Platform tilt when tiedown tracking on
The average height of the platform is kept
using the distomats (laser rangers) and the tiedowns. Every two minutes
the distomats remeasure the average height of the platform. The
will then pull or release evenly on all 3 corners of the platform to
the height close to optimal. The tiedown motion is a gradual one that
45 seconds to accelerate up, coast, and then decelerate. The tiedowns
moving in program track mode using a 24 bit optical encoder that is
to fractions of a mil.
During some of the pointing runs there were source
tracks that had jumps and then recoveries in the pointing errors.
One possibility was that the tiedowns could have been tilting the
as they moved from one position to the next. Maybe the cables were
differently. Data was taken on 29jul02 with the azimuth at 242.87
and the dome at 10 degrees za. The tiedowns made two motions of .2
and .16 inches while the tilt sensors where running.
The plot shows
the pitch and roll of the tilt sensors during the tiedown motion (.ps)
The top figure is the relative pitch of the dome (i
did not put it on the absolute scale determined by the theodolite).
The bottom figure is the relative roll of the dome.
Positive roll is clockwise looking uphill.
The red line is the motion of the tiedowns during this period. The
tilt is not affected by the tiedown motion so they must be moving at
same rate. There is a small downward drift in the roll over the entire
period of time. The oscillations at the end of the measurement is me
onto the turret floor to disable the tilt sensors (maybe i need to go
From previous computations, a pitch error of .01 degrees at za=10 is
a pointing error of 4.3 arc seconds (assuming you have pulled the
back down into focus). The motions we see in the tilt sensors
map into small pointing errors.
The jumps that were seen in the source tracks were
probably caused by the gregorian wheels sliding across the rolling
(since the side rollers are not tight).