The Engineering mode serves to move the telescope hardware without using the computer. This is done by technical staff to move the telescope for such jobs as top end changes or by the Telescope Operator for the initial setting up at the start of the night. Engineering Mode is the default mode on powering up the system; telescope control is from the Engineering Rack in the control room, placed conveniently close to a window from which the telescope can be seen. A blind is drawn over this window when the telescope is under computer control.
The telescope is protected from being driven outside a safe operating
regime by the use of
limit switches. When under computer control, the telescope is normally
protected from running into these limit switches by the software. If,
however, due to exceptional circumstances or faults (e.g. CAMAC or
encoder failure), the telescope runs into a
limit switch when under computer control, that control is lost completely.
Then, the telescope can only be driven out of the switches by changing to
the Engineering Mode and in all likelihood the zeropoint of the encoders
will be lost. Therefore, the encoders will have to be re-initialized
(section 
).