x0 = 100.7
y0 = 125.1
r0 = 25.0
im = fltarr(200,300)

nx = (size(im, /dim))[0]
ny = (size(im, /dim))[1]

xx = findgen(nx) # (findgen(ny)*0+1)
yy = (findgen(nx)*0+1) # findgen(ny)
rr = sqrt((xx-x0)^2 + (yy-y0)^2)

help, im, xx, yy, rr

within = where(rr le r0)
im[within] = 1.0

window, xsize=nx, ysize=ny
tvscl, im