/DISK_IO                                                   June 1998

This subdirectory contains IDL procedures to read popular disk formats in
astronomy.  Currently available are procedures to access

     (1) IRAF image (.imh) files  (read and write)
     (2) MIDAS image and table files  (read and update)
     (3) WFPC2 images (FITS or STSDAS)

The procedures to access disk FITS files are kept in a separate subdirectory
/FITS, and those to access STSDAS image files are in the subdirectory /SDAS.

The procedure IRAFDIR uses the non-standard system variables !TEXTOUT and
!TEXTUNIT.   These can be added to one's session using the procedure
ASTROLIB.

*********************************************************************

IDL-MIDAS Interface routines  
                             F. Murtagh, ST-ECF, January 1989.

   The following four IDL routines are available for accessing
   MIDAS .BDF and .TBL data structures:

   MID_RD_IMAGE      ... image (.BDF) read,
   MID_UP_IMAGE      ... image (.BDF) update,
   MID_RD_TABLE      ... table (.TBL) read,
   MID_UP_TABLE      ... table (.TBL) update.

   The following lower level procedure is called by MID_RD_IMAGE and
   MID_UP_IMAGE.

   MID_RD_DIRDSC     ... directory descriptor read

Annotated example of usage:

   mid_rd_image,'trial',nax,npix,data
                              [image TRIAL.BDF is accessed;
                              values will be obtained for number of 
                              dimensions in the image, NAX (assumed 1
                              or 2); numbers of pixels in these 
                              dimensions, NPIX(..); and pixel values, 
                              DATA]
   print,data                 [... for instance]
   data(0) = 8.77             [arbitrary change of value]
   mid_up_image,'trial',nax,npix,data
                              [image TRIAL.BDF is updated; values for
                              NAX, NPIX and DATA must be provided 
                              and this example indicates how this can 
                              be done using a prior MID_RD_IMSGE call]



========================================================================
Known Difficulties
========================================================================

1. A very large number of image descriptors cannot be handled.  This is
   because the offsets determined by these routines cannot cope with 
   for example 86 descriptors, as is the case with FOC images converted 
   to MIDAS c. fall 1990.  An error message indicating that NAXIS is 0
   indicates that the correct value of NAXIS is not being picked up. 

   A simple work-around here is as follows.  Get rid of all superfluous
   descriptors.  In MIDAS, this is permitted using the aux_mode keyword.
   
   help/key aux_mode             [... gives information about aux_mode]
   read/key aux_mode             [... indicates that usually the 4th 
                                  element is 1; put it to 0...]
   write/key aux_mode 1,0,0,0,0,0,0,1
   comp/ima newimage = oldimage  [... this copies the image, getting rid
                                  of unnecessary descriptors.]