The FITS definition allows for multiple extensions (or HDUs) inside one FITS file. Each HDU can have a completely independent dataset inside of it. One HDU can be a table, another can be an image and another can be another independent image. For example, each image HDU can be one CCD of a multi-CCD camera, or in processed images one can be the deep science image and the next can be its weight map, alternatively, one HDU can be an image, and another can be the catalog/table of objects within it.
The most common software for viewing FITS images is SAO DS9 (see SAO DS9) and for plotting tables, TOPCAT is the most commonly used tool in astronomy (see TOPCAT). After installing them (as described in the respective appendix linked in the previous sentence), you can open any number of FITS images or tables with DS9 or TOPCAT with the commands below:
$ ds9 image-a.fits image-b.fits $ topcat table-a.fits table-b.fits
But usually the default mode is not enough. For example, in DS9, the window can be too small (not covering the height of your monitor), you probably want to match and lock multiple images, you have a favorite color map that you prefer to use, or you may want to open a multi-extension FITS file as a cube.
Using the simple commands above, you need to manually do all these in the DS9 window once it opens and this can take several tens of seconds (which is enough to distract you from what you wanted to inspect). For example, if you have a multi-extension file containing 2D images, one way to load and switch between each 2D extension is to take the following steps 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. A different series of steps is also necessary if you the extensions are 3D data cubes (since they are already cubes, and should be opened as multi-frame). Furthermore, if you have multiple images and want to “match” and “lock” them (so when you zoom-in to one, all get zoomed-in) you will need several other sequence of menus and clicks.
Fortunately SAO DS9 also provides command-line options that you can use to specify a particular behavior before/after opening a file. One of those options is -mecube which opens a FITS image as a multi-extension data cube (treating each 2D extension as a slice in a 3D cube). This allows you to flip through the extensions easily while keeping all the settings similar. 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’ (multi-character) option should have been called like --mecube, but SAO DS9 follows its own conventions.
For example, try running $ds9 -mecube foo.fits to see the effect (for example, on the output of NoiseChisel).
If the file has multiple extensions, a small window will also be opened along with the main DS9 window.
This small window allows you to slide through the image extensions of foo.fits.
If foo.fits only consists of one extension, then SAO DS9 will open as usual.
On the other hand, for visualizing the contents of tables (that are also commonly stored in the FITS format), you need to call a different software (most commonly, people use TOPCAT, see TOPCAT). And to make things more inconvenient, by default both of these are only installed as command-line software, so while you are navigating in your GUI, you need to open a terminal there, and run these commands. All of the issues above are the founding purpose of the installed script that is introduced in Invoking astscript-fits-view.
GNU Astronomy Utilities 0.22 manual, February 2024.