Notes: in previous versions of mos-spectra many of the above files were derived from the 'prefix' input parameter. This has changed. A mandatory input event list is entered and the prefix derived from the INSTRUME and EXPIDSTR attributes within that file. All of the other input files have a 'default' as their filename and are thus optional. If the user does not enter a file name it will be derived from prefix and in most cases, if it does not exist, it will be created with the default name. E.g. if the FOV image is not given on the command line, the filename mos+expidstr+fovim.fits will be created, checked for existence, and either used (if clobber not set), or created and overwritten (if clobber is set).