function fit_vis_2src,visin,xy1,xy2,sc,PLOT=plot,XYOFFSET=xyoffset,YFIT=yfit

; PURPOSE:
;  Finds the best fit coefficients A,B for one-collimator visibility:
;  Vis = A*exp(i*k.x1) + B*exp(i*k.x2)
;     where RHESSI wavevector as a function of PA; and xy1,xy2 are vector
;     source positions

; INPUTS:
;  VISIN = visibility structure with the usual tags
;  `XY1 and XY2 are vector positions of the sources (arcsec) relative
;  to map center (unless XYOFFSET is set, dee below)  
;  SC = scalar in range 1-9 designating a subcollimator
;
; OPTIONAL INPUTS:
;   If xyoffset is a named variable, the xyoffset is subtracted from xy1,xy2
;   Default =[0,0], for which xy1,xy2 must be relative to map center
;
; OUTPUTS:
;   [A,B]
;
; HISTORY:
;   EJS Dec 2006
;   revised to use regress.pro

default,xyoffset,[0.,0.]
x1=xy1[0]-xyoffset[0] ; if xyoffset not specified, we assume xy are relative to map center
x2=xy2[0]-xyoffset[0]
y1=xy1[1]-xyoffset[1]
y2=xy1[1]-xyoffset[1]

pitch = (hsi_grid_parameters()).pitch
w=where(visin.isc eq sc,nw)
if nw eq 0 then message,'No subcollimator visibilities exist'
if nw lt 3 then message,'WARNING: 3 or fewer visibilities',/info

u=visin[w].u
v=visin[w].v
PA=atan(v,u)
i=complex(0,1)
dx=[0,1,0,-1]*0.05
dy=[1,0,-1,0]*0.05

x1=-7.25
x2= 6.5
y1=-10.5
y2= 8.3

for mm=0,3 do begin
  for nn=0,3 do begin
    x_1=x1+dx[mm] & x_2=x2+dx[nn]
    y_1=y1+dy[mm] & y_2=y2+dy[nn]
  rho=exp(i*2*!pi*(u*x_1+v*y_1))
  tau=exp(i*2*!pi*(u*x_2+v*y_2))
  ;stop

   ; REGRESS solution
  XX1=[float(rho),imaginary(rho)]
  XX2=[float(tau),imaginary(tau)]
  XX=[transpose(XX1),transpose(XX2)]
  YY=[float(visin[w].obsvis),imaginary(visin[w].obsvis)]
  chisq=-1 & const=-1 & SIGMA=sigma & yfit=-1
  AB=regress(XX,YY,CHISQ=chisq,CONST=const,SIGMA=sigma,YFIT=yfit)
  print,x_1,x_2,y_1,y_2,' chisq=',chisq
endfor
endfor
stop

if keyword_set(plot) then begin
!p.multi=[0,1,2]
  plot,pa,abs(AB[0]*rho+AB[1]*tau)
  plot,pa,abs(visin[w].obsvis),psym=6
endif
stop

return,AB
end

restore,'MEMfit_sigvis_phzradset_2003-06-17_225310-225440E30-60ff.sav',/ver
xyoffset=vis[0].xyoffset
visin=vis
xy1=[xpos[0],ypos[0]]
xy2=[xpos[1],ypos[1]]
sc=4
yfit=-1

AB=fit_vis_2src(visin,xy1,xy2,sc,/plot,XYOFFSET=xyoffset,yfit=yfit)

A=ab[0] & B=ab[1]
print,'A,B=',a,b

end