MCDONALD OBSERVATORY LINEAR POLARIMETER
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  Instrument Overview                                                         
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    The Linear Polarimeter was put into service at McDonald Observatory in    
    1978.  The main element is a Glan-Air prism, rapidly rotating to          
    counteract atmospheric transparency fluctuations.  The Glan prism was     
    chosen as an analyzer due to its very high throughput, achromatic nature, 
    optical precision, and low cost.  A slight disadvantage                   
    results from the presence of an air gap inside the prism and the resultant
    lateral image displacement (less than 0.1mm).  A slight image rotation    
    causes an instrumental polarization of 0.05% which varies slightly with   
    wavelength, is constant in time, and can easily be subtracted out.  Other 
    sources of instrumental polarization, including the aluminized telescope  
    mirrors, have turned out to be less than 10^-4.                           
                                                                              
    Tests have shown that under ideal observing conditions, the polarimeter   
    can measure accurately to a precision on the order 10^-4.  In practice,   
    therefore, the precision of the polarimeter is limited by photon          
    statistics.                                                               
                                                                              
    For more information about the instrument, including a schematic diagram, 
    see Breger (1979).                                                        
                                                                              
    Breger, M., Interstellar grain size:  A look at the deviant four,         
    Astrophysical Journal 233, 97-101, 1979.

References
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Breger, M., Interstellar grain size: A  
        look at the deviant four, Astrophysical Journal 233, 97-101, 1979.