TOPCAT is not a requirement of Gnuastro, it is a table viewer and plotter (in many input formats, including FITS, VOTable, and others). TOPCAT is therefore a useful tool to visually inspect the tables of your Gnuastro inputs or outputs (for images, see SAO DS9). In Gnuastro we have an installed script to run DS9 or TOPCAT on any number of FITS files (depending on it being an image or table), see Viewing FITS file contents with DS9 or TOPCAT (which also includes a .desktop file for GUI integration). After installing DS9, you can easily use that script to open any FITS file (table, image or cube).
TOPCAT is a very large package with many capabilities to visualize tables (as plots). It also has an extensive documentation that you can read for optimally using it. TOPCAT is written in Java, so it just needs a relatively recent (in the last decade) Java Virtual Machine (JVM) and Java Runtime Environment (JRE). Your operating system already has a relatively recent Java installation in its package manager, and there is a large chance that it is already installed. So before trying to install Java, try running TOPCAT. If it complains about not finding a suitable Java environment, then proceed to search your operating system’s package manager.
To install TOPCAT, you just need to run the following two commands. The first .jar file is the main TOPCAT Java ARchive (JAR). JAR is a compressed package of Java files and definitions that should be run with a special Java command. But to avoid bothering users with details of how to call Java, TOPCAT also provides a simple shell script (the second downloaded file below) that is easier to call and will do all the internal checks and call Java properly.
$ wget http://www.star.bris.ac.uk/~mbt/topcat/topcat-full.jar $ wget http://www.star.bris.ac.uk/~mbt/topcat/topcat $ chmod +x topcat $ ./topcat # Just for a check to see if everything works! $ sudo mv topcat-full.jar topcat /usr/local/bin/
Once the two TOPCAT files are copied in the system-wide directory, you can easily open tables with a command like below from anywhere in your operating system.
$ topcat table.fits
Install without root permissions: If you do not have root permissions, you can simply replace /usr/local/bin in the command above with $HOME/.local/bin.
If this directory is not in your