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Looking at Stellar Jets: HH30

In this example we are going to look in the HST image archive for images of stellar Jets. You will need the VOExplorer application running as well as TOPCAT and Aladin.

First we are going to search the registry for resources matching HST which provide images. In order to do that first we need to create a list of resources that we want to query. In VOExplorer click on the New Smart List. Name your search HST and add the conditions as in the figure below (hint: clicking on the '+' icon adds another condition). When this is done click on Create.

The new HST Smart List will contain a list of resources similar to the figure below. Note that this List is dynamically generated from a registry query so its contents may vary if new services are added or other ones are deleted. If you select one resource of the list you get in the Information tab a description of it and some other details. You can also add your own annotations or flags to each resource and even assign tags for easier search later. Each resource has Curator or person in charge of it which can be emailed for clarifications or if the service is not working or not returning the expected results. The next step is to query all these resources for images. With the HST Smart List selected click on the menu Edit -> Select All to select all the resources and then under Actions click on Query.

A new window will appear which is named VOScope. This is a generic interface which allows us to query the selected resources for images, catalogues and spectra. In our case we are only querying for images, so type HH30 in the Position or Object Name box and 0.01 in the Search Radius (meaning 0.01 deg). Then click on the Search button. The selected HST resources are now being queried for images containing the HH30 object. A graphical display will appear similar to the one on the figure below.

Double click on the CADC/HST Image Search and under actions Send Tables to Aladin. If you do not have that option available then it is a good time to start the Aladin browser now. The table we are sending contains the references to the actual images not the images themselves so Aladin displays a list of image files grouped by filter name. Just select two as in the figure below and click Submit.

The images are now being retrieved from the archive and loaded in the Aladin browser. You can investigate the different options to scale the images and change the colours and pixel display range.

Finally click on the RGB button to create a colour image. Choose one of the images for the red channel and another one for the blue channel leaving the green channel empty. After a few seconds you should get an image similar to the one below.

As you can see the images are slightly misaligned. You can use Aladin to correct the astrometry of one of the images so they are aligned and the resultant RGB image looks better. This is left as an exercise to the reader.

Using Python to automate the search

The procedure outlined above is fine if one wants to look at a particular object or two, but if we want to do this for several objects and also save the results then it is better to write a script.

We are going to use a Python script to search the CADC/HST archive for images available for a list of objects. In order to do that you will need the following script: conesearch_hst.py (in order to download it go to the bottom of that page and under 'Download in other formats' select 'original format'. You can modify the list of objects to be searched in lines 19 and 20.

If the Astrogrid Python module is not in your Python path then from your terminal type

export PYTHONPATH=<path-to-module>/astrogrid-0.9-py2.4.egg   # sh shells
or
setenv PYTHONPATH <path-to-module>/astrogrid-0.9-py2.4.egg   # csh shells

Launch TOPCAT and Aladin and after they load run:

python conesearch_hst.py -b

The selected HST archive (defined in line 26) will now be searched for images for each object. A VOTable with the list of images for each object will be saved to the 'hst' directory and also sent to TOPCAT.

From within TOPCAT you can operate on the tables in order to select e.g. only those images with exposure time larger than some value and send the tables to Aladin (use Interop -> Send Table To -> Send to Aladin) in order to display the images as before.