Construction of the mirror, tower and rail track
Striking to the construction are:
- the mirror with focus box and four leg;
- the tower, in which the mirror is suspended, and the control house;
- the foundation with circular rail track.
Mirror and antenna
Mirror of mesh
The mirror or reflector consists of metal mesh following the shape of a parabola as accurately as possible. The mirror has a diameter of 25 meters, a bowl depth of 3.25 meters and a focal length of 12 meters. The surface of the mirror is more than 500 square meters.
The metal mesh is fixed into 372 triangular frames, which are attached to a bowl-shaped framework of supporting beams using 210 set screws. This construction gives the mirror the required torsional rigidity.
During the building, metal mesh with a mesh size of 16 millimeters and a thickness of 1.5 millimeters was used. The ideal parabolic shape of the mirror could be adjusted to within 3 millimeters. To make the radio waves reflect well against the mirror, the mesh is tinned. With this mirror the radio telescope was suitable for observation at 21 centimeters and longer wavelengths.
In 1969, the mesh surface was renewed with preformed stainless steel mesh frames and stainless steel mesh with a mesh width of 8 millimeters and a thickness of 0.8 millimeters to keep the deviation of the parabolic shape within 1 millimeter. This made the radio telescope suitable for observing at wavelengths of about 10 centimeters and longer.
The set screws or adjustable pedestals are metal plates that are fixed to the supporting beams with an adjustable screw connection. Six triangular frames are assembled on each set screw. Thanks to this construction, the ideal parabolic shape could be approached as well as possible without having to make too many support points.
Antenna and focus box
In the focal point of the mirror are the antennas and part of the receiving equipment. During construction, these were attached to a central mast, an aluminum tube of 15 cm diameter. To reach the antenna and receiver, the mast was tiltable from the center of the reflector. The mast was held in place with three guy wires.
Tripod and four leg
Due to new developments, the receiver equipment in the frontend became increasingly heavier, from about 25 to about 150 kilograms. Therefor in 1961, a tripod construction replaced the antenna mast. The lower two legs were mounted rotatable on the mirror. With the aid of a winch, the tripod and frontend could be lowered to just above the ground and equipment could be repaired or replaced.
When heavy frontends with cooling equipment came into use, the tripod was no longer suitable. In 1974, a fixed four-leg construction with focus box replaced the tripod. In addition, this was better suited for the new solar frontend receiver. A lifting tower next to the telescope, specially purchased for this purpose, enabled reaching the receivers and antennas in the focus box.
Weight of the mirror construction
While building, the weight of the mirror including beams, counterweights and antenna mast was approximately 25 tons. With the new mesh, the four leg and focus box, the weight increased to more than 35 tons.
Tower and control house
The tower is a 15-meters-high construction in half-timbered form with platforms, stairs and control house. The mirror hangs on two axes at the top of the tower, and can move in vertical direction (height). The typical beak shape at the top of the tower is needed to bring the mirror into a horizontal position.
The tower stands on a square frame with sides of 11.7 meters. On each of the four corners of this bottom frame there is a running wheel with a diameter of 80 centimeters.
The control house on the bottom frame rotates with the telescope and is always behind the mirror. The latter ensures maximum shielding of the frontend receiver for possible radio interference by equipment in the house. The control house has three rooms: the observation room, the engine room and a working space.
Foundation and rail track
The tower and mirror together are rotatable around a vertical central axis (the spindle) over a horizontal circular rail track with a diameter of approximately 17 meters. The rail track and spindle stand on a heavy concrete foundation in the form of a cone. The center of gravity of the entire telescope is on the vertical axis.
The entire radio telescope turns around the spindle. The spindle has a cable entry. The electrical supply of the telescope is effected by slip ring contacts which are attached to the spindle. Initially, the cable entry was not used and therefore the telescope could in principle rotate in the same direction without restriction. In the end-1960s, coax and other cables were installed through the spindle. Since that time, to prevent the cables from breaking down, the radio telescope cannot turn more than one and a half round in one direction or the other .
Weight of the radio telescope
During the building, the weight of the entire radio telescope (tower with control house and mirror) was 120 tons. Each wheel carries 15 tons. The spindle in the center of the bottom frame carries the remaining 60 tons. With all the changes made over the years, the total weight is now more than 130 tons.