Alternative Reflector Geometries

Abstract

It is increasingly difficult and costly to increase the diameter of a reflector that is attached to a two-axis mount and maintains its accuracy in all attitude angles. After the completion of the Jodrell Bank giant, several ideas for a telescope of comparable or larger size, but very much cheaper, sprang up among radio astronomers and engineers. We shortly mention here the major examples.

Addendum: Remarks on Wheel-On-Track Systems

Wheel-on-track systems for the azimuth movement are a feature of large radio telescopes. Nowhere else in the field of wheel-on-track applications are such huge loads per wheel applied as in radio telescopes. Examples are 100 tons per wheel for the Effelsberg telescope and 475 tons per wheel for the Green Bank telescope. The only somewhat similar application in other technical areas exists in the field of hydraulic steel constructions, but the requirements there are very different in regard to accuracy and life cycle loads. The wheel-on-track system of a radio telescope is an important and sensitive subsystem with an impact on reliability and performance. We summarise some of its essential features.

The first huge radio telescope in Jodrell Bank used for its azimuth axis system the best available technology, a wheel-on-track system based on standard railway tracks. The problem of the large loads was solved by a large number of wheels on bogies, and by more than one track (in the final construction status three), and by two railway rails per track. The rails were mounted discontinuously in segments with bolts and gusset plates connecting the joints, as was usual in railway technology. The system has worked well over more than 50 years with adequate maintenance by the Jodrell Bank crew.

The main issues for the design of a wheel-on-track system are the optimisation of the rolling behaviour of the wheels on the track and the handling of the high stresses in the contact area between the wheels and the track. Both issues have a big impact on pointing performance and lifetime of the telescope.

A number of bogies normally located at the four or six corners of the alidade transfer the weight of the telescope via the track to the foundation. For the relatively small telescopes, only one wheel per alidade corner may be sufficient (e.g. Dwingeloo, see Chap. 3). With large telescopes, the contact stress problem forces the use of two or more wheels per corner, and the load distribution between the wheels is achieved by sequentially arranged levers (Fig. 7.20). The very large telescopes have four (Green Bank) or even eight (Effelsberg) wheels per corner.

Fig. 7.20

The lever systems for the bogies of Effelsberg and Green Bank telescopes

The rolling behaviour of a wheel on the track depends on two angles of the wheel axis; the steering angle determines the direction of the rolling movement and the chamfer angle determines the curvature of the movement. For an accurate rolling behaviour, both angles have to be precisely aligned during assembly and commissioning.

For fine alignment of the steering and the chamfer angles, it is important to understand their influence on the rolling behaviour of the wheels. A misalignment of the steering angle causes a skew symmetric runout of the wheels on the track (Fig. 7.21 top), while a misalignment of the chamfer angle causes a symmetric runout of the wheels (Fig. 7.21 bottom). This different behaviour can be easily identified during commissioning of the drive system of the azimuth axis, for instance via dial gauges between the bogie body and the load spreader .

Fig. 7.21

Influence of misalignment of the bogies on the rolling behaviour. Top—steering, bottom—chamfer

Two different design approaches for the wheel axis alignment evolved in the advancement of radio telescopes, one with self-aligning features for the chamfer angle (developed by JPL for the Deep Space antennas and used also for the GBT ) and the other with alignment levers for both angles with some similarities to the steering mechanisms of car wheels (developed by MT Mechatronics for the LMT (Chap. 5) and the Sardinia Radio Telescope . The Effelsberg bogies use a third alignment concept, based on sliding blocks that can be interpreted as a predecessor of the alignment levers.

The self-aligning features of the GBT bogies (Fig. 7.22 left) are achieved by triangular flexures, which separate the body of the bogie from the load spreader. A misalignment of the wheels against the track, caused e.g. by local inaccuracies of the track surface, is compensated by a bending of the triangular flexures. This bending causes a reaction moment in the contact area of the wheel on the track and a related edge pressure (Fig. 7.23 left).

Fig. 7.22

Two different design approaches for the wheel axes alignment. Left: GBT Green Bank Telescope; right SRT Sardinia Radio Telescope

Fig. 7.23

Comparison of chamfer alignment features. Same scale; left: GBT Green Bank Telescope; right SRT Sardinia Radio Telescope

The design philosophy of the SRT bogies is completely different (Fig. 7.22 right). It has a crowned wheel. The crowning diameter is chosen in such a way that a slight deviation of the chamfer angle does not cause edge pressure but moves the centre of the contact area a little sideward (Fig. 7.23 right). The self-aligning features of the GBT concept are not needed here.

The coarse alignment of the chamfer angle during assembly of the telescope is done in the GBT by shims and in the SRT by a tappet at the end of an alignment lever at the bogie. For the GBT system, realignment requires releasing the loads on the wheels via jacks. The SRT system allows very fine realignment by the tappet during slight movement of the telescope without jacking.

The alignment lever of the SRT system is also used for the alignment of the steering angle (Fig. 7.24 right). In the GBT , the alignment of the steering angle is done during assembly by lateral alignment of the cushions (Fig. 7.24 left). A realignment of the steering angle would necessitate removing the loads on the wheels by jacking.

Fig. 7.24

Comparison of steering alignment features drawn at same scale. Left: GBT Green Bank Telescope; right: SRT Sardinia Radio Telescope

A large runout of the wheels on the track causes a lateral restraining force between the wheels and the track that can become very high, above the limits set by friction between the wheels and the track. In such a case, the lateral forces parallel to the wheel axis (not the rolling forces) can suddenly release and cause an impulse type vibration of the complete telescope structure. Clearly, such an instance should be avoided.