function ring_model, dim, radius, width

step = 0.1

N = (dim[0] > dim[1])*sqrt(2)/2/step

x = findgen(N)/(N-1)

position = radius/float(dim[0])*sqrt(2)
width_norm = width/float(dim[0])*sqrt(2)

z=((x-position)/width_norm) > (-5) < 5

arg = (z^2*4.*alog(2))

maxval = 64
maxval = 20

index = where(arg gt maxval, count)

model1 = exp(-arg  < 64)

if count gt 0 then model1[index] = 0

xx = (findgen(dim[0])-(dim[0]-1)*0.5) #  replicate(1, dim[1])
yy = (findgen(dim[1])-(dim[1]-1)*0.5) ## rEplicate(1, dim[0])

distan = sqrt(xx^2+yy^2)

return,  model1[distan/step]

end