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 and find their way to your sensors, even if you are careful and don’t point directly at the sun, moon, or Earth (the brightest sources for most spacecraft). Preventing or reducing stray light is an important part of design and fabrication of such optical systems and can even determine the success or failure of a mission.
A case in point from a paper presented by Mr. Thierry Viard of Thales Alenia Space in sunny Cannes, France. The COROT spacecraft was launched in December 2006 to search for exoplanets with short orbital periods, especially large terrestrial (Earth-like) planets. COROT is a relatively small and low-cost spacecraft, so launching it to a dark, distant Lagrange point (e.g., L2) was not possible. An 800 km polar orbit would have to do, which meant that the very bright Earth would always be nearby (in addition to the sun and moon, though the sun would always be kept “behind” the spacecraft by pointing to a different area of the sky depending on the season).
The “transit” method of detecting exoplanets depends on recording very slight changes in the brightness of rather dim stars as a small planet passes in front of the star as viewed from our direction. The telescope had to be designed with a form that would minimize the chances of light from “off-axis” objects (those outside of the desired pointing area) getting to the detectors. Several optical criteria used in the chosen design form helped with this, in addition to extensive “baffles” (black metal rings that only allow light from certain directions to get in).
But this was not enough for COROT to succeed. The engineers also determined that cleanliness would make or break this mission. The slightest dust in the spacecraft could also scatter light to the detector, and such scattering had to kept to about 10 photons per pixel per second from the nearby Earth (which is putting out about 10^20 photons per second per pixel, a huge, huge number). This required keeping particle contamination (dust) to something under 200 parts/million, which is very clean even for a clean room used to building delicate spacecraft. And there was no way to even test this without risking even more contamination, so it had to be based entirely on simulations. Pretty scary!
But guess what? The COROT optics team succeeded. When they first imaged the starry sky, it was pitch black except for the bright, distinct stars. Just as designed. They detected their first exoplanet in May 2007, and the first Earth-like exoplanet , COROT-7b (1.7 times Earth’s radius) in 2009.
Of course there is much more to this or to any spacecraft than the optical systems. This is just an example of the clever engineering, hard work, extreme attention to detail, and ultra-high quality needed for any successful space mission. Space is not easy.
Photos courtesy of CNES.