And here is a close-in crop of M51::
AP155EDF w 4" FF at ~ F/7
12 x 10 min Luminance, 3 x 10 min RGB for a total of 210 minutes
Mount - Paramount ME
Acquired in CCDSoft5
Alignment, Calibration and Sigma Clip combine in Maxim
Final processing in PS CS4
Luminance layered in per Rob Gendler
May 8 2011
Flesherton, Ontario, Canada
Right Ascension 13 : 29.9 (h:m)
Declination +47 : 12 (deg:m)
Distance 37000 (kly)
Visual Brightness 8.4 (mag)
Apparent Dimension 11x7 (arc min)
Discovered 1773 by Charles Messier.
The famous Whirlpool galaxy M51 was one of Charles Messier's original discoveries: He discovered it on October 13, 1773, when observing a comet, and described it as a "very faint nebula, without stars" which is difficult to see. Its companion, NGC 5195, was discovered on March 21, 1781 by his friend Pierre Méchain, so that it is mentioned in Messier's 1781 catalog: `It is double, each has a bright center, which are separated 4'35". The two "atmospheres" touch each other, the one is even fainter than the other.' In addition to the description, in his personal copy of the catalog, Messier has added a small drawing, or sketch of the two "nebulae," M51 and NGC 5195, one of the rare cases a detailed drawing by Charles Messier of one of his objects is preserved.
M51 is the dominating member of a small group of galaxies, which also contains M63 and a number of fainter galaxies. As it is about 37 million light years distant and so conspicuous, it is actually a big and luminous galaxy. The value of M51's (and the whole group's) distance is still not very well known.
This galaxy was the first one where the spiral structure was discovered, in spring 1845 by Lord Rosse, who made a very careful and acurate painting. Therefore, M51 is sometimes referenced as Rosse's Galaxy or Lord Rosse's "Question Mark".
According to present understanding, the pronounced spiral structure is a result of M51's current encounter with its neighbor, NGC 5195 (the fainter one in Messier's description). Due to this interaction, the gas in the galaxy was disturbed and compressed in some regions, resulting in the formation of new young stars. As is common in galactic encounters, spiral structure is preferably induced in the more massive galaxy.
For the amateur, M51 is easy and a showpiece if the sky is dark, but is quite sensitive for light pollution which easily makes it fade in the background. Under very good conditions, even suggestions of its spiral arms can be glanced with telescopes starting from 4-inch. Low magnification is best for viewing this pair.