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For period 68 the Wide Field Imager at the MPG/ESO 2.2-m telescope at La Silla will offer, in addition to the imaging mode, a spectroscopic mode. This will be accomplished by the installation of a grism in front of the WFI triplets to allow for slitless spectroscopy. The combination of the wide field of view, the grism, and the simultaneous availability of the whole set of WFI filters, make this a unique instrument in the southern skies. The instrument will be operated in VLT mode, with the instrument package with the spectroscopic templates already released.
In a preliminary ESO internal report by Hermann Bohnhardt the followingcase was made for the spectroscopic mode of the WFI. This mode will allow survey work for stellar, nebular objects, and galaxies with special characteristics. The main goal would be the registration and identification of such sources by their spectral sigantures to establish population and distribution statistics and to prepare in-depth follow-up investigations by dedicated research programs with other telescope and instruments.
A rough estimate of the gain in sensitivity for the WFI at the MPG/ESO 2.2-m telescope as compared to objective prism spectroscopy at Schmidt telescopes gives: gain in aperture by a factor of 4-5, gain by CCD sensitivity as oposed to fotographic emulsion by a further factor of 20 or more, for a total factor of 100 in sensitivity gain. The advantage of the WFI in spectroscopic mode with respect to standard CCD spectrographs at 2-4m class telescopes is its much larger field of view, and its ability to obtain spectra of all objects therein at once.
In the preliminary draft mentioned above the following areas of research were identified (with the help of five external scientists):
The potentialities of the instrument can be gauged by looking at the survey results by Schneider, Schmidt, and Gunn (1994), where they cover a 61 degree area but with an efective exposure time of only 34s to 63s (the WFI in 4 hours integration covers an equivalent volume): they obtained spectra for 600000 objects, with approximately 800 emission line galaxies, and 90 quasars. The same authors in a deeper survey, with 300s effective exposure time, and 1.10 square degrees effective area, find approximately 50 low redshift emission line galaxies, 9 high redshift (z > 2.7) quasars (Schneider, Schmidt, and Gunn 1999).
Here we describe the instrument configuration for spectroscopic observations, and sumarize the results of two commissioning runs done during March 2001.
| | | 1 Introduction | Contents | Index |