History of the Upgrade
The surface of the main parabolic reflector of the 26-m Hartbeesthoek radio telescope comprises 252 panels. The panels installed in the 1968 had 1-cm diameter perforations, which prevented the telescope being used at wavelengths below about 2 cm, or a frequency of 15 GHz. In addition, the rms error over the surface was about 2.5 mm, which similarly prevented useful operation above that limit. A programme to replace the surface panels with solid panels with a lower surface error began in 1998. After two years of developing panel-making techniques, production of new panels got under way in 2000 and installation began in April of that year (~JD2451643). The last panel was installed on 2003 September 10 (JD 2452893).
Alignment of the panels using theodolite and tape measure was initially carried out during the day, culminating in a night alignment session on 2004 September 02-03 (JD 2453250.5). The subreflector was realigned on 2004 October 8-10 (JD 2453288.5). After measurements showed that the subreflector was level east-west to 1mm over its diameter, the four main surface panels over the alignment points were reinstalled on 2004 October 15 (JD 2453294.0).
The upgrades to the telescope have caused its mass distribution to change, and for it to become top heavy. The out-of-balance force was measured in 2005 March. Lead counterweights totalling about three tonnes were manufactured. The first set were installed on 2005 April 14 (day 104, JD 2453475). Welding of cracks in the counterweight structure was carried on 2005 April 21/22 (day 111/112, JD 2453482 / 2453483) and 2005 April 25/26 (day 115/116, JD 2453486 / 2453487). The second set of counterweights were then installed on 2005 May 6 (day 126, JD 2453486).
For calibration of microwave holographic measurements of the surface alignment, four panels were offset vertically from their nominal positions on 2005 July 18 (JD 2453569.8). A ring 1 panel was offset -3 mm, a ring 2 panel by +3 mm, a ring 4 panel by +6 mm and a ring 5 panel by -6 mm.
The rebalancing of the telescope carried out in 2005 April – May meant that the telescope pointing map in use since the early 1990’s was significantly in error. Observations of pointing calibrators, initially using the 13cm receiver and then the 3.5cm Dicke-switched system, were applied to the pointing model of W E Himwich, as described in the MKIV VLBI Field System Documentation. The new model was made operational on 2005 September 12 (day 255, JD 2453626.08).
Measuring the Improvement in Efficiency
Tests of the aperture efficiency of the telescope at 12.2 GHz after alignment of the surface panels using the theodolite and tape measure indicate that this has improved from 0.22 pre-upgrade to approximately 0.50. This implies that the efficiency is essentially the same in all frequency bands.
The aperture efficiency was measured by scans through the calibrator sources 3C123 and 3C218 (Ott et al. 1994, Astronomy & Astrophysics, 284, 331). The change in aperture efficiency from before to after the upgrade as a function of frequency is shown above.
The measured efficiency at 8.5 GHz after the upgrade is lower than the maximum possible owing to the relatively large offset of the feed from the axis of the telescope. The 1.6 GHz feed is also far off axis, but in this case the offset was compensated for by tilting the subreflector through half the feed offset angle. However this was not done for the 8.5 GHz data shown in the graph.
Steps in the upgrade process
Last new panel installed – 2003 September 10
Rings 5 and 6 complete – 2002 October 15
Rings 3 and 4 complete – 2001 October 15
Ring 2 complete – 2001 March 30.
The Panel Manufacturing Process
Steps in the production process are illustrated below.
Click on the images for more detailed views.
Removing panels from the telescope
Cleaning the telescope structure
Welding new ribs into the frames
Cutting out the new solid surface panels
Attaching the frame to the solid surface panel
Painting the panels
Installing the panels
Last Updated on March 20, 2023