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Cavendish Astrophysics


We have heard the sad news that Derek Vonberg has died at the age of 93.  Derek Vonberg was one of the earliest pioneers of radio astronomy and with Martin Ryle established the interferometric approach to radio astronomy here in Cambridge.  After leaving Cambridge in 1949, Derek did not continue in astronomy, instead becoming a very distinguished medical research scientist for which he was awarded the CBE.

Despite his short time in radio astronomy his impact has been far reaching as this extract written by Malcolm shows.

"Ryle returned to Cambridge supported by an Imperial Chemical Industries (ICI) fellowship and joined Derek Vonberg.   Their first project was to measure the properties of the radio emission from the Sun.  There was scarcely any money for equipment, but they were able to buy considerable amounts of surplus War electronics very cheaply and also acquire large amounts of high quality German radar equipment which had been requisitioned after the War. They took away five truckloads of surplus equipment from the Royal Aircraft Establishment (RAE) at Farnborough, including several 3m and 7.5m steerable Wurzburg radio antennae which were to be used for many years.

The angular resolving power of the radio antennae available at that time was not sufficient to resolve the disk of the Sun.  Ryle and Vonberg therefore adapted the surplus radar equipment and developed new receiver techniques for metre wavelengths to create a radio interferometer, the antennae being separated by several hundred metres in order to provide high enough angular resolution. Only later was it realized that they had invented the radio equivalent of the Michelson interferometer. A massive sunspot occurred in July 1946, and their observations showed conclusively that the radio emission originated from a region on the surface of the Sun similar in size to that of the sunspot region."

Importantly the initial measurements of the active sun clearly showed that the brightness temperature of the radiation  was too high to be explained by any thermal process and they concluded that the emission process must be non-thermal.  To test this conclusion they hypothesised that polarised emission would clearly indicate a non-thermal or coherent phenomenon.  They then undertook further experiments on the active sun and demonstrated that the emission was strongly circularly polarised.  This is all reported in one paper in 1946 (emission from the quiet sun, use of a multi-baseline interferometer in radio astronomy, measuring the angular size of sunspot emission [10 arcminutes], non-thermal origin of emission from the active sun and polarised emission from the sun).  The paper is a wonderful example of clarity and brevity - it is only one page in length in nature!

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