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Next VLT Instrument Ready for the Astronomers

FORS2 Commissioning Period Successfully Terminated

8 lutego 2000

The commissioning of the FORS2 multi-mode astronomical instrument at KUEYEN , the second FOcal Reducer/low dispersion Spectrograph at the ESO Very Large Telescope, was successfully finished today. This important work – that may be likened with the test driving of a new car model – took place during two periods, from October 22 to November 21, 1999, and January 22 to February 8, 2000.

The overall goal was to thoroughly test the functioning of the new instrument, its conformity to specifications and to optimize its operation at the telescope. FORS2 is now ready to be handed over to the astronomers on April 1, 2000. Observing time for a six-month period until October 1 has already been allocated to a large number of research programmes.

Two of the images that were obtained with FORS2 during the commissioning period are shown here. An early report about this instrument is available as ESO Press Release eso9948.

The many modes of FORS2

The FORS Commissioning Team carried out a comprehensive test programme for all observing modes. These tests were done with "observation blocks (OBs)" that describe the set-up of the instrument and telescope for each exposure in all details, e.g., position in the sky of the object to be observed, filters, exposure time, etc.. Whenever an OB is "activated" from the control console, the corresponding observation is automatically performed. Additional information about the VLT Data Flow System is available in ESO Press Release eso9930.

The FORS2 observing modes include direct imaging, long-slit and multi-object spectroscopy, exactly as in its twin, FORS1 at ANTU. In addition, FORS2 contains the "Mask Exchange Unit", a motorized magazine that holds 10 masks made of thin metal plates into which the slits are cut by means of a laser.

The advantage of this particular observing method is that more spectra (of more objects) can be taken with a single exposure (up to approximately 80) and that the shape of the slits can be adapted to the shape of the objects, thus increasing the scientific return. Results obtained so far look very promising.

To increase further the scientific power of the FORS2 instrument in the spectroscopic mode, a number of new optical dispersion elements ("grisms", i.e., a combination of a grating and a glass prism) have been added. They give the scientists a greater choice of spectral resolution and wavelength range.

Another mode that is new to FORS2 is the high time resolution mode. It was demonstrated with the Crab pulsar, c.f. ESO Press Release eso9948 and promises very interesting scientific returns.

Images from the FORS2 Commissioning Phase

The two composite images shown below were obtained during the FORS2 commissioning work. They are based on three exposures through different optical broadband filtres (B: 429 nm central wavelength; 88 nm FWHM (Full Width at Half Maximum), V: 554/111 nm, R: 655/165 nm). All were taken with the 2048 x 2048 pixel 2 CCD detector with a field of view of 6.8 x 6.8 arcmin 2; each pixel measures 24 µm square. They were flatfield corrected and bias subtracted, scaled in intensity and some cosmetic cleaning was performed, e.g. removal of bad columns on the CCD. North is up and East is left.

Tarantula Nebula in the Large Magellanic Cloud

30 Doradus, also known as the Tarantula Nebula, or NGC 2070, is located in the Large Magellanic Cloud (LMC), some 170,000 light-years away. It is one of the largest known star-forming regions in the Local Group of Galaxies.

It was first catalogued as a star, but then recognized to be a nebula by the French astronomer A. Lacaille in 1751-52. The Tarantula Nebula is the only extra-galactic nebula which can be seen with the unaided eye. It contains in the centre the open stellar cluster R 136 with many of the largest, hottest, and most massive stars known.

Radio Galaxy Centaurus A

The new FORS2 image of Centaurus A, also known as NGC 5128, is an example of how frontier science can be combined with esthetic aspects.

This galaxy is a most interesting object for the present attempts to understand active galaxies . It is being investigated by means of observations in all spectral regions, from radio via infrared and optical wavelengths to X- and gamma-rays. It is one of the most extensively studied objects in the southern sky.

FORS2, with its large field-of-view and excellent optical resolution, makes it possible to study the global context of the active region in Centaurus A in great detail. Note for instance the great number of massive and luminous blue stars that are well resolved individually, in the upper right and lower left in ESO Press Photo eso0005b.

Centaurus A is one of the foremost examples of a radio-loud active galactic nucleus (AGN). On images obtained at optical wavelengths, thick dust layers almost completely obscure the galaxy's centre. This structure was first reported by Sir John Herschel in 1847. Until 1949, NGC 5128 was thought to be a strange object in the Milky Way, but it was then identified as a powerful radio galaxy and designated Centaurus A .

The distance is about 10-13 million light-years (3-4 Mpc) and the apparent visual magnitude is about 8, or 5 times too faint to be seen with the unaided eye.

There is strong evidence that Centaurus A is a merger of an elliptical with a spiral galaxy, since elliptical galaxies would not have had enough dust and gas to form the young, blue stars seen along the edges of the dust lane.

The core of Centaurus A is the smallest known extragalactic radio source, only 10 light-days across. A jet of high energy particles from this centre is observed in radio and X-ray images. The core probably contains a supermassive black hole with a mass of about 100 million solar masses.

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