CRAO FIVE-CHANNEL PHOTOPOLARIMETER
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  Instrument Overview                                                         
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    The Crimean Astrophysical Observatory (CrAO) Five-Channel Photopolarimeter
    is capable of simultaneous polarimetric measurements in UBVRI colors, with
    four dichroic filters splitting the optical paths of the UBVRI components 
    of white light in such a way as to feed five separate photomultipliers    
    that measure each color simultaneously.  A half-wave plate in front of a  
    calcite block splits the incoming radiation into two components with      
    polarization planes mutually orthogonal to each other.  Rotation of the   
    half-wave plate by steps of 45 degrees and corresponding measurements of  
    the intensities of the two rays allow the observer to derive the values of
    the Stokes parameters and hence the value of the degree of linear         
    polarization and the position angle. The construction of the polarimeter  
    allows a direct elimination of the sky background polarization.           
                                                                              
    For more information, see:                                                
                                                                              
    Piirola, V., Simultaneous five-color (UBVRI) photopolarimeter, in Coyne,  
    G.V., A.M. Magalhaes, A.F.J. Moffat, R.E. Schulte-Ladbeck, S. Tapia, and  
    D.T. Wickramasinghe (eds.), Polarized Radiation of Circumstellar Origin,  
    Univ. of Arizona Press, Tucson, pp. 735-746, 1989.                        
                                                                              
    This book may be difficult to obtain, so a more accessible publication    
    with information about the instrument is:                                 
                                                                              
    Scaltriti, F., V. Piirola, A. Cellino, E. Anderlucci, L. Corcione, G.     
    Massone, F. Racioppi, and F. Porcu, The UBVRI photopolarimeter of the     
    Torino Astronomical Observatory, Mem.  Soc. Astron. It. 60, 243-246, 1989.
                                                                              
    See also:                                                                 
                                                                              
    Korhonen, T., V. Piirola, and A. Reiz, Polarization measurements at La    
    Silla, ESO Messenger, Dec. 1984, p, 20-24.                                
                                                                              
    For methods of observation and data processing, see:                      
                                                                              
    Shakhovskoy, N.M. 1994. The methods for analysis of polarization          
    observation.  Izvestiya Krimskoy Astrophisicheskoy Observatorii 91,       
    106-123. (In Russian).

References
==========

Korhonen, T., V. Piirola, and A. Reiz,  
        Polarization measurements at La Silla, ESO Messenger, Dec. 1984, p,   
        20-24.

Piirola, V., Simultaneous five-color    
        (UBVRI) photopolarimeter, in Coyne, G.V., A.M. Magalhaes, A.F.J.      
        Moffat, R.E. Schulte-Ladbeck, S. Tapia, and D.T. Wickramasinghe       
        (eds.), Polarized Radiation of Circumstellar Origin, Univ. of Arizona 
        Press, Tucson, pp. 735-746, 1989.

Scaltriti, F., V. Piirola, A. Cellino,  
        E. Anderlucci, L. Corcione, G. Massone, F. Racioppi, and F. Porcu, The
        UBVRI photopolarimeter of the Torino Astronomical Observatory, Mem.   
        Soc. Astron. It. 60, 243-246, 1989.

Shakhovskoy, N.M., The methods for      
        analysis of polarization observation, Izv. Krimskoy Astrophis. Obs.   
        91, 106-123, 1994.