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The UVES Spectrograph Sees the Sun

New VLT Instrument Nearing Completion

18 Novembre 1998

The third astronomical instrument to be mounted on the ESO's Very Large Telescope (VLT) - following FORS1 and ISAAC - is now nearing completion at the ESO Headquarters in Garching (Bavaria, Germany). Housed in a modernistic building, some 15 km north of Munich, this is a major science and technology centre with advanced laboratory facilities.

The VLT UV-Visual Echelle Spectrograph (UVES) is a complex assembly of advanced opto-mechanical systems and the associated control electronics and motors. The entire system is remotely computer controlled and will perform a spectral analysis of the light from celestial objects that is collected by the giant telescope mirrors. The design of UVES was begun in 1992 and its realization involved scientists and engineers at ESO and the Observatory of Trieste (Italy), as well as companies from all over Europe.

Spectroscopic observations serve to separate the light from an object, be it a flame or a lamp in the laboratory, a star or a galaxy observed through a telescope, into its constituent colours - blue on one side, red on the other. The resulting band of coloured light is called a spectrum . A glass prism produces this effect; in nature small raindrops in the air may cause a rainbow to be seen in the sky.

Various types of spectrographs are used in many contexts, e.g. for environmental studies or process control. They allow researchers to perform a detailed analysis of light emitted or absorbed by test specimens. As every element has its own unique spectral "fingerprint", this technique allows for instance to detect minute traces of pollutants in material samples.

Astronomical spectrographs like UVES are used to probe celestial objects that are distant in space and time. This new instrument will be mounted on the second 8.2-metre VLT Unit telescope (UT2) in the middle of 1999. The light-efficient design of UVES, when combined with the great light-gathering capability of the UT2 will provide a unique facility for many front-line astronomical observations.

A UVES spectrum of the Sun

ESO Press Photo eso9854c shows part of a test spectrum of the Sun, taken by UVES in the Laboratory in Garching on October 19, 1998. The bright (nearly) horizontal lines correspond to adjacent, small wavelength ranges of the solar spectrum. Within these, many vertical dark lines indicate the presence of certain elements in the solar atmosphere, for instance gaseous hydrogen or iron. From the line shapes and their depths, it is possible to determine the temperature, pressure and chemical abundances of this gas. Small shifts in wavelength of such lines, when measured in other stars, indicate velocity changes due to the Doppler effect that may be caused by invisible planets circling the observed star. In ESO Press Photo eso9854d is shown a tracing of a small part of this test spectrum. The dips correspond to absorption caused by particular elements in the solar atmosphere, as indicated.

One of the first "observations" with UVES in Garching consisted in registering a small part of the solar spectrum. For this purpose, an optical fibre was attached to the spectrograph's entry slit (to which the light from celestial objects will later be directed) and simply pointed towards a roof window in the laboratory. A part of the solar spectrum recorded during a short exposure is seen in ESO Press Photo eso9854c. A tracing of a small wavelength region is shown in ESO Press Photo eso9854d. These data are used to test and optimize the performance of UVES before it will be installed at the telescope.

Future observations with UVES

When it becomes fully operational at the UT2 next year, UVES will be used to tackle some of the most exciting and elusive problems of modern astrophysics.

For example, it will measure the gas which filled cosmic space in the early phases of the Universe when it was just one tenth of its present age. It will study the motion of gas and stars in energetic nuclei of galaxies, trying to reveal the properties of the gigantic black holes that are thought to reside there. It will map the chemical composition of stars - both in the Milky Way and in other galaxies - providing systematic data which can give us new insights into the nuclear processes that take place deep inside the stars.

Video News Reel with UVES sequences available

ESO has produced ESO Video News Reel No. 2 with sequences illustrating the current work around UVES. It includes interviews with two of the involved scientists, and is accompanied by detailed, written information, including a shot list and suggested narration. The sound track contains original sound, only.

An extract of ESO Video News Reel No. 2 is available as ESO Press Video eso9854, as MPEG (Video and Video+Audio) and in a RealMedia-version that requires the RealPlayer software.

ESO Video News Reels are produced specifically for broadcast use . They are available on VHS, Betacam SP and MII. The footage is free to air ("Conditions for Use of ESO Video Footage" are enclosed with the video tapes), if credit is given to the European Southern Observatory (© 1998). The tapes are only available on request from the ESO EPR Department.

Ulteriori Informazioni

Technical Information:

• This image was obtained by combining 45 1-min exposures, taken with the telescope randomly offset by small amounts in between ("jittering") to allow subtraction of the bright sky emission at this wavelength. The seeing was 0.5 arcsec and the limiting magnitude is around Ks = 22. The field measures 2.5 x 2.5 arcmin with N at the top and E to the left.

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