July 26, 2006
ALMA Pointing
21
Accelerometer Measurements of BUS Pointing Errors
(cont.)
±For
stationary pointing conditions under high wind loads the BUS rim wind shake is
dominated by elevation motion for both antennas.
±For
sidereal tracking during low wind conditions the tracking jitter of both antennas
is largely due to elevation motion, with large contributions in the 3-6 Hz
range. The largest jitter is
observed at low elevation, while minimum tracking jitter is seen while
crossing the meridian.
±For BUS
pointing stability during fast (0.5 deg/s) OTF scanning:
°VertexRSI performance is strongly dependent upon the
(Az,El) at which the scan is performed.
This is reflected in the standard deviation for this measurement.
°AEC performance is affected by apex rotation feedback
into the BUS motion. This apex
motion died out after about 1 minute in most cases, but also caused azimuth drive
shutdown under certain conditions.
The spread of 0.3 arcsec (1s) reflects this variable pointing stability.
±For the
AEC antenna the contribution due to apex motion to all pointing measurements
has been investigated, but not included in the results presented. Accelerometer measurements could not
properly distinguish between subreflector translation, which impacts pointing,
and subreflector on-axis rotation, which has no affect on pointing.