pro centrproj,x,y,theta0,phi0,theta,phi
; This routine returns the zenith (theta) and azimuth (phi) of a
; point on the sky when the dihedral angles x (azimuth) and y (zenith),
; and the center
; of the field of view in zenith and azimuth (theta0, phi0) are known.
; All angles are called out in degrees.  Zenith is converted to latitude
; inside the procedure.
pi=3.1415926535
rad=pi/180.
alph=x*rad
beta=y*rad
f0=(90.-theta0)*rad
lamb0=phi0*rad
f=atan((sin(f0)+beta*cos(f0))/sqrt(alph^2+(cos(f0)-beta*sin(f0))^2))
lamb=lamb0+asin(alph/sqrt(alph^2+(cos(f0)-beta*sin(f0))^2))
f=f/rad
theta=90.-f
phi=lamb/rad
return
end