- VIRUS-W Views the Sky with 250+ Eyes
MUNICH, Germany, June 28, 2010 — At the end of May, the VIRUS-W spectrograph of the Max Planck Institute for Extraterrestrial Physics and the University Observatory Munich was completed and made ready for installation at the McDonald Observatory near Ft. Davis, Texas. VIRUS is an acronym for Visible Integral-field Replicable Unit Spectrograph.
The VIRUS-W was specifically designed to observe nearby spiral galaxies. Its relatively large field of view, combined with a high spectral resolution, makes it ideally suited for this purpose. Operated in high-resolution mode, it will allow scientists to study the bulk motion of stars in spiral galaxies down to velocity dispersions of only about 20 km/s.
Assembly of the optical fibers of VIRUS-W in the focal plane of the telescope. (Photos: M. Fabricius, Max Planck Institute for Extraterrestrial Physics/University Observatory Munich)
An additional medium-resolution mode was developed for studying the chemical composition of galaxies. The field of view of the spectrograph will be about 150 × 75 arcsec, which means that the kinematically very interesting central regions of spiral galaxies can be studied in just one or two exposures. Depending on its apparent brightness and relative size in the sky, a galaxy can be observed in just a few hours.
The actual spectrograph is mounted on an optical bench and is connected to the telescope with a fiber optic cable. From the focal plane of the telescope, the light is guided through 267 glass fibers, with 150 μm inner diameter, to the optical components, which disperse the light into the different wavelengths. The main elements of the spectrograph are two holographic gratings, which can be swapped as necessary. Holographic gratings are particularly efficient and produce only a very small fraction of stray light.
“This bench mounting makes the instrument very robust; the direction, in which the telescope is pointing for an observation, has no influence on the measurement,” said Maximilian Fabricius, who developed and built VIRUS-W for the most part. “After all, we are talking about 500 kg that you would need to maneuver for each new pointing of the telescope.” Ensuring mechanical and optical stability is a big challenge for instruments mounted directly on the telescope.
With this mounting, the spectrograph also can be fitted to other telescopes much more easily. “Since VIRUS-W is connected with the telescope simply with optical fibers, for each new telescope only the relatively small end piece of the fiber bundle has to be adapted,” Fabricius said. This makes VIRUS-W a mobile instrument; further observations are planned with the MDM observatory at Kitt Peak in Arizona and the future 2.1-m Fraunhofer telescope on Wendelstein mountain in Bavaria, Germany.
“One of the advantages of modern 2-m-class telescopes is the feasibility of long observation runs,” said Niv Drory of the Max Planck Institute for Extraterrestrial Physics, who is studying galaxy formation and evolution in various projects. “While on an 8-m- or 10-m-class telescope you get typically only a few nights of observing time, on a 2-m-class telescope a project might get several dozen nights of observing time per year. Moreover, smaller telescopes have a larger field of view, which in our case is an added bonus.” The large field of view and high resolution of VIRUS-W therefore opens a new window onto the exploration of star and gas kinematics in nearby galaxies, shedding light on their evolutionary history.
In the VIRUS-W spectrograph, light from the optical fibers first shines onto the (curved) collimator mirror and is then reflected onto the high-resolution grating (in the angular black box; the low-resolution grating has not yet been installed). The dispersed light spectrum is then recorded with the cooled-CCD camera in the round casing.
VIRUS-W is the refined version of a similar instrument being developed for an ambitious survey at the 9.2-m Hobby-Eberly telescope of the McDonald Observatory. The VIRUS instrument, which will consist of some 150 individual spectrographs, will blindly survey 420 square degrees of the whole sky (about 1000 times the apparent size of the moon) over the course of three years starting in 2012.
A prototype of these spectrographs, VIRUS-P, is operating at the smaller 2.7-m Harlan J. Smith telescope of the McDonald observatory since 2007. In a pilot survey of local spiral galaxies over 30 nights, this prototype impressively demonstrated the capabilities of this instrument. With its relatively low spectral resolution, however, VIRUS-P is not suitable to study stellar motions in spiral galaxies.
The first observations with VIRUS-W at the Harlan J. Smith telescope in West Texas are planned for the end of June. Apart from studies of spiral galaxies by the scientists at the Max Planck Institute for Extraterrestrial Physics and the University Observatory, other research groups can then apply for using the instrument as well.
For more information, visit: www.mpe.mpg.de
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