The major mechanical modification has been the removal of the plateholder and replacement by an adapter ring to which the cryostat can be clamped. The ring incorporates the standard clamping and location arrangements common to RGO cameras. The ring is held onto the casing via 7 x M6 studs with nuts and washers. The ring is precisely located on the surface of the casing by three metal pads permanently bolted to the casing. No attempt should be made to disturb the pad locations.
The cryostat can be rotated about the optical centre in order to align the CCD pixels relative to the dispersion direction. The 7 x M6 nuts, securing the ring, should be loosened and fine control of rotation is achieved using a micrometer tangent screw included in the modification. The radius on which the tangent screw operates is 14l,mm.
The shutter mounted within the casing adjacent the slit has been removed because it was not compatible with the CCD control system. Also the shutter limited grating rotation. A different shutter (Compur model 141) has been mounted within the filter spacer ``above'' the slit. This location is not ideal because, when the shutter is ``shut'', the slit can not be viewed, but the physical size of the unit restricted mounting at any other location. The shutter is interfaced for control by the CCD software and operates from fully open to closed within 4 msecs. The shutter is not permanently allocated to the spectrograph but it is now necessary for normal operations.