;
; WRITTEN   14Nov2001
; MODIFIED  07Jul2004  for B0 effect
;
pro br1800a_bl1800a,bl1800a,br1800a,B0=B0,BSZ=bsz

    if N_params() lt 1 then begin
       print,'pro br1800a_bl1800a,bl1800a,br1800a,B0=,BSZ='
       return
    endif

    if not keyword_set(B0) then B0=0
    mgsz=size(bl1800a)
  ; li=findgen(mgsz(2)+2) & dcth=2.0/(mgsz(2)+1)
  ; cth=1.-li*dcth & thr=DOUBLE(acos(cth(1:mgsz(2)))) ; sth=sin(thr)
  ; thr0=(90+B0)*!DTOR
  ; cth0th=COS(thr0-thr)
  ; phd=(findgen(mgsz(1))-mgsz(1)/2)/10.
  ; crph=COS(phd*!DTOR)
    dxsz=2*mgsz(1)
    zgrid,dxsz,mgsz(2),phd,thd,lad,cth,sth
    phr=(phd(dxsz/4:(dxsz*3/4-1))-phd(dxsz/2-1))*!DTOR
    cph=COS(phr)
    th0r=(90-B0)*!DTOR
    cth0=COS(th0r) & sth0=SIN(th0r)
    br1800a=bl1800a
    for j=0,mgsz(2)-1 do $
        for i=0,mgsz(1)-1 do $
            if finite(bl1800a(i,j)) eq 1 then $
               br1800a(i,j)=bl1800a(i,j)/(cth(j)*cth0+sth(j)*sth0*cph(i))
    if keyword_set(bsz) then begin
       idn=where(finite(br1800a) ne 1,cc)
       br1800a(idn)=0.0
       br1800a=SMOOTH(br1800a,bsz,/edge_truncate)
    endif

end