gethex

 function [ra, dec]=gethex(racenter, deccenter, lside, nx, ny)

 Return RA, DEC coordinates for the hex grid centered on racenter, deccenter.

 Inputs:

   racenter  -- The center pointing RA, in radians (use htor() if you
                want to pass RA as a hh:mm:ss.s string)

   deccenter -- The center pointing DEC, in radians (use dtor() if you
                want to pass DEC as a dd:mm:ss.s string)

   lside     -- The pointing separation, in arcminutes
  
   nx        -- The half-width of the hex grid.  An integral number of
                pointings, spaced along a great circle tangential to the 
                center DEC ring.

   ny        -- The half-width of the hex grid.  An integral number of
                pointings, spaced along a great circle passing through the 
                center RA and the poles

 Example:

    [ra, dec] = gethex(htor('03:00:00'), dtor('45:00:00'), 600, 1, 1);

    would return the seven ra, dec pointing ceters for a simple hex, 
    centered on 3h, 45d, spaced by 10 degrees.

0001 function [ra, dec]=gethex(racenter, deccenter, lside, nx, ny)
0002 
0003 % function [ra, dec]=gethex(racenter, deccenter, lside, nx, ny)
0004 %
0005 % Return RA, DEC coordinates for the hex grid centered on racenter, deccenter.
0006 %
0007 % Inputs:
0008 %
0009 %   racenter  -- The center pointing RA, in radians (use htor() if you
0010 %                want to pass RA as a hh:mm:ss.s string)
0011 %
0012 %   deccenter -- The center pointing DEC, in radians (use dtor() if you
0013 %                want to pass DEC as a dd:mm:ss.s string)
0014 %
0015 %   lside     -- The pointing separation, in arcminutes
0016 %
0017 %   nx        -- The half-width of the hex grid.  An integral number of
0018 %                pointings, spaced along a great circle tangential to the
0019 %                center DEC ring.
0020 %
0021 %   ny        -- The half-width of the hex grid.  An integral number of
0022 %                pointings, spaced along a great circle passing through the
0023 %                center RA and the poles
0024 %
0025 % Example:
0026 %
0027 %    [ra, dec] = gethex(htor('03:00:00'), dtor('45:00:00'), 600, 1, 1);
0028 %
0029 %    would return the seven ra, dec pointing ceters for a simple hex,
0030 %    centered on 3h, 45d, spaced by 10 degrees.
0031 
0032 
0033 % Convert the spacing from arcminutes to radians
0034 
0035 lside = lside*60.0/206265;
0036 
0037 % Draw a globe
0038 
0039 globe;
0040 
0041 % Get the hexgrid in a coordinate system centered on the pointing position
0042 
0043 xyz = hexgrid(lside, nx, ny);
0044 res = zeros(size(xyz));
0045 
0046 % Calculate the rotation matrices needed to rotate into the coordinate
0047 % system aligned with the celestial pole
0048 
0049 rotMat = rot(racenter, deccenter);
0050 
0051 for it=1:size(xyz,2)
0052   res(:,it) = rotMat * xyz(:, it);
0053 end
0054 
0055 % Plot 'em
0056 
0057 plot3(res(1,:),res(2,:),res(3,:), 'c.')
0058 
0059 % Now we have xyz coordinates for the pointing centers.  Convert back
0060 % to RA and DEC in radians
0061 
0062 dec = asin(res(3,:));
0063 ra  = atan2(res(2,:), res(1,:));
0064 
0065 for iRa=1:size(ra,2)
0066   while(ra(iRa) < 0)
0067     ra(iRa) = ra(iRa) + 2*pi;
0068   end
0069 end