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The ultimate aim is to quantify the motion of the telescope beam in two
coordinates, azimuth and elevation. These motions are functions of the
yaw, roll and pitch of the elevation axis, which are caused by
the motion of the wheels over the track.
The yaw, roll and pitch of the elevation axis are stored as (F1,F2,F3)
in
the track model MODEL1, and the following relations apply between them
and the pointing errors, daz and del (see MTUN005 and MTUN025) :
del = pitch
daz = roll * sin(el) + yaw * cos(el)
Roll, pitch and yaw are assumed determinable from suitable inclinometry
measures :
Roll would seem measureable directly by placing an inclinometer
on the elevation axis, parallel to the axis. Unfortunately, the elevation
axis does not exist as
a solid object : the elevation bearings are real enough, but the space between
comprises the receiver cabin. To date we have assumed that the bearings,
the cabin, and the TMU platform, which lies on the axis, move as if part of
a rigid whole, and we have placed an inclinometer on the TMU platform in order
to measure roll. The relationship between the measured tilt and roll
is assumed to be 1:1.
Pitch is similarly measured by placing an inclinometer on the TMU
platform perpendicular to the elevation axis and operating the telescope
with the SERVO system ON. The relationship between the
measured tilt and pitch is also assumed to be 1:1.
Yaw is not measureable directly using inclinometers. Yaw is the
motion of the elevation axis in the horizontal plane, which we assume to
be the sum of the components in the horizontal plane of the motions of the
tops of the A-frames. We further assume that these are determinable from the
motions of the two A-frames, which we also assume, initially, to be rigid.
We assume further that inclinometers placed in the middle of the bottom
beams of the A-frames measure the rotation of the entire, rigid, A-frames.
The conversion of these measured rotations to yaw depends upon the
geometry of the antenna and assumptions concerning its rigidity. The
geometry is a matter of specifying a few key dimensions :
The distance between wheels on the same A-frame = 7320 mm.
The height of the elevation bearing above the wheels = 8150 mm.
The length of the elevation axis = 8000 mm.
With these dimensions and assumptions
a 1" rotation of the A-frame causes the top of the A-frame (the
end of the elevation axis) to move 17.7
m in the vertical plane, and
39.5
m in the horizontal plane. The angular effects upon the elevation
axis are then 0.46" of roll and 1.02" of yaw, respectively. At any
azimuth the total roll and yaw are the additions of the contributions from
the two A-frames, with some appropriate sign convention.
Next: Application within the
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Previous: The real telescope
Iain Coulson
Last update : Fri 05 Nov 2004