How to make a simple, 16-bit integer FITS file
                      Michael Richmond
                       Nov 27, 1996
     
 
       Here's the prescription for making a minimal FITS file.
 I hope that it gives enough of the details for you to implement it.
 
       1. The file must be a multiple of 2880 bytes long.
          If necessary, pad with NULLs at the end of the file.
  
       2. The first 2880 bytes are called the "header".  This
          header is arranged (conceptually) into 36 lines,
          each of which is 80 characters long.  The kicker is
          that there are NO end-of-line characters, just
          spaces.  Thus, character 80 might be a space,
          and character 81 might be a 'E' and 82 'N' and
          83 'D' if the keyword in (conceptual) line 2 of the 
          were to be "END".
          If you want to store more than 36 header lines, just
          extend the header to another multiple of 36 lines, and
          make sure that the data starts at the next 2880-byte
          boundary after the last header line.
 
       3. The basic form of lines in the header is
 
                KEYWORD = value
 
          where "KEYWORD" must be <= 8 characters long, starting
          in (conceptual) position 1, the equals sign must
          be in (conceptual) position 9, and value can be
         
            a number:      GAIN    =     12
                           EXPTIME =    100.3
                           CAMTEMP =    -23.3e+00
 
            a string:      OBJECT  = 'Ring Nebula'
                           OBSERVER= 'Richmond'
 
            a 'T' or 'F':  SIMPLE  = T      
                           REDUCED = F
   
           Comments can be placed after the "value", separated from
           the value by a slash character:
 
                           OBJECT  = 'Ring'    / aka M57
 
        4. One can make a whole line a comment by using the special
           COMMENT keyword, which requires no equals sign or 
           value, like so:
 
                           COMMENT   This is a comment line
                           COMMENT You can start here
                           COMMENT       or have plenty of spaces
 
        5. The last (conceptual) line of the header must be
 
                           END
 
        6. The typical minimal header gives just enough information
           to allow someone to decode it: it tells the data type,
           and the number of rows and columns in the image.  For
           16-bit signed integers, which are pretty widely used,
           the minimal header of a 200x100 image would look like this:
 
           SIMPLE  =     T             / this must be the first line
           BITPIX  =    16             / 16-bit signed integers
           NAXIS   =     2             / a 2-d image
           NAXIS1  =   100             / 100 columns of data
           NAXIS2  =   200             / 200 rows of data
           END
 
       7.  Okay, that was the header.  Now, starting with the 2881'th
           byte in the file, you put the data.  Use 2 bytes per pixel,
           placing the high-order byte first.  Thus, if the first pixel
           value in the first row is 259, then 
 
                byte 2881 = 0x01
                byte 2882 = 0x03 
 
           Just keep writing row after row, placing data values one
           after the other.  Don't skip any space at the end of a row;
           begin placing the value from the first column in the next
           row immediately afterwards.
 
        8. When done with all the data, write out enough NULLs (zeros)
           to pad the length of the entire file to a multiple of 2880
           bytes.
 
   There!  You're done.
 
   The tricky part is remembering NOT to place an end-of-line character
 at the end of each (conceptual) line of the header.  
 
   For the REAL definition of FITS, read the FITS archive at NRAO:

       http://fits.cv.nrao.edu/www/fits.html