;+
; NAME:
;   oc_star_apply_pm
;
; PURPOSE:
;   Apply proper motion to a star
;
; DESCRIPTION:
;   Apply proper motion to a star
;
; CALLING SEQUENCE:
;   star_et = oc_star_apply_pm(star, et)
;
; INPUTS:
;   star - star structure or array of structures
;   et - ephemeris time (seconds TDB after J2000)
;
; OPTIONAL INPUT PARAMETERS:
;  None
;
; KEYWORDS:
;  None
;
; OUTPUTS:
;  If et is a scalar and star is a single structure then
;  return a star structure
;
;  If et is an array and the star is a single structure
;  then return [star_0,star_1,...] where star_i is for et[i]
;
;  If et is a scalar and the star is an array of structures
;  then return [ star_0, star_1, ... ] where star_i is for star[i]
;  
;  If both et and the star are arrays
;  then return [ star_0, star_1, ... ] where star_i is for et[i], star[i]
;  (if they are different lengths, then do up to the minimum)
; 
;  REVISON HISTORY:
;   2006 Mar 08 Leslie Young.  
;   2006 Mar 21 Henry Throop.  Modified to return proper ET.
;-

function oc_star_apply_pm, starin, et

    net = n_elements(et)
    nstar = n_elements(starin)

    star = starin

; Call the routine to calculate the new RA/DEC/ERR.
; That routine returns the RA/DEC/ERR only, but does *not*
; populate the structure.

; Populate the structure with the new RA/DEC/ERR.

; One ET, one star

    if (net eq 1) and (nstar eq 1) then begin
      radec = oc_star_radec(et, starin, dradec, radecerr)
      for i = 0L, nstar-1 do begin
        star[i].ra     = radec[0,i]
        star[i].dec    = radec[1,i]
        star[i].raerr  = radecerr[0,i]
        star[i].decerr = radecerr[1,i]
      endfor
      star.et	= et
    end

; Many ET's, one star

    if (net gt 1) and (nstar eq 1) then begin
      star	= replicate(star, net)
      radec 	= oc_star_radec(et, starin, dradec, radecerr)
      for i = 0L, net-1 do begin
        star[i].ra     = radec[0,i]
        star[i].dec    = radec[1,i]
        star[i].raerr  = radecerr[0,i]
        star[i].decerr = radecerr[1,i]
	star[i].et	= et[i]
      endfor
    end

; One ET, many stars

    if (net eq 1) and (nstar gt 1) then begin
      radec = oc_star_radec(et, starin, dradec, radecerr)
      for i = 0L, nstar-1 do begin
        star[i].ra     = radec[0,i]
        star[i].dec    = radec[1,i]
        star[i].raerr  = radecerr[0,i]
        star[i].decerr = radecerr[1,i]
	star[i].et	= et
      endfor
    end

; Many ET, many stars

    if (net gt 1) and (nstar gt 1) then begin
      radec = oc_star_radec(et, starin, dradec, radecerr)
      for i = 0L, max([nstar-1, net-1]) do begin
        star[i].ra     = radec[0,i]
        star[i].dec    = radec[1,i]
        star[i].raerr  = radecerr[0,i]
        star[i].decerr = radecerr[1,i]
	star[i].et	= et[i]
      endfor
    end

    return, star

end