In order to make it easier to use lots of different telescopes, astronomers decided to create a particular kind of computer file that all telescopes could use. They called this kind of file a FITS file, which stands for the Flexible Image Transport System.
As well as storing the actual images, the files also have other information in them, such as the date and time of the observation, the name and location of the telescope, the weather at the time of the observation and so on.
There are now large archives of FITS files, bringing together observations from many telescopes and allowing astronomers to make use of millions of images.
The NSO uses a special compressed version of the FITS file, called HFIT. This allows for faster downloads, but does mean that the HFIT versions cannot be opened in other FITS-viewing software. However, users can opt to download a pure FITS version of their observations. If you want to convert a HFITS file to a FITS file, you can do this in LTImage by opening the HFITS file, and re-saving whilst choosing the FITS format from the drop down menu in the "Save Image Data" function.