proper motion regimes

In between work supporting our alpha testers and writing other people's papers, I thought a bit more about detecting faint source proper motions. There are really four regimes: slow, in which stars move less than the PSF over the duration of the survey (total epoch range); medium, in which stars move substantially more than the PSF, but less than the size of individual images; fast, in which the sources move a distance comparable to the size of any individual epoch image between epochs, and many times this over the duration of the survey; and streaked, in which the sources make trails even in individual images, and probably have orbits that are non-trivially curved over the duration of the survey. Each of these regimes requires different statistical equipment and techniques for optimal detection and measurement. I realized I need to re-read papers by Lepine.