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For easy target acquisition it is important to determine the optical centre of LDSS-2 in the LEXT coordinate system. By convention this is taken as the position of the centre hole in the matrix mask (Section 1.2). To determine this position, take a direct exposure through the mask with a calibration light source and then use the CURSOR/GFIT command in LEXT. Though it is possible to use the DMS or Sparcstation to determine the LDSS-2 centre, it is then necessary to convert this to the coordinates as measured by LEXT - they are not the same and will depend on the options used with the LEXT READ command.