New instrument eyes the southern skies
Roland Roux
Early images from the Very Large Telescope (VLT) science-verification program show the new telescope in Paranal, Chile, living up to expectations and working with spectacular efficiency. Just a few days after having been mounted on the first 8.2-m VLT Unit Telescope (UT1), the focal reducer and spectrograph (FORS 1) saw first light. A second instrument, FORS 2, will eventually be added by the European Southern Observatory, which operates the VLT.
The FORS was designed to investigate the faintest and most remote objects in the universe. It first detects visual images of distant galaxies and immediately thereafter obtains recordings of their spectra. This allows, for instance, the determination of their stellar composition and distances.
A composite image shows the spiral galaxy NGC 1232, which was obtained during a period of good observing conditions in September 1998 (see image right). It is based on three exposures taken with three different filters--10 min at 360 nm, 6 min at 420 nm, and 2.3 min at 600 nm. The fields shown measure 6.8 p 6.8 arc min and 1.6 p 1.8 arc min. North is up, east is to the left.
NGC 1232 is located 20° south of the celestial equator, in the constellation Eridanus (the River). The distance is about 100 million light-years, but the optical quality of the VLT and FORS allow astronomers to see a wealth of details.
Later in September a color image of the Dumbbell planetary nebula was obtained in mediocre viewing conditions (see image below). The Dumbbell is a typical planetary nebula, which is located in the constellation Vulpecula (the Fox). Astronomers believe it is about 1200 light-years, although they are not certain.
Despite its name, the Dumbbell nebula has nothing to do with planets. It consists of very rarefied gas that has been ejected from the hot central star nearing the end of its life. The gas atoms in the nebula are heated by the intense ultraviolet radiation from this star and emit strongly at specific wavelengths.
For the three-color composite, a short exposure was first made through a wideband filter registering blue light from the nebula. It was then combined with exposure through two interference filters at the wavelengths of double-ionized oxygen atoms and atomic hydrogen. They were color-coded as blue, green, and red, respectively, and combined to produce this picture that shows the structure of the nebula in "approximately true" colors.
The FORS uses a charge-coupled-device camera with 2048 p 2048 pixels, each covering 24 p 24 µm; the sky fields shown measure 6.8 p 6.8 arc min.
The study is being carried out by a consortium of three German astronomical institutes: Heidelberg State Observatory and the university observatories of Göttingen and Munich.