In the little bit of night photography I have done, I have had this problem.

The camera sits on a motorized mount because the exposure time is so long, and the Earth is moving agianst the backdrop of stars.

A piece of dust on the lens will then move along with the sky-seeming to follow it.

Given your name, Im well certain you do quite a bit of proffessional quality night photography and so are causally familiar with such things.

But to those who don't have that experience at all to fall back on, it simply doesn't occur to them.

To those readers who aren't Im pointing this out to clarify how a speck of dust can be significant.