Blue sky would be good. Makes a good "flat" too
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The spot looks too big for a dead pixel. I'd be going for dust. Quote from "The New CCD Astronomy"
Quote: "The visual impact of the dust varies with its distance from the focal plane. Dust motes that are very close (to) the CCD chip will throuw small, tight, and dark shadows. Dust motes further up the optical pathway, such as on a filter a few inches in front of the CCD chip, will throw larger, more diffuse shadows." End Quote.
If its a dust mote it looks like its pretty close to the CCD surface.