Fig. 7.12. Computed residual deformations (in mm) of the reflector from the best-fit paraboloid in horizon and zenith position, assuming a perfect setting at 50" elevation angle. The rms deformation is 55 mm.

Fig. 7.12. Computed residual deformations (in mm) of the reflector from the best-fit paraboloid in horizon and zenith position, assuming a perfect setting at 50" elevation angle. The rms deformation is 55 mm.

The residual structural deformation as function of elevation, the deviation from perfect homology, is illustrated in Fig. 7.12, where the surface error is shown for horizon and zenith pointing, whereby we assume that the surface was set to the theoretical paraboloid at elevation angle of 50 degrees (See Sec.7.2). This will be successful only if the surface can be measured and hence set at that angle. It was achieved for the MRT by holographic measurement of the surface at 22 GHz with the aid of a strong radio source at the appropriate elevation (Sec.6.1).

There remains to describe the methods used to control the influence of temperature variations. It is clear from a simple calculation, and experienced widely in practice, that the asymmetrical heating of the antenna structure by the Sun can cause large pointing errors and significant deterioration of the surface shape, often in the form of large-scale, comatic or astigmatic deformations. Thus it was clear from the onset that for the MRT these should be kept below 0.1 mm, which means controlling temperature gradients to about 1 K. The first measure was to cover the entire outside of the telescope with insulating panels of polyurethane (Fig. 7.11). Because the site was known to exhibit icing storms, heating elements were embedded in these panels, which are switched on during icing conditions. The concrete pedestal and the massive steel azimuth and elevation structures have a long thermal time constant and maintain a constant temperature to within one degree Celsius during a 24 hour period. Slow seasonal changes are allowed without influencing the performance of the telescope.

The situation is different with the reflector and BUS. During daytime, the Sun will often illuminate (part of) the reflector and despite a thin, white, heat-reflecting paint, a considerable heat flow through the panel must be expected. To keep this heat from entering the BUS volume, insulation was also placed between the panel surface and

Fig. 7.13. Change in focal length as function of the difference In temperature between BUS and yoke. The stable, linear relationship is routinely used in the focus control software.

the BUS space-frame. The connections from the panels to the BUS are invar, low heat-conducting adjusters. Calculations indicated that the thermal time constant of the BUS was only of the order of one hour and hence a significant daily thermal deformation of the BUS could be expected, despite the insulation around it. We also determined that a difference between the BUS and its supporting yoke-cone structure leads to a symmetrical deformation of the BUS and a concomitant change in focal length. This was later confirmed by measurement and shown in Fig. 7.13. Thus the BUS is kept at the temperature of the yoke by a system of regulated airflow, created by five large fans and a ducting system which creates a slowly circulating airflow to maintain a homogeneous temperature field. The air can be heated or cooled as required. The quadripod is also insulated and maintained at constant temperature by regulated liquid flow through a spiraling tube around the legs.

This system contributes significantly to the stable performance of the antenna. Over the years with long and careful measurements and analysis, some weak points have been identified and improvements were implemented. These are described in an interesting paper by Greve et al (2005). In particular it could be demonstrated that a Finite Element Model of the antenna, loaded with a certain temperature distribution, accurately predicts the resulting deformations. Thus it is now possible to predict by temperature measurements how the surface will deform and which corrections to the panel positions would be needed. Unfortunately, the MRT panels do not have motorised adjusters. But these deformations are normally large scale and correction should be possible by a deformable secondary or tertiary mirror - an example of adaptive optics applied to a radio telescope (Greve et al, 1996).

Summarising, the MRT has surpassed in all aspects the original specifications. For more information see Baars et al., 1987 and 1994. Over the 20 years of its operation the telescope has been improved in performance on the basis of long term measurements and analysis; examples were mentioned above. It is the most powerful instrument for the short millimeter wavelength range (100-350 GHz), but it is being challenged by the 50 m diameter Large Millimeter Telescope (LMT), which we will discuss below.

The MRT uses predominantly the traditional materials steel and aluminium with one exception: the secondary reflector of 2 m diameter employs carbon-fiber reinforced plastic (CFRP). This material was chosen primarily for its low weight, because of the need for a chopping secondary to suppress atmospheric fluctuations (see Sec. 6.3.5). A second great advantage is its very small coefficient of thermal expansion. The mirror was designed and fabricated by Dornier in the form of a composite structure of CFRP skins bonded to an aluminium honeycomb core. The specification of 25 mm could not be guaranteed by Dornier and we agreed to accept the mirror if it was not worse than 50 mm. The fabricator met the challenge magnificently and delivered a secondary with 15 mm rms surface quality. After 20 years of service and more than 5 million chopping cycles, there is no indication of any deterioration in the reflector.

We will now move on to the discussion of a telescope, in which the use of CFRP was unavoidable to satisfy the extreme specifications.

0 0

Post a comment