The FITS definition allows for multiple extensions inside a FITS file,
each extension can have a completely independent data set inside of
it. If you ordinarily open a multi-extension FITS file with SAO ds9,
for example by double clicking on the file or running
foo.fits, SAO ds9 will only show you the first extension. To be able
to switch between the extensions you have to follow these menus in the
SAO ds9 window: File→Open Other→Open
Multi Ext Cube and then choose the Multi extension FITS file in your
computer’s file structure.
The method above is a little tedious to do every time you want view a
multi-extension FITS file. Fortunately SAO ds9 also provides options
that you can use to specify a particular behavior. One of those
options is -mecube which opens a FITS image as a
multi-extension data cube. So on the command-line, if you run
$ds9 -mecube foo.fits a small window will also be opened,
which allows you to switch between the image extensions that
foo.fits might have. If foo.fits only consists of one
extension, then SAO ds9 will open as usual.
Just to avoid confusion, note that SAO ds9 does not follow the GNU style of separating long and short options as explained in Arguments and options. In the GNU style, this ‘long’ option should have been called like --mecube, but SAO ds9 does follow those conventions and has its own.
It is really convenient if you set ds9 to always run with the -mecube option on your graphical display. On GNOME 3 (the most popular graphic user interface for GNU/Linux systems) you can do this by taking the following steps:
[Desktop Entry] Type=Application Version=1.0 Name=SAO ds9 Comment=View FITS images Exec=ds9 -mecube %f Terminal=false Categories=Graphic;FITS;
$ cp saods9.desktop ~/.local/share/applications/
In case you don’t have the directory, you can make it yourself:
$ mkdir -p ~/.local/share/applications/
In case you are using GNOME 2 you can take the following steps: right
click on a FITS file and choose Properties→Open
With→Add button. A list of applications will show up, ds9
might already be present in the list, but don’t choose it because it
will run with no options. Below the list is an option “Use a custom
command”. Click on it and write the following command:
-mecube in the box and click “Add”. Then finally choose the command
you just added as the default and click the “Close” button.