Accurate pointing of the 14m antenna is accomplished through the use of a multi-parameter physical model of the structure and its response to gravity. This model is updated once or twice each year to account for any long term changes in the behavior of the structure. However, to allow for small displacements that may arise between pointing runs, the tracking programs recognize special constant offsets in AZ and EL which are typically updated by the observer through observations of standard sources in the course of an observing run. The APOINT program is typically used to determine AZ and EL pointing offset constants and the optimum focus setting of the subreflector. For a complete description of the APOINT program, see Chapter 4. For the APOINT program to work, the tracking programs must be in PCNORMAL mode. In this mode, the pointing offsets are entered using the AZPC and ELPC keywords. The APOINT program modifies these values during the course of its operation.
There is a mode in which users can enter offsets from the digiswitches on the control panel. To enable the digiswitches, the tracking programs must be in PCMANUAL mode. This is mode is strongly discouraged! !!! In PCNORM mode, the thumbwheel digiswitches have no effect on the pointing of the telescope. Check SCREEN 1 if you are in doubt about which mode you are in.