The design of an optical system is intended to direct all of the light at the focal plane where it can be absorbed by the sensor. But sometimes the light takes another path, reflecting off of something and eventually getting to the sensor in another way, other than that intended. This is a reflection and can occur in even a well designed optical system. The effect is a halo or other undesirable image artifact that can be seen in the image.

A reflection off of the side or wall of the optical path will usually result in arcs or lines of light across the image. Eliminating these is usually a matter of properly baffling the optical train to eliminate areas where light can strike the wall and reflect off to the sensor.

Not all of the light focused on the CCD is absorbed and converted to electrons, a substantial percentage, as much as 20 to 30%, is reflected off the silver silicon. In any camera there must be some glass over the CCD to protect it, either a cover glass attached to the CCD package or the optical window of the chamber. This glass can reflect some light back at the CCD. The result is a halo or glow around a bright object like a bright star.

Diagram showing the reflection path that can occur between the optical window and the CCD surface

Internal reflection in an SBIG ST8 from the CCD cover glass in a photo of M45.
Photo courtesy Dean Salman

Internal reflection in a Starlight Express MX916 camera beside an image of Zeta Orionis
Photo courtesy Steve Ratts