PRO IM_ROT,az,el,ra,RA1,RA2,t1,t2,t3
; Gives the image rotation as a function of telescope
; coordinates and observation table at the Swedish Solar
; Observatory, La Palma.

; Looking at the projected primary image the rotation is
; clockwise during the morning up the meridian passage
; and then counterclockwise during the afternoon.

; Input parameters: AZ  azimuth of Sun in radians
;		    EL  elevation of Sun in radians
;			 TC  table constant in radians
;			     a constant depending on which observation
;			     table is used. TC is about 48 to give
;			     the angle between the table surface
;			     and the N-S direction at the first
;			     observation table.

; Output parameter: RA  rotation angle in radians

; Adapted from Goran Hosinsky

LAT=28.758*!dtor      ;observatory latitude (La Palma)
TC=318.0*!dtor    ;table constant

; According to spherical astronomy the angle between the N-S
; great circle and the vertical great circle in an AZ-EL
; telescope varies as:

t1=((EL-LAT)/2.0)
t2=((!pi-EL-LAT)/2.0)
t3=tan(0.5*(!pi-AZ))
t3=t3<1.E20
ra1=atan(cos(t1)/cos(t2),t3)+atan(sin(t1)/sin(t2),t3)

; In the image plane the angle of the movement in Elevation
; varies as:

ra2=AZ+(atan(cos(EL),sin(EL)))+TC

RA=ra2-ra1

END