Rotator Drive
The rotator drive has two drive speeds. The point at which the drive changes form fast to slow, when driving to a new position, is determined by the Task Common variable TTS2F. This parameter may need to be adjusted occasionally. In order to ensure that the turntable does not spontaneously rotate when the telescope is moved (until now this has never been observed), the system also checks that its position angle is within an error TTERR of the requested one.
Encoder Zeropoint
For the IDS/FOS check that with the rotator nominally at
90 or 270 a star trailed in RA moves accurately along the slit.
A misalignment of 1 is easily detectable. In general, the rotator offset
is defined so that East-West (i.e. PA 90 or 270) is along
one of the detector axes, in particular the IDS slit.
An allowance can subsequently be made for any offset/rotation
between this and the A&G box coordinate system (see section
). Note that any
gross change in the rotator zeropoint requires a change in the
software limits. (If the zeropoint changes, the limits appear to
be at a different Position Angle.)