What cluster of stars is this with a "dark donut" to one side? In the 2011 Sixty Symbols video Spy Satellites (from Deep Sky Videos) after about 01:44 amateur astrophotographer Nik Szymanek shows an image of a cluster of stars and I think I see what looks like a dark donut shape (dust?) to the right of the center of brightness, as shown in the screenshots below.

The vertical streak is a satellite trail, the topic of the video but not related to the present question.

Question: Can someone recognize what object this is and explain what is thought to cause the dark donut?

Screenshots from video, click for full size:

 

Cropped and shamelessly processed to enhance the dark donut's visibility:
OK, having (finally) actually looked at the video, it's clear that Szymanek is looking at the center of M33. There is in fact a nuclear star cluster in the center of that galaxy; not knowing the field of view or the resolution, I can't tell how much of the central condensation is simply the unresolved, smeared-out nuclear star cluster (which is small -- you really need something like HST to resolve it) and how much is that plus the surrounding central region of the disk.

The "dark donut" is undoubtedly the diffraction shadow due to a dust grain on the filter or other optical element in front of the detector; see the "Dust Rings" subsection of this page for an example and brief discussion.






  Dust on either the filter, the window protecting the CCD, or any of the corrective optics will leave little donut shapes on an image like the one below. They appear as rings because the dust grains lie on optical surfaces above the focal plane so when they cast a shadow on the CCD, it is out of focus. Astronomers can measure the size of a dust ring and tell exactly where in the optics the grain of dust lies. Dust grains on the CCD itself leaves little dark spots. Calibration frames completely remove dust spots and rings from images.