Thanks David, I appreciate your comments.
After entering the coordinates and slewing to the target, the left-hand object was centred in the field of view in a series of 20 sec test exposures and the right-hand object was moving off screen to the right.
So, I assume that the acquired target was the Ariane Rocket Booster, and the right-hand object was the now separated JUICE spacecraft?
So much easier using the Horizons website than trying to do the calculations via slide rule, or worse still, my old Logarithmic Table.
Cheers
Dennis