An 80cm, three mirror telescope provides a wide, flat field of view (see for example Korsch 1977), and by re-imaging the entrance pupil, a Lyot stop is used to reduce the scattered light. The star field is imaged onto a mosaic of CCD detectors. A mosaic provides easy coverage of the wide field and ensures that there are no single point failure modes, as should a detector fail or become noisy, only a fraction of the data is lost. The image is blurred on the detector by a combination of defocus and controlled aberration of the telescope. This serves two purposes: firstly the blur spreads the image of the star over many pixels (a few hundred) so as not to saturate any single pixel which has a full well depth of about 5105 e-. Secondly the spread reduces the sensitivity of the instrument to pointing jitter, as we average over the inhomogeneity of many pixels (see equation 3.4).