The telescope subreflector is positioned according to an algorithm that has
been determined through a series of RF measurements on the LES-8 and LES-9
satellites. This algorithm accounts for the basic average motion of the
subreflector with respect to the main reflector, but it does not account for
possible time or frequency dependence of the subreflector position. These are
accounted for in normal operation by introducing small offsets in the
subreflector position and tilt from the nominal model.
The subreflector has four degrees of freedom under the control of the
observer. The radial position of the subreflector with respect to the main
reflector is defined to be the FOCUS coordinate and offsets from the nominal
subreflector model in the radial direction may be
made with the FOCUS command.
To enter a FOCUS offset of 1mm, for example, the observer types:
FOCUS 1
Typically, observers need only alter the FOCUS during observations since our
experience shows that most variations in antenna performance are caused by
changes in the focus of the antenna. The APOINT program (see Chapter 4)
is typically used to find the best focus from a pointing measurement of an
SiO maser or planet.
!!! It is recommended that observers check the focus position of the subreflector often during their run, especially during times of rapid temperature variation, such as sunrise and sunset.
The three remaining subreflector degrees of freedom are called AZSB, ELSB, and YAXIS. These correspond to tilts in the subreflector in the azimuth direction (AZSB) and elevation (ELSB) directions and to a vertical translation of the subreflector (YAXIS). These values are typically not changed by normal observers, but they are occasionally checked and updated by the observatory staff.
!!! Please do not make changes to the AZSB, ELSB, or YAXIS position unless you know what you are doing and alert subsequent observers to your action. Altering the tilt and vertical position of the subreflector will introduce large pointing shifts.
The current values of the subreflector offset position may be viewed by entering the SUBREFLECTOR command. The actual position of the subreflector including the nominal model and all offsets is shown on SCREEN 1. truein