Problem:

This exposure of a bright star shows the signature of internal reflections as doughnuts around, but not centered on, the bright star.

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).

The fact that the centers of the doughnuts are not centered on the star is most likely due to the fact that the star is not centered on the CCD. Thus the internal reflections will re-image the out-of-focus doughnut away from the star in a radial direction from the CCD center. See below for links to bright stellar images and internal reflections in which either the reflective surfaces in the optical path are tilted, or in which the star is in the center of the CCD and thus causes the doughnuts to be centered on the star.

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.

The diffraction spikes in this image are caused by (the Fourier Transform of) the support struts which hold the secondary mirror in place. The very same support struts also cause the dark lines in the doughnuts. The reason why there are only four dark lines in the doughnut, and six bright diffraction spikes (which are not aligned with the dark lines in the doughnut) is not fully understood, but it might have to do with the fact that the Fourier Transform of the support struts, seen from off-axis, may cause an appearance like this.

Fix: