Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.atnf.csiro.au/projects/mnrf1996/technical_memos/subreflector.ps Äàòà èçìåíåíèÿ: Tue Aug 19 06:03:16 1997 Äàòà èíäåêñèðîâàíèÿ: Sun Dec 23 11:31:58 2007 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: arp 220

Mechanical Stability of the AT Subreflectors
M.Kesteven and B.Parsons
August 19, 1997
at doc : AT/39.3/070
This note describes some recent investigations into the mechanical stability of the subreflectors on the
AT antennas. These experiments followed the movement of the subreflector as the antenna tipped
from the zenith to the horizon. We find, relative to a coordinate frame attached to the receiver
platform :
ffl The axial movement (axial defocussing) is 0.7mm
ffl The lateral movement, (in the focal plane, normal to the elevation axis) is 2mm, and no move­
ment parallel to the elevation axis.
ffl A rotation of 3 arcminutes about the elevation axis.
These results are based on five experiments. The coordinate frame is attached to the feed rotator; the
z­axis is normal to the vertex room floor (positive is up), the x­axis is parallel to the elevation axis.
``North'' is at positive y, ``East'' is positive x. The experimental setups are sketched in figure 1.
1. A telescope attached to the feed rotator tracked a target at the subreflector vertex. This gave
the movement in the focal plane; the results are shown in figure 2.
2. A laser was attached to the feed rotator; its beam was reflected off a mirror attached to the
subreflector vertex back to the vertex. The movement of the laser spot was tracked. This gave
the rotation of the subreflector, shown in figure 3.
3. A laser was installed at the subreflector vertex, directed to the feed rotator. The laser spot was
tracked. This experiment was a control on the previous two, as the movement is the sum of the
reflector translation and the rotation. The closure is well within the measurement errors of ¸
0.2mm.
4. Two thin wires were attached to the subreflector rim at the highest and lowest points; the wires
were kept at constant tension. The movement of markers fixed to the wires relative to a scale on
the vertex room roof was tracked. This allowed us to determine the axial movement (figure 4)
and the subreflector rotation This is shown in figure 3, as the trace labelled ``CA03''.
5. A thin wire at constant tension was attached to the subreflector rim at the East point. This
measured the axial movement.
The first three experiments form a consistent set (within the measurement errors of 0.2mm). The last
two experiments are consistent within their measurement errors of 0.1mm. The rotation results are
1

#1 #2 #3
#4 #5
Figure 1: Organization of the five experiments
not as well defined over the entire set, with the optically determined values about 30 % higher than
the mechanical values. Since the optical and mechanical observations were made on different antennas
(Narrabri antennas CA01 for the optical, CA03 for the mechanical) one possibility is that there are
antenna­to­antenna variations. We note that the stretched cables supporting the subreflectors were
not adjusted to any rigorous standard, so that it is likely that the tensions vary vary from antenna to
antenna. It might be prudent to develop a scheme to ensure that the tension in the cables is consistent
over all the antennas.
Discussion
The optical measurements were made on a Narrabri antenna (CA01) and the antenna at Mopra. The
stretched wire experiment was done on antenna CA03, Narrabri.
These results are encouraging as they suggest that the mm operations may not require active sub­
reflector control. It must be recognised, however, that the deformation of the structure has yet to
be factored into the debate ­ we don't know whether the reflector's focus shifts as the antenna tips.
Preliminary 22 GHz observations (R. Gough) show that the axial focus shift is less than 1mm, which
indicates that the structure's contribution is small. Detailed experiments to measure the structural
deformation are planned.
file: /epp/source1/mjk/subreflector/txt/subreflector.tex
2

­1.5
­1
­0.5
0
0.5
1
1.5
0 20 40 60 80 100
Elevation (degrees)
(mm)
CA01 down
CA01 up
Mopra down
Mopra up
Figure 2: The lateral subreflector shift in the y­axis direction. Postive is ``North'' (ie. uphill)
­2
­1.5
­1
­0.5
0
0.5
1
1.5
2
2.5
0 10 20 30 40 50 60 70 80 90
Elevation (degrees)
(arc
minutes) CA01
Mopra
CA03
Figure 3: The rotation of the subreflector about the x­axis. Positive is counter­clockwise when viewed
looking ``East''.
3

­0.2
­0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0 10 20 30 40 50 60 70 80 90
Elevation (degrees)
(mm)
Figure 4: The axial subreflector shift. Postive is away from the vertex
4