Showing extended objects on astronomical images with Aladin & Simbad

Edited May 2017


If you have an astronomical image with point sources be they stars, quasars or other star like objects the interactive star atlas Aladin with an overlay from Simbad or a more detailed catalogue for fainter objects does a good job of identifying the objects shown in the image.


If your image has extended objects such as Nebulae, dark clouds, supernova remnants , etc. Simbad will still place a point marker at the centre of the object but if there are several objects of different sizes it is not obvious which extended feature the point marker applies to.


M6 with Simbad Overlay
This image shows M6 from the DSS survey overlaid with the Simbad identifiers. They are mostly point sources but there may be some extended sources though it is difficult to tell.


This page explains how to show the size and orientation of extended objects in Aladin and how to over come the problems caused by the 'less important' objects not being shown by Simbad as you zoom out. The 'less important' object may well be the centre point of your large extended object !  As a result of this extended objects are shown perhaps partially in a close up view but disappear as you zoom out to see the full extent of the extended object.



If your image is not in FITS format and does not contain WCS co-ordinates you will need to calibrate it first. I have written an explanation of how to do this using that is available here. This explanation assumes reasonable familiarity with Aladin, if you need to get to know Aladin have a look at my Virtual Observatory exercises first.


For extended objects the Simbad database has several pieces of data that are required to show the position, size and orientation of the object. The first is the position of the centre of the object, this is where Aladin shows the point marker for an extended object. The other three items are the dimension of the major and minor axis plus the orientation of the ellipse that bounds the extended object. 


One of the tools available in Aladin is the 'Filter', The title of this is a bit misleading as it not only allows you to select or filter objects in the display using specified criteria but also allows you to draw shapes and label objects. This feature is what we will use to show the position and size of extended objects and label them.


 After loading your image and the Simbad catalogue you need to create a filter. A new filter is created by clicking on the 'Filter' button on the right hand side of the image area. The window that pops up will open showing the 'Beginner mode' tab which has a number of pre-set filters that can be selected but none are appropriate to show the size of extended objects. To load or enter the filter required to show the size of objects you need to click on the 'Advanced Mode' tab. You now have two options, you can download my filter from here (file:Extended Objects Only.ajs) and put it in an appropriate folder on your computer and then use the 'Load filter' button. Alternatively you can copy and paste the filter text below into the filter box box and then save it to your computer. In both cases you need to click the 'Apply' button to activate the filter.


Filter Text


$[phys.angSize.smajAxis] > 0 {
draw orange
draw blue $[src.class]
draw blue ellipse(0.5*$[phys.angSize.smajAxis],0.5*$[phys.angSize.sminAxis],$[pos.posAng])


The text may be wrapped in the filter box depending on how wide you have made it.


Filter explanation

The first line of the filter following the dollar sign selects those objects that are extended and have an angular size of the major axis greater than 0 arc seconds, this can be edited if required to only show larger objects. This suppresses the stars and quasars as they generally have no size for the major axis in Simbad.

The section in the curly brackets instruct Aladin what to do for each object in the image that has a major axis greater than zero.

The second line 'draws an orange box at the centre of the extended object..

The third line labels the centre of the object with the object class in blue

finally the third line instructs Aladin to draw a blue ellipse the size of the major and minor axes with angle determined by the position Angle.


It possible to change the colour to contrast with your image by editing the colours above. Green seems to be quite a good choice as it contrasts with most images. The field descriptions in square brackets should be common to all the catalogues in the virtual observatory though some may not contain values for them.


Viewing the result

Having clicked the apply button ellipses (or circles) should be shown for any extended objects in the field or view. I suggest you now try this with your image and also loading other objects into Aladin and looking at the changes as you zoom in and out. One interesting area to look at that illustrates some of the problems is the area around  'Gamma Cygni'  that contains lots of overlapping extended objects as well as stars, quasars and galaxies.


The following page will discuss some of the apparent problems and a way of getting round them.


Next Page (page 2)

(c) John Murrell 2017



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