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Building the Radio Telescope

by Richard Baker

The Mount

Cardiff undergraduate student Richard Baker and observatory technician Hugh Lang remove the old mount

Cardiff undergraduate student Richard Baker and observatory technician Hugh Lang remove the old mount.

One of the first considerations when designing the mount was the location of the Radio Dish. The obvious choice was as high up as possible. This is to limit the amount of interference from ground-based signals and to provide a clear line of sight from horizon to horizon. For this reason, we decided to mount the dish on the roof of the North building in the Physics and Astronomy department. The concern with this site is that we have to be certain that the structural integrity of the roof is capable of withstanding the forces that the dish exerts.

The mount for the new dish in place

The mount for the new dish in place.

It is important that the dish is resistant to the elements and that it has a secure footing in order for it to point precisely, given that the dish was to be located where the weather was at its most severe.

The first part of our project was to calculate how much force would be imposed on the dish/mount given certain wind velocities in order to design a mount that could accept these forces. We tried several values for wind velocities and calculated the forces and torques on both the base of the mount and at its anchor points. The values of these are shown in Table 1.

Table 1: Calculation of force on mount for various wind speeds.
Wind
(km/h)
Force
(N)
Weight
(tonnes)
Torque
(Nm)
Force per bolt
(N)
50 1636 0.163 2863 716
100 6544 0.654 11452 2863
150 14726 1.472 25770 6442
200 26179 2.619 45813 11453
250 40906 4.096 71585 17896
300 58904 5.890 103082 25770

Windspeed Calculations

Observatory technician Hugh Lang prepares for the arrival of the new dish by removing one of the old radio antennas

Observatory technician Hugh Lang prepares for the arrival of the new dish by removing one of the old radio antennas.

These values were checked against an existing wind loading program and found to be consistent. Assuming the wind speed in Cardiff will not reach velocities of over 200kph (which is reasonable) the dish design must exceed the allowance stated for this speed. The design we produced is shown above. The telescope manufacturers have tested the telescope to wind speeds of 200kph and the bolts that anchor the mount to the roof have been pull-tested to the equivalent of 1 tonne of force. Unless we receive wind speeds of over 200kph (which has never been recorded in Cardiff), the dish will resist all weather conditions.

It is also extremely important to weigh the overall dish/mount system and compare it to known load bearing specifications for the roof as the dish and mount were subject to weight constraints. The only way we could physically accomplish this was to weigh the unassembled pieces of the dish by hand, using some scales. We weighed the mount using a combination of physical weighing and volume times density calculations. The results are well within the safety limits for the roof. We then bolted the mount to the existing supports on the roof. The supports had been used for an existing antenna array, which we disassembled earlier in the year.