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include <math.h>
define SZ_CDMTX 4
# EQTOPIX -- procedure to obtain pixel coordinates from the equatorial
# ones, giving the center position and scale of the frame.
procedure eqtopix (plt_ra_cen, plt_dec_cen, plt_x_cen, plt_y_cen,
cdmtx, ra, dec, x, y)
double plt_ra_cen # Plate centre (radians)
double plt_dec_cen # " "
double plt_x_cen # Plate center in x (pixels)
double plt_y_cen # Plate center in y (pixels)
double cdmtx[SZ_CDMTX] # CD Matrix (cd11, cd12, cd21, cd22)
double ra # Objects RA position (radians)
double dec # Objects DEC position (radians)
double x # Position from centre (pixels)
double y # "
double xi # Standard coordinate (degrees)
double eta # "
double det, cd11, cd12, cd21, cd22
double xi_arcs, eta_arcs # Standard coord. in arc seconds.
begin
cd11 = cdmtx[1]
cd12 = cdmtx[2]
cd21 = cdmtx[3]
cd22 = cdmtx[4]
det = (cd11*cd22 - cd21*cd12)
call treqst (plt_ra_cen, plt_dec_cen, ra, dec,
xi_arcs, eta_arcs)
xi = xi_arcs/3600.0 # To degrees
eta = eta_arcs/3600.0 # To degrees
# Calculate x[i] and y[i] by solving the linear equations:
# xi = cd11(X-Xo) + cd12(Y-Yo)
# eta = cd21(X-Xo) + cd22(Y-Yo)
x = (cd22*xi - cd12*eta)/det + plt_x_cen
y = (cd11*eta - cd21*xi)/det + plt_y_cen
end
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