Feature

A Clearer View of Space

A powerful spectrograph will focus on new planets.

Chanel Hughes

Researchers in Canterbury University's Department of Physics and Astronomy will get a clearer view of space with the installation of Hercules (High Efficiency and Resolution Canterbury University Large Echelle Spectrograph) at the university's Mount John Observatory.

Designed and built on campus, Hercules -- as its name suggests -- will be one of the world's most powerful spectrographs, and will enable researchers to record and analyse the light from distant stars.

Spectrographs receive starlight through a telescope, split the light into a rainbow spectrum of colours and record the intensity and distribution of those colours. This information can be used to determine the quantities of various chemical elements in a star, which in turn reveal the star's velocity, temperature, pressure and rotational rate. The velocity is of particular interest, as it can show whether a star has planets orbiting it.

"The presence of a planet makes the star wobble," says Professor John Hearnshaw. "A small planet like Earth, for example, makes the sun wobble a little, while Jupiter makes it wobble a lot."

This method of detecting planets was first perfected five years ago. Since then, 20 planets have been discovered in the northern skies and their details published, with a further 10 known of, though unpublished. Hearnshaw is particularly interested in discovering planets in the southern skies.

"It's very exciting. Up until five years ago we had a very local view of the universe; it was only five years ago that we had the first real evidence that many more planets existed."

It is estimated that 5-10% of stars have orbiting planets. There are a large variety of different star types, but so far all those with known planets fall into the same category as the sun. Hearnshaw would like to begin examining stars that vary slightly from the sun, such as those with a more intense heat.

Other projects that Hercules will be used for include the examination of variable stars -- stars that change in brightness and pulsating stars which expand and contract at varying rates from a space of seconds to hundreds of years.

Home Team Design

The spectrograph was designed by Hearnshaw; Gary Nankivell, a leading optical engineer; and optical designer Norman Rumsey. Rumsey and Nankivell worked with Hearnshaw to build the university's first spectrograph in 1975. Technical officer Graeme Kershaw undertook the mechanical construction.

They have incorporated several novel features to increase precision, adding a vacuum tank to house the spectrograph. When starlight travels through air, the colours are slightly affected; a much more exact measurement of the spectral lines may be made by passing it through a vacuum chamber. This concept has only been developed in the past 10 years, prior to which the spectrograph was attached directly to the telescope.

Light will be carried from the observatory's McLellan telescope to the spectrograph through a 20-metre optical fibre that is 0.1mm in diameter. This means the spectrograph can also be kept in a separate room where the temperature may be maintained at a stable level and to within 0.1^oC. Temperature, like air, affects the precision of the image.

The crucial elements are the 25cm x 25cm prism made from extremely pure glass, and the echelle defraction grating -- a sheet of glass with 12,000 grooves cut into it. It is these components combined with several mirrors and lenses that split the light into a colour spectrum.

"The photons of light will travel through the universe for hundreds of years and spend their last millionth of a second speeding through the optical fibre and bouncing around these mirrors and lenses."

The design team is currently awaiting the arrival of the glassware before proceeding with installation and several months of testing. Almost a quarter of a million dollars has been spent on the design -- a mere drop in the bucket for a spectrograph that is in fact worth $2 million; Hearnshaw hopes to raise a further $40,000 in funding. Aside from the fact that labour costs and overheads have not been factored in, Professor Hearnshaw attributes this great economising to Kiwi ingenuity.

"We know how to make things out of ceiling wax and string!"

Professor John Hearnshaw (front) and Graeme Kershaw at work on Hercules.
Photo: Duncan Shaw-Brown

Chanel Hughes is with External Relations at Canterbury University.