The data in this paper are published in the form of raw pixel coordinates which were determined by the centre-finding tool of the IRAF (Image Reduction and Analysis Facility) software. Publication of the data in this format will enable future investigators to make direct use of the original CCD images if they so wish.
We use 
to denote the column coordinate and 
to denote the row coordinate
in the CCD array. In the data tables, the satellite positions are given as 
pairs.
The orientation of the CCD device changes from one year to another, depending on how the device was mounted by the Observatory staff. However, one axis of the device is always aligned roughly north-south. We therefore use x to denote a direction parallel to the axis of the CCD which is nearest to the east-west direction, with x increasing eastwards (i.e. in the direction of increasing Right Ascension). Likewise, y is parallel to the axis of the CCD which is nearest to the north-south direction, with y increasing northwards (i.e. in the direction of increasing Declination).
Table 3 shows the approximate directions of the CCD coordinate axes in each
year, together with the transformation from 
to (x,y).
Table 3: Orientation of the CCD device and transformation from 
coordinates to (x,y)
Table 4: Calibration parameters for the observations
Let 
denote the position of satellite B
with respect to satellite A in raw pixel coordinates. Further, let 
denote
the scale of the CCD device in units of arc-seconds per pixel. This
assumes that the pixels are square; we have run calibration trials with
different scales in the 
and 
directions but have found no
significant difference between the two scales. Let 
denote
the position angle of the north-south axis of the CCD with respect to the
true pole of date, measured positive towards the East.
Table 5: Example transformations from pixel coordinates to true equator and equinox
of date
First calculate 
and 
according to Table 3.
Then the differential coordinates 
and 
referred to the true equator and
equinox of date are calculated from
and 
The parameters 
and 
have been determined empirically
for each observing campaign by comparing the observations with the
orbital theories of Harper & Taylor (1993, 1994) and determining
the values of 
and 
which yield the best fit between
the observations and the theories. Work is in progress to obtain independent
values of these parameters from observations of globular clusters and other
fiducial star fields (Jones 1996).
The values which we have adopted at present are given in Table 4. They are based upon measurements of the positions of Tethys, Dione, Rhea and Titan.
The 1990 data has been divided into three subsets. Set (a) covers 10-18 July and
represents the main observing programme in that year. Set (b) covers 26 July to
3 August, whilst set (c) covers 15-17 August. The CCD
detector was removed between sets (b) and (c) while a different instrument was in use.
In addition, it is not possible to guarantee that
the CCD was not rotated between sets (a) and (b); the difference
in the 
values for these sets is 
, which is eleven times
the standard error of set (a), suggesting that some alteration of the alignment may
have occurred.
In order to assist the reader in checking the validity of the formulae, we present four sample calculations in Table 5. In each case, the example is taken from the first image listed for the given year.