The file name is generated from the prefix and the sequence number. A FITS file may contain several extensions each of which may result in a MIDAS file. If more than one file of the same type ( e.g. image or table) is associated to the same FITS file, an additional letter is appended to the name. The fourth table file in the FITS file ' obs0023.mt' would get the name ' obs0023c.tbl'.
The original file name is normally available in the descriptor FILENAME which can be used for renaming it (see RENAME/FILE). This is not done by default because it could overwrite exsisting files.