The main problem when observing the major satellites of Saturn is the proximity of the planet and its rings. It is almost always impossible to exclude the planet and rings from the field of view if one wishes to obtain images of the satellites interior to Rhea. In order to reduce the brightness of the planet, we employed a Gunn Z filter which has a central wavelength of 930 nm and a full width at half maximum of 150 nm. This coincides with the wavelength of methane in the atmosphere of Saturn and hence it reduces the brightness of the globe of the planet. It cannot, of course, affect the rings since these shine with the spectrum of reflected sunlight.
We used an exposure time of 4 seconds. The images of the satellites were circular and well-defined when seen at some distance from the planet, and hence it was possible to use IRAF to measure their centres reliably. Nevertheless, the planet and rings were always saturated. This meant that it was impossible to measure the positions of any satellites which lay close to the planet or to the ansae of the rings.
As another consequence of the over-exposure of the planet and rings, we were unable to measure the positions of the satellites with respect to the centre of the planet. We therefore analysed the observations as positions of one satellite relative to another. This required that there must always be at least two measurable satellite images in each CCD frame.