PDS_VERSION_ID         = PDS3
RECORD_TYPE            = STREAM
OBJECT                 = TEXT
  PUBLICATION_DATE     = 2004-12-07
  INTERCHANGE_FORMAT   = ASCII
  NOTE                 = "Description of offset directory contents."
END_OBJECT             = TEXT
END
   
Offset directory:
-----------------

The offset directory contains shutter offset information for each camera as
a function of temperature. 

Shutter Offset Files

The actual exposure time differs slightly from the commanded shutter time 
because the shutter is mechanically activated.  This difference is called 
the shutter-offset. Because the shutter blades travel in a horizontal 
direction, the shutter-offset is image sample dependent.

T(s) = t - t0(s)

where 
s is the sample number
T is the true exposure time
t is the commanded exposure time
t0 is the shutter-offset

The shutter-offset files are VAX-VMS REAL*4 data format in VICAR format. 
There is one line of 1024 samples representing the shutter-offset for each 
sample. The values have the units of milliseconds. The VICAR labels contain 
the characteristics of the image sequence that generated each shutter-offset 
file and are based upon those of one of the input image files. However, the 
table below also correlates filename with characteristics.

Optics
Temp    Camera   Filename

-10 C   NAC      nacfm_so_m10.img
+5 C    NAC      nacfm_so_p5.img
+25 C   NAC      nacfm_so_p25.img

-10 C   WAC      wacfm_so_m10.img
+5 C    WAC      wacfm_so_p5.img
+25 C   WAC      wacfm_so_p25.img