Just a quick added note. The point of the barlow is not just to create a blurred laser beam allowing a shadow of the centre dot to be projected back. I don't understand the optical side of it, but once you fit the barlow lens, the beam returned from the primary becomes immune to wobble or slop of the laser in the eypiece holder
.
I'm sure we've all be frustrated by the act of careful laser collimation only to find the laser wobbling in the holder making a nonesense of our effort. To wiggle the barlowed laser and see the projected shadow of the centre dot onto the base of the laser, sitting rock solid, is something that has to be seen to be believed.