Instrument Information
IDENTIFIER urn:nasa:pds:context:instrument:eso-paranal.vlt_kueyen_8m2_ut2.forsi::1.1
NAME ESO VLT FOCAL REDUCER AND SPECTROGRAPH #1 (FORS1)
TYPE IMAGER

SPECTROMETER
DESCRIPTION Two versions of the visual and near-UV Focal Reducer and low-dispersion Spectrograph(FORS) have been built. Until June 1, 2004 FORS1 was mounted at the Cassegrain focusof Unit Telescope 1 (Antu); after this date it was moved to Unit Telescope 2 (Kueyen)of the Very Large Telescope (VLT) at the European Southern Observatory (ESO). FORS1is an all-dioptric instrument for the wavelength range from 330 nm to 1100 nm andprovides an image scale of 0.25 arssec/pix (or 0.125 arsec/pix with thehigh-resolution collimator) in standard readout mode (2x2 binning). FORS1 has amosaic of two 2K x 4K E2V CCDs with 15 micron pixels. FORS1 has the followingobserving modes:IMA - imagingOCC - imaging with occulting barsLSS - longslit spectroscopyMOS - multi-object spectroscopy (moveable slits)IPOL - imaging polarimetryPMOS - multi-object spectropolarimetryThe IPOL mode is used to measure linear polarization in imaging mode. Themeasurements are perfomed by using a remotely controlled rotatable half-wave retarderplate in front of the Wollaston prism. Rotation of the half-wave plate by steps of22.5 degrees and corresponding measurements of the intensities of the two rays allowthe derivation of the values of the Stokes Q and U parameters and hence the value ofthe degree of linear polarization and the position angle.For more information about the FORS1 instrument, see Appenzeller et al. (1998). Formore information specifically about the IPOL mode of FORS1, see Bagnulo et al. (2006)and Fossati et al. (2007).Appenzeller, I, K. Fricke, W. Furtig, W. Gassler, R. Hafner, R. Harke, H.-J. Hess, W.Hummel, P. Jurgens, R.-P. Kudritzki, K.-H. Mantel, W. Meisl, B. Muschielok, H.Nicklas, G. Rupprecht, W. Seifert, O. Stahl, T. Szeifert, and K. Tarantik, Successfulcommissioning of FORS1 - the first optical instrument on the VLT, Messenger 94, 1-6,1998.Bagnulo S., Boehnhardt H., Muinonen K., Kolokolova L., Belskaya I., and Barucci M.A.,Exploring the surface properties of transneptunian objects and Centaurs withpolarimetric FORS1/VLT observations, Astron. Astroph. 450, 1239-1248, 2006.Fossati, L., S. Bagnulo, E. Mason, and E. Landi Degl'Innocenti, E., Standard Starsfor Linear Polarization Observed with FORS1. In: The Future of Photometric,Spectrophotometric and Polarimetric Standardization, ASP Conference Series, Vol. 364,Proceedings of a conference held 8-11 May, 2006 in Blankenberge, Belgium. Edited byC. Sterken. San Francisco: Astronomical Society of the Pacific, p.503-513, 2007.
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REFERENCES Bagnulo S., H. Boehnhardt, K. Muinonen, L. Kolokolova, I. Belskaya, and M.A. Barucci,Exploring the surface properties of transneptunian objects and Centaurs withpolarimetric FORS1/VLT observations, Astron. Astroph., v. 450, No. 3, p. 1239-1248,2006.

Appenzeller, I, K. Fricke, W. Furtig, W. Gassler, R. Hafner, R. Harke, H.-J. Hess, W.Hummel, P. Jurgens, R.-P. Kudritzki, K.-H. Mantel, W. Meisl, B. Muschielok, H.Nicklas, G. Rupprecht, W. Seifert, O. Stahl, T. Szeifert, and K. Tarantik, Successfulcommissioning of FORS1 - the first optical instrument on the VLT, Messenger 94, 1-6,1998.

Fossati, L., S. Bagnulo, E. Mason, and E. Landi Degl'Innocenti, E., Standard Starsfor Linear Polarization Observed with FORS1. In: The Future of Photometric,Spectrophotometric and Polarimetric Standardization, ASP Conference Series, Vol. 364,Proceedings of a conference held 8-11 May, 2006 in Blankenberge, Belgium. Edited byC. Sterken. San Francisco: Astronomical Society of the Pacific, p.503-513, 2007.