The Great Whirlpool Galaxy. This is the first galaxy in which a spiral structure was resolved - by Lord Rosse employing a 72-inch (~1.83 m) reflecting telescope which he constructed at Birr Castle, Ireland.

M51 is actually a pair of colliding galaxies referred to as M51A (NGC 5194) and M51B (NGC 5195). Source wikipedia

exposure details

approx 24 hours luminescence in 15 minute subs
4 hours each or red, green and blue

total exposure time 36 hours -)