A double-channel polarimeter designed by Professor Ben Gascoigne for the 50-inch (127-cm) telescope at Mt Stromlo Observatory in 1963. It is on display at Sydney Observatory. Courtesy Powerhouse Museum

Recently I wrote about a visit to Mt Stromlo Observatory. The Powerhouse Museum has a number of historic instruments from Stromlo that were acquired in 1989. The objects were regarded as significant at the time, but since the disastrous fire of 18 January 2003 that destroyed much of Stromlo’s heritage, their importance has become much greater.

Astronomers are gradually starting to understand the important role that magnetic fields play in the dynamics of galaxies and in star formation. Today there are a variety of techniques in their armoury to detect and plot magnetic fields in different parts of the sky. An important project for the Australian Square Kilometre Array Pathfinder instrument currently being built in Western Australia will be a survey that will sample magnetic fields in three million places over a large section of the sky.

Some of the pioneering studies in measuring magnetic fields were made with an instrument from Mt Stromlo that is on display at Sydney Observatory. This is the double-channel polarimeter designed by Professor Ben Gascoigne and built at Stromlo in March 1963. In use the instrument was placed on the 50-inch (127-cm) telescope that was the first reincarnation of the Great Melbourne Telescope at Mount Stromlo.

The instrument measures the polarisation of starlight. Light consists of the fluctuations of electric and magnetic fields at right angles to the direction of travel. Ordinarily the directions of the fluctuations are random, but if there is a tendency to fluctuate more in one direction than in another at right angles then the light is said to be polarised. For example, Polaroid brand sunglasses reduce glare by cutting out one direction of fluctuation or polarisation. By holding two such glasses towards the light and rotating one with respect to the other to right angles it is possible to completely darken the view.

Mt Stromlo astronomer, Natarajan Visvanathan, known to astronomers as Vis, was the first user of this instrument while he was working on a PhD thesis. In a subsequent paper he explained the workings of the instrument. It uses a special prism called a Wollaston prism made of calcite that splits light into two polarisations at right angle to each other. With the help of mirrors and lenses the two beams are fed to the cathodes of photomultipliers, devices that convert and amplify the light into electrical signals. These signals were then further amplified and integrated over a short period and then plotted on the output of two separate chart recorders. Any differences in the tracings indicated polarisation.

Polarisation is linked to magnetic fields for between us and the distant stars there are clouds of interstellar dust particles. If there is a magnetic field present then these particles line up with the field. As light from a star passes through these lined-up dust grains it becomes polarised so that the polarisation of the starlight indicates the direction of the magnetic field.

Vis studied the polarisation of light from the Large Magellanic Cloud (LMC), a nearby galaxy that can only be seen from the Southern Hemisphere. Over a period of 11 nights from September 1963 to February 1965 he examined 30 stars in the LMC. His results were in agreement with the idea that ‘magnetic lines of force are parallel to spiral features and that the Large Magellanic Cloud is a highly flattened disk’.

A point of interest is that the LMC appears as an irregular galaxy yet Vis’ results indicated a spiral structure. In the late 19th century Sydney Observatory astronomer Henry Chamberlain Russell was the first to notice spiral structure in the Cloud.

The idea of a spiral structure for the LMC stands today. In a 2004 paper Professor Bryan Gaensler of Sydney University and collaborators study the magnetic field of the LMC using modern radio techniques and conclude, inter alia, ‘The LMC has a coherent magnetic field with a spiral geometry’.

The double-channel photometer was used for important pioneering work in studying the Universe. It is on display in the first room of the Sydney Observatory exhibition and is certainly worth looking at on a visit to the Observatory.

References

N Visvanathan, ‘Interstellar Polarization in the Large Magellanic Cloud’, Monthly Notices of the Royal Astronomical Society, vol 132, pp 423-432, 1966.

Bryan M. Gaensler et al, ‘The Magnetic Field of the Large Magellanic Cloud: A New Way of Studying Galactic Magnetism’, Proceedings of The Magnetized Plasma in Galaxy Evolution, Sep. 27th – Oct. 1st 2004, Jagiellonian University, Krakow, Poland, Eds. K.T. Chy?zy, R.-J. Dettmar, K. Otmianowska-Mazur and M. Soida.