Launching the Task Launcher
Since the Task Launcher is a component of the Workbench, the first step is to start the Workbench; information on how to start the Workbench can be found here. Once the Workbench has been started, click on the "Task Launcher" button to launch the Task Launcher component (see the figure below, the Task Launcher button has been highlighted with a red circle).
While the AstroScope enables one to search multiple data sets and catalogues in one go, the Task Launcher is designed to perform more targeted queries of individual catalogues/data sets or run applications such as S-Extractor (an aperture photometry program). If you are unfamiliar with the Task Launcher, please read the overview (available here) before continuing with this example case.
2.2 Finding the HST Preview Images Service
Searching the database of resources (or the registry as it is otherwise known) is simply a case of entering some search terms into the "Find" text box and clicking on the "Search" button (logical operators such as "AND" and "OR" are supported); in a relatively short time a list of matching services should appear. Since we wish to search for a specific service, it is useful to enter multiple keywords, for the "Hubble Preview Images" service we would enter "Hubble AND Preview AND Images" which in this case reveals just one resulting service. The image below is a screenshot of the Task Launcher window after such a search has been performed.
For such a specific search we find only one result. There are hundreds (if not thousands) of services in the Virtual Observatory so it is beneficial at times to provide as much information for the search as possible.
2.3 Querying the HST Preview Images Service
To query the HST Preview Images service (a Simple Image Access Protocol or SIAP service) we must select it by clicking the tick box next to the service name in the Task Launcher results pane. When this is clicked, a small window asking "Which interface?" will appear. Some services provide multiple methods for searching a dataset, often consisting of a simple search interface whereby the user searches the data based on a central RA and Dec and a radius about this co-ordinate and, a much more powerful search interface enabling the user to search multiple fields within the data; for the HST Preview Image service, these interfaces are referred to as "Minimal" and "Full" respectively. For this demonstration we only wish to search by co-ordinate, so select "Minimal" from the drop-down list and click the "OK" button; the window will now change to the Parameter view (with the Parameter tab highlighted) which will appear similar to the image below.
The "Value" of each parameter may be edited by double-clicking on the appropriate "Value" text box, entering the desired text, and then pressing enter when finished. The "Position" coordinate should be comma separated RA and Dec of the centre of the field in decimal degrees. The easiest way of determining these values is to open the AstroScope from the Workbench, enter your target name in the "Position or Object Name" field and then pressing enter; the AstroScope will resolve the coordinates into a form that can be copy and pasted into "Position" field of the Task Launcher (for DQ Her one would enter "271.875708,45.858861" into the "Position" field). Size is the radius of the search in decimal degrees; for this tutorial enter a "Size" of 0.01.
This query will generate a VOTable containing a list of the images which match this search; we wish to save this output file to our MySpace area. To do this, first click on the "Ref?" tick box in the "Output" row. This will open a file browsing dialog which by default will be viewing your home directory. From the pull-down "Location" list select "MySpace", if you have not already logged in then you will need to select your community and enter your username and password. Enter a name for the output file (such as DQHer-HST.vot) and press "OK". Focus will be returned to the Task Launcher and the "Value" of the output will now be updated.
Now that all the parameters have been entered, we are ready to submit the query. Click the 
button to execute the query.
3 The VO Lookout
The purpose of the VO Lookout tool is to enable the user to monitor their progress of any jobs they might have running (such as data queries) and to view the results. Please click here for an introduction to the VO Lookout before proceeding.
When a query is executed in the Task Launcher, the VO Lookout will launch automatically. The image below shows the VO Lookout with the HST Preview Image query of DQ Her selected; note that in this image the VO Lookout shows that the query has run correctly and that the results are now available in the file DQHer-HST.vot.
4 Topcat
4.1 What is Topcat
Topcat stands for Tool for OPerations on Catalogues And Tables and is effectively a basic spreadsheet like application designed for, but not restricted to, astronomical tables. It is a very powerful tool and like AstroGrid, has been written in Java so it can be used on most operating systems. For more information on Topcat and how to launch it, please visit the web site by clicking here. This tutorial will only describe the commands necessary to complete the example case but Topcat is an extremely powerful application and one is encouraged to read the documentation (links are on the main web page for multiple versions) for further information.
4.2 What is PLASTIC
The current version of Topcat supports PLASTIC which stands for PLatform for AStronomical Tool InterConnection. This communication protocol enables astronomical applications to "send files to each other" thus making it easier for the user to work between applications. When Topcat is started, one should see a message on the screen, similar to 
. One would also see the Topcat window appear, which will appear similar to the image below.
4.3 Sending the table to Topcat
To send the results of the HST Preview Images query from your MySpace area to Topcat, simply left-click the filename in the VO Lookout (indicated by the small pair of spectacles) and from the menu that appears, select To Topcat -> as VOTable. Within a very short time the table should appear in the "Table List" of Topcat (the name of the table in Topcat will not the original file name that you saved the table with). To view the table, click the 
button on Topcat's toolbar which will open the table browser (see image below).
4.4 Removing the unwanted images in the table
Scrolling down in the table browser reveals that for this query, 8 images were returned. The web addresses (known as Uniform Resource Locators or URLs) for these 8 images are to be found in the column labeled "url". Scrolling horizontally reveals a number of details about these images, such as their size and the date they were observed. There is also a column called "format" which lists the format of the image - for these images they are either images/gif or images/fits. It is this column that we will use to remove the GIF images from the table. To remove the GIF images from the table, we actually want to create a row subset of the table (using Topcat terminology - click here for more information) which contains just those images which are of the format "image/fits".
The first step in creating/defining the row subset is to open the "Row Subsets" window by clicking the 
button on the main Topcat window. The window that appears will be similar to the image shown below.
A new subset is defined by clicking the 
button which will open a new window asking for a name for the subset and an expression used to define the subset (read this page for more information). The name can be anything but "FITS" would be appropriate since we are creating a row subset of only FITS files. For the expression, we want to match all rows where the "format" column is equal to "images/fits" so the algebraic expression would be of the form "format.equals("image/fits"); the notation of these expressions is that used in Java and C so if you program in those languages then you may be familiar with the syntax. Otherwise read this page for more information. With these values entered the "Define Subset" window should appear similar to the image below.
