parallactic_angle

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 parallactic_angle function                                             ephem
 Description: Calculate the parallactic angle of an object.
              The parallactic is defined as the angle between the local
              zenith, the object and the celestial north pole measured
              westwerd (e.g., negative before, and positive after the
              passage through the southern meridian).
 Input  : * Set of three input argument: [RA, Dec], LST, Lat
            or alternatively four input argument: RA, Dec, LST, Lat.
            Where RA, Dec and Lat are in radians, and LST in 
            fraction of days. Lat is observer the geodetic latitude.
            LST can be either a scalar, matrix of the same size as
            RA and Dec, or a vector which have a common dimension
            as RA and Dec.
 Output : - Parallactic angle
            If object in the zenith then NaN.
 Tested : Matlab 5.3
     By : Eran O. Ofek                   October 2001
    URL : http://wise-obs.tau.ac.il/~eran/matlab.html
 Reliable: 1
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0001 function [PA]=parallactic_angle(varargin)
0002 %------------------------------------------------------------------------------
0003 % parallactic_angle function                                             ephem
0004 % Description: Calculate the parallactic angle of an object.
0005 %              The parallactic is defined as the angle between the local
0006 %              zenith, the object and the celestial north pole measured
0007 %              westwerd (e.g., negative before, and positive after the
0008 %              passage through the southern meridian).
0009 % Input  : * Set of three input argument: [RA, Dec], LST, Lat
0010 %            or alternatively four input argument: RA, Dec, LST, Lat.
0011 %            Where RA, Dec and Lat are in radians, and LST in
0012 %            fraction of days. Lat is observer the geodetic latitude.
0013 %            LST can be either a scalar, matrix of the same size as
0014 %            RA and Dec, or a vector which have a common dimension
0015 %            as RA and Dec.
0016 % Output : - Parallactic angle
0017 %            If object in the zenith then NaN.
0018 % Tested : Matlab 5.3
0019 %     By : Eran O. Ofek                   October 2001
0020 %    URL : http://wise-obs.tau.ac.il/~eran/matlab.html
0021 % Reliable: 1
0022 %------------------------------------------------------------------------------
0023 
0024 if (length(varargin)==3),
0025    Coo = varargin{1};
0026    LST = varargin{2};
0027    Lat = varargin{3};
0028 
0029    RA  = Coo(:,1);
0030    Dec = Coo(:,2);
0031 else
0032    RA   = varargin{1};
0033    Dec  = varargin{2};
0034    LST  = varargin{3};
0035    Lat  = varargin{4};
0036 end
0037 %HA = 2.*pi.*LST - RA;
0038 HA = bsxfun(@minus,2.*pi.*LST,RA);
0039 
0040 TanQ = sin(HA)./(tan(Lat).*cos(Dec) - sin(Dec).*cos(HA));
0041 PA   = atan(TanQ);