Image artifacts – Internal reflections
Light reflecting inside the telescope optics, and diffracting off edges within the telescopes, can produce some interesting effects.
Consider this series of images showing the planet Venus leaving the field of view of the HI1-B telescope between Janary 26-31, 2009. As Venus approaches the edge of the field-of-view, a ring shape is seen apparently coming out of the planet.
This is caused by reflections of the bright planet off of the camera barrel. (If you look closely at the full-field version of the January 26, 2009 image above, you’ll see a large, faint bubble on the left side of the image, which is also an internal reflection of Venus.) The ring grows progressively larger as time goes by. On January 31, a horizontal streak appears near the position where Venus disappeared. This latter effect is caused by diffraction off of the optical baffles.
Source and author: Skyywatcher88 (youtube)