First things first, I replicated an Arago spot as per the instructable, mostly to get a feel for how it behaves with different separations and different sized balls/discs. That allowed me to confirm a suspicion I had. The Wikipedia page for Arago spots shows that the spot is the same intensity as the brightness at the rim of the disc, and the spot is an Airy disk whose size is determined by the lens disc size (in the same way as the diffraction limit of the circular aperture of a telescope). So the peak intensity is independent of disc diameter but the larger the lens disc the smaller the Airy disk, so larger lens discs ‘focus’ less total light onto the image plane. Therefore going very large with the disc will give very little light at all, but going small gives low resolution. The image is an extended set of Arago spots from each ‘point source’ in the image plane, which are always formed, regardless of separation, so the distances don’t need to be focused, it will always image at the diffraction limit. Magnification can be calculated by similar triangles, just like a single lens set up. The ratio of the distance from the disc to the image plane over the distance from the subject to the disc gives the magnification. Exposure can be set up by trial and error, but you need the light source to illuminate the disc sufficiently that a spot of that brightness will be detectable on the sensor, both in terms of absolute brightness and in terms of brightness above the stray light illumination of your environment.

Cheers

Ben

Mark

Steve