The structure of the 30-m telescope is made of steel and the design aimed from the beginning at the minimisation of its thermal deformations. For this purpose the antenna steel structure is covered with thermal insulation. In addition, the Reflector Backup Structure (BUS) is equipped with an active cooling/heating thermal control system (TCS) maintaining the temperature homogeneity of the BUS within 1 K.
The initial TCS controls only the temperature of the BUS. Five ventilators apply a tangential air circulation with a heating/cooling capacity of 30/22 kW. A temperature sensor installed in the upper part of the Yoke, below the membrane, measures the reference temperature to which the temperature of the BUS and the feed legs is slaved. The initial TCS does not apply temperature control to the Yoke and the Counter Weights. During the past years, 174 temperature sensors were installed in the BUS, Yoke and Counter Weights and also in the TCS fluids of the feed legs in order to monitor the temperature of the whole antenna. In a further step, a Finite Element Model (FEM) was developed to interpret the measured antenna temperatures with respect to the antenna performance. The predictions of the FEM have been confirmed from dedicated focus and holography measurements. The FEM permits a decomposition of the calculated temperature-induced reflector surface deformations into large-scale contributions represented in Zernike polynomials; from these we found that the dominant components of reflector surface deformations are astigmatism (of amplitude ), and 3rd order defocus (of amplitude ). It was recommended to improve the thermal homogeneity of the upper part of the Yoke, just below the membrane.
An independent forced ventilation system, consisting of 4 ventilators moving
the air in a circular way, was installed in the upper part of the Yoke in
autumn 1999. The temperature of this part of the telescope structure was
improved. Nevertheless, the reflector astigmatism still showed a permanent
high value though with less variation than previously observed, which was
due to temperature inhomogeneities of the yoke.
When using the FEM to analyse the reflector astigmatism versus the antenna temperature distribution for a long period of time we noticed that occasionally the amplitude of the astigmatism is small. This situation occurs when the air temperature drops rapidly due to some particular meteorological condition. The BUS and Yoke temperatures decrease faster than the temperature of the Counter Weights due to their lower inertia, and at a certain moment the temperatures of these three components are equal. At this moment the astigmatism disappears.
According to the FEM a certain temperature gradient in the Counter Weights has a smaller effect than a similar gradient in the Yoke. On the other hand, the temperature difference between the Counter Weights and the reference temperature is much higher than the temperature difference between the Yoke and the reference temperature. We decided to modify the TCS by applying heat to the Counter Weights in order to reproduce the noticed condition of small astigmatism, i.e. of equal temperatures in the BUS, Yoke and Counter Weights.
In September of this year (during the yearly maintenance) we have made the following changes in the antenna TCS (Figure 5):
The new TCS operates since 17 October 2002. We have analysed the data of 5 days with clear sky, low wind, and with the antenna observing all the time (last VLBI session) to understand its behaviour. Results are shown in Figures 8 and 9 which illustrate the improvement reached with the new TCS. Besides the 5 days analysed so far, the new TCS is working well since its beginning of operation.
Figure 10 is similar to Figure 9 but including two more days. Between day 29.5 - 30.5 the new TCS was intentionally switched back to the initial TCS to facilitate the comparison of the performances.
With the operation of the new TCS the thermal behaviour of the antenna has
improved. Several more months of data must confirm the preliminary results.
However there is no reason to expect a degradation of the new TCS after
working well under different meteorological conditions during 4 weeks. The
goal to keep the antenna from the Reflector Backup Structure to the Counter
Weight on the same temperature has been reached and so far the antenna
shows a more stable thermal behaviour. Under this improved condition a new
efficiency measurement as well as holography should be considered in the