I was told that CASA is used extensively in Radio Astronomy. However, softwares like SAO DS9 and also python (Astropy) are also used for radio astronomy data analysis. So, what makes CASA different as compared to other softwares? Is there any other alternative software that can be used for radio astronomy data analysis?
CASA is very specifically designed for analysis of data from radio telescopes and in particular interferometers, notably, ALMA and the VLA. This means it has a very large suite of radio- and interferometer-specific tools and capabilities which DS9 does not. As a very basic example, CASA can implement the CLEAN algorithm, performed using the
tclean commands. You can also use it to perform numerous other interferometer-specific calibration steps. I'm not aware of DS9 being able to do this without a good deal of inconvenient modifications, if at all.
It's true that with some tinkering you can inspect radio maps in DS9 that have already gone through the standard process of calibration/deconvolution/etc., but even then there are plenty of not-so-subtle reminders that CASA was built for this sort of thing while DS9 was not. For instance, the CASA viewer automatically displays the interferometer's synthesized beam shape, making it easier to see whether a particular region of emission is compact - which would be the case if the source doesn't appear larger than the synthesized beam.
There are certainly other radio interferometry tools out there, such as Difmap, Miriad, GILDAS, UVMULTIFIT and of course CASA's predecessors, AIPS/AIPS++. Many of these were designed to be used by specific telescopes.