Showing extended objects on astronomical images with Aladin & Simbad

Page 3

New October 2016


 


Providing a list of extended objects

 

In the previous pages we used Simbad to provide the list of objects to be filtered, as we have seen the Simbad list in Aladin is dynamic - as you zoom out the number of the objects loaded falls so some 'unimportant' objects will not be loaded and the extended objects associated with them will disappear from view as you zoom out.

 

The cure for this is to provide your own list containing only extended objects. As there are a lot less of these the same problems of suppression of 'unimportant' objects when you zoom out do not occur.

 

The extended objects in Simbad are generally classed as  'ISM' for Interstellar Matter in Simbad. Running a query on the number of objects classified as ISM shows that in 2016 there are around 60k objects classed as ISM. The complete list of object sub-classifications for ISM can be found here. The number of objects causes a problem as the Simbad search for objects by criteria at CDS is limited to 20k objects. As a result we have to select a sub-set of the objects appropriate to the area we are interested in. There are two ways of doing this either by selecting over a range of both RA & Dec or else using the 'Range' method to find all the ISM within a specified radius or box from a given position. These will be discussed in more detail later. We will use Simbad to compile a 'catalogue' of ISM objects in the area we are interested in. This will be exported from Simbad as a VOTABLE which can be opened as a catalogue in Aladin.

 

To compile our own list of ISM objects we need to access Simbad via the web interface rather than via Aladin as previously. This page has access to documentation about Simbad as well as a summary of the current database statistics. For the purpose of compiling the ISM catalogue the next step is to click on the 'by criteria' tab.

 

Simbad: Query bu Criteria
Simbad: Query by criteria page (top of page only)

 

Before we look at the query there are a couple of other things we need to set. On the Right Hand side there is a section marked 'Return:' This allows you to select whether the query returns a count of the number of objects that meet your criteria in the case of the top radio button or in the case of the second returns the actual information with one object per line. The drop down box allows you to select various numbers of lines to be returned from 10 to 20k. The lower limits are useful when you are view a sample of the output to confirm your query is working as expected.

 

The second thing we need to set are the output options, click on the yellow 'Output Options' tab and you will be presented with the following page:

 

Simbad Output Options
Simbad:Output options - (top of page only)

 

There are two important functions on this page - the Yellow section 'Output Format' selects where the output from the query will go. If you select 'HTML' it will be suitable for reading in your browser. Thus while you are testing your query and want to see the results this needs to be set to 'HTML'. When you have a query that you think works (and contains less than 20k objects) the option in the box needs to be set to 'VOTABLE' and the 'file output' box ticked. The results will then be returned in a VOTABLE file.

 

The second part of the form selects what data will be shown both on the screen and in the VOTABLE. You need to ensure the ICRS & FK5 co-ordinates are selected, the main identifier, and further down the 'angular size' so the data is available for the filter in Aladin to plot. The others are not really required for plotting ISM but select any you think you might find useful.

 

Finally you need to click the 'save' button at the bottom left to activate your options. The options are saved between sessions so if you leave the output directed to a file don't be surprised if there is no visible output when you try another query.

 

Leave the output to HTML & ensure the 'file output' box is not ticked before pressing save until the query is tested.

 

Entering and running your query

 

In the Search expression box enter the following:

 

otypes = ISM & dimmajor > 0.1

 

If you select Return: Object Count and then press the 'submit query' button you should get a count of the number of objects in the database which have an object type of 'ISM and have a major dimension of the bounding ellipse of greater than 0.1 arc minutes. The count should be close to 50k - too many to export as I explained earlier the limit is 20k lines.

 

Use your back button to return to the query and now select Return display - 100 objects and submit the query again.

 

Truncated ressult for items of type ISM
Example of the results for objects with OTYPE = ISM and dimension > 0.1 arc seconds (part page only)

 

The image above shows the results - your columns may be different depending what you selected in the output options. The results shown in the rows may well be different as they are only the first few rows it has found of the 50k ISM objects.

 

To get the number of objects below the 20k limit there are several options. The first is to set a larger threshold for objects - if you set dimmajor > 5 (arc minutes) the number of objects returned falls to about 16k which could be exported. Of course if you do this the smaller ISM objects will not be shown in Aladin when you load the catalogue.

 

If you want the smaller objects you need to constrain the number of objects found by selecting part of the sky. This can be done either by selecting the region you are interested in or by setting bounds on the RA & Dec.

 

Adding an area to the query using the region command to show the ISM around Gamma Cygni gives the following query

 

otypes = ISM & dimmajor > 0.1 & region(box,gamma cygni,10d 10d)

 

The region command has various shapes, in this case we have selected a box, you can either enter the centre of the region as RA & Dec coordinates or as we have done in this case enter the object name (The RA & Dec co-ordinates are resolved by Simbad). The final two numbers give the box size in this case 10 degrees. (you can also use m for arc minutes or s for arc seconds) Beware that there is no comma between the two 10d's but there needs to be a space. The same applies if you provide an RA & Dec coordinates for the centre.

 

This method works best if you are looking in a particular area. Note: if you are using a large area ensure it does not extend past 90 deg. in declination as it is not clear what will be returned. I am not sure what the situation is if you exceed the RA limits of 0 & 360 degrees.

 

If you want to use RA & Dec limits the query should look something like this:

 

otypes = ISM & dimmajor > 0.1 & ra > 10 & ra < 20 & dec > 20 & dec < 50

 

When you have entered you query you first need to use the Return: object count to check that the number of objects is less than 20k and then display the first 100 or 1000 to check the results are sensible.

 

When you are happy with your query it is time to direct the output to a VOTABLE !

 

Return to the output options tab and select the Output Format as VOTABLE and check the file output box. Also set the maximum number of objects to 20k then save the options.

 

Your query should still be open in another tab in your browser and you can return to it by clicking on the tab.

 

We are almost there ! Now set Return to 'display' and the maximum number of objects to 20k.

 

Finally submit the query again and your browser should show you a file is being downloaded. Finally download the file and move it to a convenient folder on your computer. It is best to rename it at this point to give some indication of it's contents and give it a file extension of .VOT. ( Aladin will load the file whatever it's extension - giving it a VOT extension just reminds you you what it contains.

 

The next and final page will open this file in Aladin and plot the extended objects.

 

The query I submitted is:

 

otypes = ISM & dimmajor > 0.1 & region(box,gamma cygni,20d 20d)

 

 

The resulting VOTABLE file is available here (file: Gamma Cygni 20deg box ISM objects.VOT),

 

Feel free to use this to experiment with but be aware that it will not be updated to show any changes to objects in Simbad since October 2016.

 

 

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(c) John Murrell 2016

  


 

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