Our June seminar took the form of a visit to the Mullard Radio Astronomy Observatory (MRAO) on the outskirts of Cambridge and was organised by CETC Committee Member, Jamie Murray.

Stafford Withington, who is Professor in Analytical Physics at the University of Cambridge and Fellow of Downing College, opened the evening with his presentation on “Observing the Invisible Universe”, giving a short review of the structure of the cosmos.  He then moved on to his special area of interest, which is concerned with the development of techniques and equipment for observing at wavelengths between 1mm (300 GHz) and 100 microns (3 THz).

Radiation at these wavelengths, which are to be found between the radio and far-infrared part of the spectrum, is of particular interest because it is generated by bodies that are cooling and can be traced back to the beginning of the universe.  A grant to the Cavendish Laboratory from the Particle Physics and Astronomy Research Council has enabled instruments to be developed by Professor Withington and his team to detect so-called gravity waves generated in the first second (1x10^-35 sec) of the Big Bang.  These instruments, with detectors that have to be cooled to -269ºC, are being deployed at the James Clerk Maxwell Telescope on Mauna Kea in Hawaii and the Atacama Large Millimetre Array located high in the Atacama Desert on the Chajnantor Plain in Chile.

After Professor Withington’s stimulating presentation, the audience were conducted around the MRAA site by three PhD students; Dr Bodie Senata, Jon Zwart & Jeremy Taylor. 

Bodie Senata demonstrated the Cambridge Optical Aperture Synthesis Telescope, a multi-element optical interferometer with baselines of up to 100 metres, designed to observe stars with angular resolution as high as one thousandth of one arcsecond.  Unlike the rest of the telescopes at MRAO, it is, as its name suggests, is an optical Telescope, and gets round the need for a large image catching dish (which is technically difficult to make and to move) by combining the output of 4 of the 5 small telescopes into a single image.  The main problem to overcome is combining the images in the same timeframe; given the very small differences between the times it takes for the image of the galaxy being viewed to reach each telescope.  An intricate sequence of mirrors arrayed inside a temperature-controlled bunker is used in varying combinations to achieve synchronous imaging.

Jon Zwart demonstrated the Arcminute Microkelvin Imager (AMI).  The unique feature of the AMI, MRAO’s most recent telescope, is that, owing to the Sunyaev-Zel’dovich effect (the scattering of cosmic microwave background photons by hot, ionised gas in clusters of galaxies), it is able to count all the galaxy clusters in a given area of the sky whatever their distance.  Built in a bunker designed to shield AMI from outside, man-made radio waves, each of the ten telescopes has a helium compressor that keeps the superconductive sensors at 17K (-258^oC).  Not being an optical telescope, AMI can be used round the clock, although it is adversely affected by atmospheric conditions, especially rain which leaves the telescope unusable.  Jon pointed out, however, that given that all data is recorded on disc, it is reasonably easy to remove useless or compromised images.  AMI solves the problem of synchronising the images it records by the use of complex electronic delays. 

The Club is very fortunate in benefiting from the sponsorship of the following organisations:-

There are also other companies who give us generous help with specific meetings and services.

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