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Niels Bohr Institute > Who, What, When > > Surveying the sky > Development of photoel...

23 November 2010

Development of photoelectric astrometry 

Erik Høg, an astronomer at the University of Copenhagen, worked for 15 years at the observatory in Hamburg. Here they had plans for an expedition to Perth in Western Australia with an old meridian circle.

But in 1960 Erik Høg got a good idea for photoelectric astrometry for the expedition in Australia. He proposed counting photons for the measurement of light and using a computer for calculations. You let the star glide across some small slits, while constantly measuring the light coming through the slits. This measures both the position of the star and its brightness. He knew about the technique for electronic counting before other astronomers because he had encountered it as a soldier when they had measured radioactivity in dust from the atmosphere, which originated from the detonation of hydrogen and atomic bombs by the superpowers.

The best calculating machine at the time was the Danish GIER, seen here in Australia in 1970.

The director of the observatory in Hamburg, Otto Heckmann, was immediately on board with his ideas about photoelectric astrometry and he was confident that Erik Høg would endure all of the development years that lay ahead. It actually took seven years in the end. They bought the best computer for the job, called a calculating machine then, and the best machine was the Danish GIER, which also came with to Australia. It was a large astronomical expedition by the standards of that time, with a staff of six to ten people making observations and calculations over five years. The results for a total of 25,000 stars were published in 1976.

Great strength for faint stars

The photoelectric method for astrometry was more accurate than the old visual method and you could measure fainter stars using the meridian circle. Moreover, the method was entirely electronic and therefore more efficient, because the observations were punched directly onto the punched paper tape of the time, which could then be fed into the computer, GIER.

The photoelectric method was ideal to use in a satellite and the only practical option, which was fully automatic. It gives an impression of the strength of the method that the faintest stars in the Tycho-2 catalogue are based on the registration of only around 50 photons from the star and that these photons were gathered through a combination of 100 measurements spread over a period of three years. It was enough to provide greater accuracy for the star's position than could be reached from Earth and at the same time a good measurement of the brightness in two colours. Of course, the bright stars had much greater accuracy, since many photons are synonymous with great accuracy.

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