The gravity deformations in the Compact Array antennas
The Compact Array at Narrabri is being upgraded to operate at millimetre wavelengths. As part of this upgrade, we have initiated an effort to understand the deformations, due to gravity, in the antenna structure with elevation. The plan is to measure the structural gravity deformations and relate these to the elevation dependence of the antenna gain and far-field beam patterns. We will then design and implement a scheme that will make elevation dependent corrections to the wave front and consequently recover the gain loss and avoid coma lobes.
After the first mm receivers were installed on Compact Array antennas, we began making measurements of the gain-elevation curves and of the antenna far-field beam patterns. It was soon clear that at 3 mm, the antenna forward gain dropped off at low elevations and also at high elevations. The beam patterns showed a pronounced coma lobe at low elevations and high side-lobes at high elevations. Gravity deformations were, clearly, degrading the antenna performance at elevations away from the intermediate values at which the optics had been aligned and the panels of the main reflector had been set.
In February 2002, Danny Brizzi and Harry Hanley from Vision Metrology Services Unit, University of Melbourne, brought a new measurement technique to Narrabri: they brought a V-STARS photogrammetry system consisting of retro-reflective targets, a high-resolution digital camera and a notebook computer with software. A total of about 600 targets were stuck on the Compact Array antenna CA02; they were placed over the main reflector, the sub-reflector, its support cage and quadripod structure, and on a frame fixed to the feed turret. With the antenna set at elevations 90, 75, 60, 45, 30 and 15 degrees, about 100 images were taken of the antenna from atop a Cherry-Picker while the antenna rotated in azimuth. The software then provided relative positions of all the targets, at each elevation, with 30-50 mm accuracy. Their measurement report is available as AT Technical report AT 39.3/109. Anyone who visited the Compact Array site at Narrabri after February 2002 might have noticed "spots", which are the retro-reflectors, all over the main reflector and other structural members of CA02. If you have not done so already, go over to CA02 after dark and shine a torch or headlights at the antenna when it is tipped.
The gravity deformation of the main reflector surface was described using polynomial surfaces at the different survey elevations; the coefficients were obtained via fits to the deviations measured at the target locations. Viewed face-on, the deformation of the main reflector from the design shape is such that at low elevations, the outer panels at the top of the antenna deform forwards whereas at the bottom the surface is displaced backwards. This results in a large coma lobe to the south of the main lobe. At high elevations, the displacements are reversed and the top parts of the surface deform backwards while the bottom part deforms forwards. Higher-order deformations are also observed and are, presumably, related to the form of the backup structure. The gravity displacements perpendicular to the local surface are shown in Figure 1 as surface plots.
Figure 1: Gravity displacement perpendicular to the local surface. The antenna is viewed face-on in these plots and the top end of the antenna face is at the top of the plots. The panels are for elevations 90 (top), 75, 30 and 15 (bottom) degrees (60 degrees was selected as the reference elevation; the panels show the displacement relative to the 60-degree surface). The rms surface displacement is 0.4 mm at 90-degrees elevation and 0.5 mm at 15-degrees elevation.
As the antenna tips to low elevations, the sub-reflector moves 0.3 mm axially away from the main reflector; it is displaced by 0.4 mm parallel to the aperture plane, towards the bottom of the antenna, and rotates through 0.03 degrees about an axis parallel to the elevation axis. The movements/rotations with elevation that we infer are tiny: as compared to the 7-m distance between the sub-reflector and main reflector, these movements are a few parts in 105 and are within the accuracy of the photogrammetry. We subtracted the measured displacements of the sub-reflector with respect to the main reflector from the displacements of the top of the quadripod to obtain the displacement of the sub-reflector with respect to the quadripod: this was independently measured with transducers and the concordance confirms the accuracy of the photogrammetry!
The displacement of the feed, as the antenna is tipped in elevation, is perpendicular to the elevation axis and downhill parallel to the antenna aperture plane: the total displacement is about 2.3 mm over the entire elevation range.
We have analyzed the effects of these deformations and displacements via geometric optics computations at 90 GHz. The dominant cause of gain loss is the gravity deformation of the main reflector; the measured movements of the sub-reflector and feed are, on their own, not expected to cause a gain loss exceeding 1%. Assuming that we align the optics and set the panels at 60-degrees elevation, the expected gain loss from all of the displacements and deformations together is about 7% at high elevations and 30% at low elevations; this is similar to the observed gain loss. The computations also predict a significant coma lobe at low elevations and high side-lobes at high elevations; these are also observed in measurements at 3 mm. The analysis is in AT 39.3/115.
A study of the effects of tilts and displacements in the feed and sub-reflector (AT 39.3/113) had related the movements to aperture-plane phase patterns. The gravity deformation is also essentially an error pattern in the aperture phase. Analysis (whose details are in AT 39.3/117) has found that the loss in power gain arising from gravity deformations may be recovered, to within 2 - 3% of the optimum value, by elevation dependent (1) axial repositioning of the sub-reflector or the feed, and (2) either one of the following: lateral displacement of the feed, lateral displacement of the sub-reflector, or a tilt to the sub-reflector.
We have decided to modify the sub-reflector support structure on the compact array antennas so that the sub-reflector may be tilted, through precise angles and under computer control, about an axis parallel to the elevation axis. We are implementing a scheme where a tilt of the sub-reflector together with an axial repositioning would correct for the gravity dependent optics deformations.
For those interested in details, the AT technical documents referred to herein are available on the web at www.atnf.csiro.au/observers/memos/index.html
Ravi Subrahmanyan, Mike Kesteven, Clive Murphy and Barry Parsons
(Ravi.Subrahmanyan@csiro.au)