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The commands for controlling the telescope are detailed in the WHT manual; and any of these commands except use of the handset can be issued from the ICL interface instead of the Telescope Control System (TCS) interface. If these commands are typed at the ICL interface the telescope D-task relays them to the TCS, and they are echoed on the TCS control terminal. The position information is input to the TCS by one of the methods described in the WHT manual. If a particular sky position angle is required, for an extended object or for two or more objects in a line this is specified by the TCS command ROT SKY <position angle>. When observing point sources for which the sky position angle is not important, the command ROT VFLOAT will ensure that the sky position angle is set to the parallactic angle when the object is acquired. If spectrophotometric accuracy is of prime importance, and the observer does not wish to autoguide, the command ROT VERTICAL will put the slit vertical and leave it there, i.e. the rotator will not track. The sky position angle and the parallactic angle will then always coincide. All position angles are measured in the conventional sense: from North through East.
If the observer has very precise positions, and the pointing of the telescope has been calibrated correctly with the TCS CALIBRATE procedure then the objects should appear close to the Cassegrain rotator centre. In this case it is best to define an aperture with the TCS ENTER APERTURE command, and move the objects directly into the slit with the TCS APERTURE command.
It is normally best to acquire directly onto the slit at a scale of 4.5 arcsec/mm. If the TV focus has been set correctly it should be possible to check the focus on the slit if the object is bright enough. If there are two objects which are further apart than the TV field of view at this scale it may be necessary to go to 12 arcsec/mm to get both onto the slit. When acquiring more than one object on the slit it is best to use the TCS TWEAK command, which will allow small adjustments of position and position angle, whether the telescope is autoguiding or not.