I recently attended a small technical conference on stray light
in Toulouse, France. It was sponsored by CNES (the French space
agency) and by several European aerospace companies. It was
basically an optical engineering conference on a very
specialized area of optics called “stray light.” As you can
imagine, this means light that you don’t want showing up on the
detectors of your optical system. A familiar example is a “ghost
image” that you might see on a photo that you shoot towards the
sun. You might see rings or hexagonal shapes or patches in your
picture, obscuring part of the actual picture. These are
internal reflections from the lenses or apertures that normally
would be too faint to see unless the source is very bright (like
when the sun is partly or fully in the frame). In spacecraft
optical systems (telescopes or imaging systems for Earth or
other planets), this is a big problem. The sensors for these
systems are very sensitive, and there are many ways for photons
you don’t want to bounce around a