  In order to fully exploit the physics potential of the ILC, it will be
necessary to measure (and control) beam parameters to a permille level
precision. In case of the beam polarisation, this can only be achieved with
dedicated high energy Compton polarimeters and by improving the detector
linearity, as well as the calibration of the analyzing power.
  This note summarises results of an early testbeam period with the Cherenkov
detector of the SLD polarimeter, linearity measurements of readout electronics
and photodetectors and compares simulation results of the SLD Cherenkov
detector with those of a new `U-shaped' prototype.
