ESO VLT FOCAL REDUCER AND SPECTROGRAPH  
        #1 (FORS1)
======================================

  Instrument Overview                                                         
  ===================                                                         
    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 focus of 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).  FORS1 is an all-dioptric        
    instrument for the wavelength range from 330 nm to 1100 nm and provides an
    image scale of 0.25 arssec/pix (or 0.125 arsec/pix with the               
    high-resolution collimator) in standard readout mode (2x2 binning).  FORS1
    has a mosaic of two 2K x 4K E2V CCDs with 15 micron pixels.  FORS1 has the
    following observing modes:                                                
                                                                              
    IMA - imaging                                                             
    OCC - imaging with occulting bars                                         
    LSS - longslit spectroscopy                                               
    MOS - multi-object spectroscopy (moveable slits)                          
    IPOL - imaging polarimetry                                                
    PMOS - multi-object spectropolarimetry                                    
                                                                              
    The IPOL mode is used to measure linear polarization in imaging mode.  The
    measurements are perfomed by using a remotely controlled rotatable        
    half-wave retarder plate in front of the Wollaston prism. Rotation of the 
    half-wave plate by steps of 22.5 degrees and corresponding measurements of
    the intensities of the two rays allow the derivation of the values of the 
    Stokes Q and U parameters and hence the value of the degree of linear     
    polarization and the position angle.                                      
                                                                              
    For more information about the FORS1 instrument, see Appenzeller et al.   
    (1998).  For more 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, Successful commissioning 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 with polarimetric FORS1/VLT observations, Astron. Astroph.   
    450, 1239-1248, 2006.                                                     
                                                                              
    Fossati, L., S. Bagnulo, E. Mason, and E. Landi Degl'Innocenti, E.,       
    Standard Stars for 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 by C.     
    Sterken. San Francisco: Astronomical Society of the Pacific, p.503-513,   
    2007.

References
==========

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,        
        Successful commissioning of FORS1 - the first optical instrument on   
        the VLT, Messenger 94, 1-6, 1998.

Bagnulo S., H. Boehnhardt, K. Muinonen, 
        L. Kolokolova, I. Belskaya, and M.A. Barucci, Exploring the surface   
        properties of transneptunian objects and Centaurs with polarimetric   
        FORS1/VLT observations, Astron. Astroph., v. 450, No. 3, p. 1239-1248,
        2006.

Fossati, L., S. Bagnulo, E. Mason, and  
        E. Landi Degl'Innocenti, E., Standard Stars for 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 by C. Sterken. San Francisco: Astronomical Society of 
        the Pacific, p.503-513, 2007.