Problem:

This exposure of Betelgeuse shows a series of bright rings offset from the stellar image. Most likely, these are a consequence of internal reflections inside the optical system.

Part of the light hitting the CCD surface is reflected back toward the dewar window and the filter, then reflected back toward the CCD surface where it is perceived as out of focus (thus the doughnuts). There seem to be multiple internal reflections in this example here. In general, the farther away from the CCD a reflecting surface is, the larger of an image it will produce. Depending on this particular setup, the smaller doughnut might be a reflection off the dewar window and the bigger ones from the filter. The dark lines in the doughnuts are the re-imaged support struts which hold the secondary mirror in place above the primary and thus block some of the incoming light.

The fact that the centers of the doughnuts are offset from the star indicates that either the reflecting surface is slightly tilted, or that the CCD itself is tilted. If the latter is the case there should be a focus gradient across the chip in the direction of the tilt (e.g., stars in the middle of the image are in focus, on one edge the focus is below true focus and above true focus on the other edge). See below for a link to an image in which the doughnuts are centered on the star, and a link to an image in which the offset between the centers of the doughnuts and the star is due to the star's location on the CCD.

Although this might at first glance appear as a collimation problem, the fact that the "hole" in the doughnuts is exactly in the center indicates that the collimation is not a problem.

Fix: