function psf_1_gauss_arg, d, xcen, ycen, rad 

    nxnyn, d, nx, ny

    ; ----------- isolate the data to fit, prepare depend. var. structure
    ; find the bounding x and y indices of the bounding box
    x0 = 0>(floor(xcen)-rad)
    x1 = (ceil(xcen)+rad) < (nx-1)
    y0 = 0>(floor(ycen)-rad)
    y1 = (ceil(ycen)+rad) < (ny-1)
    mx = 1L + x1 - x0
    my = 1L + y1 - y0
    
    ; make the indices
    x = lindgen(mx, my)
    y = lindgen(mx, my)
    for i = 0,my-1 do x[*,i] = findgen(mx) + x0
    for i = 0,mx-1 do y[i,*] = findgen(my) + y0
    ok = where( ( (x-xcen)^2. + (y-ycen)^2.) lt rad^2, nok)
    if nok eq 0 then begin
        print, 'psf_1_gauss_arg: no points in fit radius'
        stop
    end
    x = x[ok]
    y = y[ok]
    indx = x + y * nx
    arg = {nx:nx, ny:ny, indx:indx}

    return, arg

end