Definition of WFPC IDT FORMAT as used in EJG software.

Each image is contained in two files:

device:[directory]name.HDR
samedevice:[samedirectory]samename.IMG

The .HDR file is a fixed length 80 byte per record file
containing a simple FITS header.

The .IMG file is a fixed length 512 byte per record file
containing the binary image data.

The VMS file attributes of these files are given by the sample
directory listing below:

W80547.HDR;2                  File ID:  (1492,753,0)       
Size:            2/4          Owner:    [GRAVITY,GROTH]
Created:  13-SEP-1991 10:05:21.00
Revised:  22-NOV-1991 17:20:32.52 (30)
Expires:  13-JUL-1992 17:27:20.54
Backup:    <No backup recorded>
File organization:  Sequential      <************ Note
File attributes:    Allocation: 4, Extend: 0, Global buffer count: 0, No version limit
Record format:      Fixed length 80 byte records   <********* Note
Record attributes:  Carriage return carriage control   <********* Note
RMS attributes:     None
Journaling enabled: None
File protection:    System:RWED, Owner:RWED, Group:RE, World:RE
Access Cntrl List:  None

W80547.IMG;3                  File ID:  (1493,659,0)       
Size:         3907/3908       Owner:    [GRAVITY,GROTH]
Created:  13-SEP-1991 10:05:21.00
Revised:  22-NOV-1991 17:30:19.00 (28)
Expires:  22-NOV-1991 18:30:19.00
Backup:    <No backup recorded>
File organization:  Sequential   <********* Note
File attributes:    Allocation: 3908, Extend: 0, Global buffer count: 0, No version limit
Record format:      Fixed length 512 byte records    <******* Note
Record attributes:  None    <********** Note
RMS attributes:     None
Journaling enabled: None
File protection:    System:RWED, Owner:RWED, Group:RE, World:RE
Access Cntrl List:  None

By "simple FITS" is meant a rectangular array of data - tables,
groups, and all the wondrous extensions developed after the
initial FITS "standard" are not supported.

The .IMG contains binary data in VAX byte order with pixels in
the same order that they are found on a standard FITS tape.
EXCEPTION: In the case of NAXIS=3 and NAXIS3>1, so the file may
be thought of as an array of two dimensional images, gaps are
left between images if necessary so that the first byte of an
image starts at the beginning of a 512 byte block. NAXIS>3 is not
defined in WFPC IDT format.

The .IMG file may contain I*1, I*2, I*4, R*4, or R*8 data.
The correspondence between data type and SIMPLE and BITPIX is as
follows

I*1	SIMPLE=T	BITPIX=8
I*2	SIMPLE=T	BITPIX=16
I*4	SIMPLE=T	BITPIX=32
R*4	SIMPLE=F	BITPIX=32
R*8	SIMPLE=F	BITPIX=64

Note that the I*1 data type is a signed data type in conflict
with FITS (it is FORTRAN BYTE or C char).

The lines in the .HDR file must be legal FITS lines, in legal
FITS order ending with END.

SIMPLE  =                    T / Required
BITPIX  =                   16 / Required
NAXIS   =                    2 / Required
NAXIS1  =                  800 / Required
NAXIS2  =                  800 / Required
BLANK   =               -32768 / For example
COMMENT A comment, etc.
HISTORY some history, etc.
FLOAT   =           1234.56789 / A floating point value
INTEGER =                  123 / an integer value
STRING  = 'CHARACTERS'         / a string value
END