# 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:

• I've added both globular-cluster and star-cluster tags because I don't really know what the object is. Feel free to improve the tagging.
– uhoh
Sep 30, 2019 at 6:28

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.

• Thanks, I wish it would be possible to find a better explanation for how that works. I see the link and the explanation there but it's not helping be understand exactly what's happening. Is it something like a penumbra? Perhaps something here? Is the donut-like shape of the ring caused by the donut-like shape of the circular aperture with circular secondary obstruction for example?
– uhoh
Sep 30, 2019 at 11:33
• The cloudynight.com thread seems to be mostly about something else. There is in fact a "dust donut" in the image, but it's removed by the flat-fielding and so isn't a concern. Sep 30, 2019 at 11:47
• I believe the donut shape may in fact be related to the secondary obstruction; the "dust donut" is in effect an out-of-focus shadow (since the dust grain causing it is probably sitting on the filter or something else above the focal plane. Sep 30, 2019 at 11:48
• P.S. I tried running the larger of the two screenshots through astrometry.net, but it couldn't get a solution, so I've still no idea where in the sky that is... Sep 30, 2019 at 11:49
• Good catch; I went and watched the video (which I should have done earlier) and it's clear that M33 is the target for all the images. I've updated my answer accordingly. Sep 30, 2019 at 12:07