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Author Message   Jeffk Tinkerer Username: Jeffk Post Number: 61 Registered: 10-2011 Rating: N/A Votes: 0 Posted on Saturday, March 10, 2012 - 07:23 pm:    Flatness of the mirrors isn't critical here, they just need to be flatter than the divergence of the beam over the small area of the beam, so that the beam expands. Just bouncing one off the length of a typical house a few times off 2-3 mirrors is enough, a laser is spatially coherent but the divergence isn't zero. And if the lens is focused for red or green, it will be focused within the chromatic aberation range for other colors too - better than this isn't meaningful (or defined, really) for white light through a real lens. But agreed, if you want to build one then an autocollimator would be the best way to go, reflecting light off a piece of actual film placed inside the camera.