Opening an existing image frame is done via a call to STFOPN
(SCFOPN). This routine will open the file on disk and store the
relevant information in the internal MIDAS file table.
Again a `file id' is returned for later usage by other interfaces.
If an application opens a frame with a different data type than the one with which it was created, the data are converted automatically to the format specified in the File Open routine. When writing the data back to the frame it is converted again to the original data format. Except a few specialised tasks, all MIDAS core applications open image frames with the real data format.