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When TAURUS is used with the f/4 camera, a bright ghost image is visible of the telescope pupil. This is the result of a reflection between the back of the last element in the camera and the front of the CCD cryostat, imaging the telescope pupil onto the detector with the intensity reduced to a few tenths of one per cent. The ghost image is easily recognisable, since the shadow of the secondary mirror and its support structure are clearly visible. the ghost image can be flatfielded out, and it is hoped in the future that the f/4 camera will be modified to remove it entirely.
In the meantime, the intensity of the ghost can be reduced by using the iris to reduce the fieldsize to that required for a particular observation, rather than just letting the fieldsize be determined by the size of the chip.
Note that the f/2.1 camera does not suffer from this problem, and so this camera should be used if high spatial resolution is not important.