A familiar sight to all, the Whirlpool Galaxy, M51, is interacting with its neighbor, NGC 5195. I found this striking story on
DP Review:
For a long time astronomers were uncertain which of the two galaxies as foreground and which was background until an image taken by the Hale Telescope on Mt Palomar, then the largest telescope in the world, finally settled the question by showing that some of the dust lanes in M51 were obscuring portions of NGC 5195, thus proving that M51 is the closer galaxy from Earth's perspective. That required a forty-five minute exposure on a 200" telescope in 1948 with gas hypered Kodak Techpan film. Today, approximately the same level of sensitivity, if not detail, can be achieved by amateurs in a five minute exposure using an 80mm telescope from a suburban back yard.
Don't know if it the story is true, but it is certainly true that a small modern telescope and today's cameras can show clearly that M51's dust lane obscures part of NGC 5195, as this image does.
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