Astronomical Society of Coonabarabran

SIDING SPRING TECHNOLOGY SHINES A NEW LIGHT ON COMETS

by Liz Cutts

The force of the Deep Impact probe crashing into the Comet Tempel 1 has been likened to a mosquito running into a 767 airliner; but when it hit a shower of gas and dust was released from beneath the surface that has given scientists the opportunity to study in detail for the first time material probably unchanged since the formation of the solar system.

The Anglo-Australian Telescope (AAT) and the UK Schmidt telescope at Siding Spring were better placed than telescopes in Europe and North or South America to observe any changes to Comet Tempel 1 after the 360kg impactor hit Comet Tempel 1 on Monday, 4th July.

The unique spectrograph technology installed at the AAT and the Schmidt telescopes was used for the first time to observe a single object in the sky. The spectrograph measured chemical abundances at many spots on the comet simultaneously providing data will provide a major contribution towards a better understanding and knowledge of this astronomical phenomenon.

A spectrograph is an instrument used to obtain and record an astronomical spectrum. The spectrograph splits or disperses the light from an object into its component wavelengths so that it can be recorded and analyzed.

"The Anglo-Australian Observatory (AAO) in Coonabarabran is rather exceptional because we have two of the world's largest field of view Spectrographs; a 2DF at the AAT and a 6DF at the UK Schmidt," explained Dr. Rob Sharp, support scientist for the AAO monitoring Deep Impact at the Schmidt Telescope.

"Normally 6DF and 2DF are used to do whole galaxy surveys and specific software has been developed for the Red Shift Galaxy Survey. However, this is the first time that we have used the spectrograph technology on a single object such as a comet and everything had to be worked out manually."

Light entering the 2DF and 6DF is split or dispersed into a spectrum using prisms. As the rays pass through a prism, they undergo refraction, a change in velocity due to the change in medium. If the light falls incident to the prism at an angle other than 90 degrees it will also change direction. Red light has a longer wavelength than blue light so its angle of refraction is lower, both at entry to and exit from the prism.

"We use a robot to place fibre optics magnetically in a certain pattern on a metal plate, which is placed it into the telescope," Dr. Sharp clarified. "This enables us to observe an area of six degrees in the sky at the one time. The light emitted from the comet was captured by the button-like prisms on the 6DF plate; then travelled along the fibre optics into a spectrograph where everything was reformatted.

"The light spectra received from Comet Tempel 1 will reveal the principal gaseous components of the comet allowing astronomers to measure the amount and type of materials released by the impact.

"The plan was to take a spectrum of the comet nucleus, the area surrounding it; the tail other regions, to give us calibrations of spectra so that we can take out the effects of the earths atmosphere and other intrusive factors."

Wilhelm Tempel discovered this comet named in his honour in 1867. The comet has made many passages through the inner solar system orbiting the Sun every 5.5 years. This makes Tempel 1 a good target to study evolutionary change in the mantle or upper crust.

Scientists know that the spectra of comets consist of a reflected solar spectrum from the dusty clouds surrounding the comet nucleus, and also sharp emission lines emitted by the gas component of the comet's coma. The molecules in the coma absorb some light from the Sun and re-emit it at specific wavelengths, producing a typical signature. This physical process is known as "resonance-fluorescence" and it is the same process that is used in neon bulb lights.

Because Tempel 1 moved across the sky faster than the background stars, scientists at the Schmidt telescope had to use special observing techniques.

"We had to drive the Schmidt telescope manually at a predetermined rate," added Dr. Sharp. The Schmidt has not been used in this way for quite some time so we relied on the expertise of the telescope operators.

"It is the spherical mirror and very large lens that gives the Schmidt its enormous field of view and provides excellent imaging over large areas of the sky. However, the Schmidt is not normally used to track moving objects, it is not the kind of science it does, so some things had to be re-engineered for this project.

"The Schmidt's contribution to the Deep Impact project, was, rather than look at a tiny patch of sky containing the comet, to observe the all components that make up the complete comet.

"We are very fortunate in that the AAO take part in world class science, as both the AAT and Schmidt have enormous field of view capabilities; probably ten times bigger than the nearest competitor. A technology market has developed as a result of our knowledge about depth of field; we are very good at doing it and we are now building and providing depth of field instrumentation to the rest of the world."

Dr. Sharpe stated that he was very pleased with the results received using the 6DF spectrograph technology.

"It has all worked very nicely; we received spectra from the nucleus of the comet; although we did not acquire the signal levels we hoped for from the extended coma and tail of the comet, because it was a bit fainter than expected.

"However, we certainly received the nucleus spectrum very well and we had no problems tracking the comet which is a credit to the staff up here. In the long term it will be a few months before all the data has been examined and published in a scientific paper on the Deep Impact project.

"Although there is no stunning new data, the results from Siding Spring could prove to be a very valuable resource, simply because nobody else has monitored Comet Tempel 1 in the same way, so we have made a considerable contribution to the whole process."


6DF Spectrograph plate: The button-like prisms and optical fibres set in the required pattern to receive spectrum information from Comet Tempel 1 using 6DF technology at the Schmidt Telescope.	Donna and Rob: Donna Burton, astronomer operating the Schmidt Telescope for the Deep Impact project, and Dr. Rob Sharp examine the spectrograph data received from Comet Tempel 1.