Scanning mirror position system

A radiation scanning system comprises a detector 12 which forms part of an assembly 11 maintained at a temperature substantially below ambient, an imaging means 18 for forming an image of radiation from a field of view, a scan member 15 and a relay lens 14 arranged to scan an image of the detector 12 across the radiation image and an arrangement for applying a characteristic radiation feature to the detector 12 at a selected part of the detector image locus 19 lying outside the normal scanned field of the system comprises a mirror 20 arranged to reflect radiation from the cooled assembly 11 onto the detector 12.