Showing extended objects on astronomical images with Aladin & Simbad
New October 2016
Problems with the filter and object display in Aladin
In the following images I have set the objects drawn by Simbad to red, the filter labels to orange and the extended object ellipses to blue.
The first thing you will notice is that there are a large number of Simbad objects shown that don't appear to have a size in the major axis and in my view should be suppressed by the filter. (These are the objects in red). You may think you can fade these by using the green slider below the SIMBAD icon in the stack but if you try this will fade the output of the filter as well so is no use for suppressing the unwanted objects. The orange labels appear to be correct - some QSO's are shown but they have a (very small) but non-zero size so pass the filter criteria on the previous page.
The second problem is shown in the two images below - in the top image of the area around gamma cygni there are two diagonal curved lines that form part of an ellipse bounding a large extended object. If you then zoom out as in the second image below in an attempt to find the central object to identify this extended object the ellipse disappears ! Note the smaller circle which is partly shown in the top image is shown completely in the second image. We will now discuss how to cure these problems.
Curing the image problems
Suppressing the Simbad objects shown in red on the above images is surprisingly easy. First highlight the 'SIMBAD' row in the stack by clicking on it. Then click on the 'prop' (for properties) button. You should then get the properties window up for the SIMBAD plane. Part the way down you will see a section 'Dedicated Filter' which should have the 'All Objects' radio button highlighted. To hide the SIMBAD objects click on the 'no-filter-' radio button. This removes the filter in the SIMBAD plane. You might think that removing the filter would show all the objects but in fact the opposite is true - if there is no filter no objects are drawn ! QED.
If you want to see what the SIMBAD filter does with the 'All Objects' radio button highlighted click on the 'This filter on the stack for editing' and another window will open showing the 'SIMBAD' default filter. This is a set of instructions of the form for this type of object draw this type of symbol. It's probably easier to start at the bottom as the top couple of lines for stars are quite complicated.
You might think you can load the extended object filter into the SIMBAD plane but that does not appear to work either if you load it or cut and paste the filter definition from page 1.
To show the extended objects that disappear when you zoom out you can use the bottom correction on the 'SIMBAD' plane properties labeled 'Density correction' this will show most if not all of the extended objects but makes the process of zooming in and out very slow as Aladin has to load more Simbad objects in the view if the density is set too high and then suppress them as you have instructed that they are not displayed using the 'no-filter' option above. It makes for some interesting flickering effects until all the objects are loaded and hidden. While this shows more it still does not appear to show all the smaller objects when you zoom out.
The next page will show a way that performs better but is more complicated.
Note: while we are discussing the SIMBAD plane properties the 'Scaling Factor for Filters' changes the size of the filter text. The ellipses stay the same size as they are defined in the filter.
(c) John Murrell 2016
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