In particular, the FITS document I got from NASA's FTP site is kind of vague on what data is actually in a FITS and examining the FITS it's format is rather strange. In particular, the FITS document I got from NASA's FTP site is kind of I'm not aware of any FITS readers for Amiga. The first point to remember is that FITS is not a simple format for pure pictures but a generalized data transport format in which pictures can be stored in the form of a multidimensional matrix, but such a matrix need not represent a digital image. How to interpret headers is described in the on-line NASA document.
Proper software can read the header and extract the stored data that follows.
In the case of HST, it is useful to read their accompanying documentation. What HST does is to combine the four images into one matrix, with the first two axes being the two axes of the picture and the third axis representing the separate chips. Interpreting the numbers requires understanding the header keywords.