M51 Whirlpool Galaxy

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2012/04/15 - This is the first light of my new QSI cooled CCD camera. It is also my first CCD image. I did a quick (and bad) focus on M51, started PHD guiding and took some LRGB subs. It was a bit windy, so my setup wasn't very stable. The first results were quite good, but there was a gradient because of light pollution and twilight. During the session some clouds passed by, but guiding stayed on target. I didn't take flats.

The Whirlpool Galaxy is an interacting grand-design spiral galaxy that is estimated to be 23 ± 4 million light-years from the Milky Way Galaxy in the constellation Canes Venatici. It is one of the most famous galaxies in the sky. The galaxy and its companion (NGC 5195) are easily observed by amateur astronomers, and the two galaxies may even be seen with binoculars. The Whirlpool Galaxy is also a popular target for professional astronomers, who study it to further understand galaxy structure (particularly structure associated with the spiral arms) and galaxy interactions.

What was later known as the Whirlpool Galaxy was discovered on October 13, 1773 by Charles Messier, and is designated as M51. Its companion galaxy, NGC 5195, was discovered in 1781 by Pierre Méchain. It was however not until 1845 that the Whirlpool became the first to be recognized as a spiral. This was achieved by Lord Rosse employing a 72-inch (~1.83 m) reflecting telescope which he constructed at Birr Castle, Ireland. Sometimes M51 is used to refer to the pair of galaxies, in which case the individual galaxies may be referred to as M51A (NGC 5194) and M51B (NGC 5195).

With the recent SN 2005cs derived estimate of 23 Mly distance, and an angular diameter of roughly 11.2′, it can be inferred that M51's bright circular disk has a radius of about 43,000 light-years. Its mass is estimated to be 160 billion solar masses. A black hole, surrounded by a ring of dust, is thought to exist at the heart of the spiral. The dust ring stands almost perpendicular to the relatively flat spiral nebula. A secondary ring crosses the primary ring on a different axis, a phenomenon that is contrary to expectations. A pair of ionization cones extend from the axis of the main dust ring.

Located within the constellation Canes Venatici, M51 is found by following the easternmost star of the Big Dipper, Eta Ursae Majoris, and going 3.5° southeast. Its declination is +47°, making it a circumpolar for observers located above 43°N latitude; it reaches high altitudes throughout the northern hemisphere making it an accessible object from the early hours in winter through the end of spring season, after which observation is hindered in lower latitudes. M51 is visible through binoculars under dark sky conditions and can be resolved in detail with modern amateur telescopes. When seen through a 100 mm telescope the basic outlines of M51 and its companion are visible. Under dark skies, and with a moderate eyepiece through a 150 mm telescope, M51's intrinsic spiral structure can be detected. With larger (>300 mm) instruments under dark sky conditions, the various spiral bands are apparent with HII regions visible, and M51 can be seen to be attached to M51B. As is usual for galaxies, the true extent of its structure can only be gathered from inspecting photographs; long exposures reveal a large nebula extending beyond the visible circular appearance.


Object information
Name : M51 Whirlpool Galaxy
Type : Galaxy
Constellation : Canes Venatici
Distance : 23M light-year
Apparent dimensions : 11'.2 x 6'.9
Apparent magnitude : 8.40

Image information
Image date : 2012/04/15
Right ascension : 13:30:15.151
Declination : 47°10' 4.33"
Focal length : 1359.06 mm
Focal ratio : f/5.35
Image resolution : 0.820 arcsec/pixel
Field of view : 43' 9.8" x 28' 46.8"
Sky quality : 19.75 magnitude/square arcsecond
Sensor temperature : -20°C
Light frames : 2 hours, 50 minutes total exposure time
8x 600 sec. luminace unbinned
4x 450 sec. RGB 2x2 binned
Image acquisition with Nebulosity v3
Auto-guiding with PHD
Bias frames : 100x unbinned
50x 2x2 binned
Dark frames : Bap pixel map
25x 600 sec. unbinned
20x 450 sec. 2x2 binned
Flat frames : No flats
Processing : PixInsight for calibration (bias frames, bad pixel map and flat frames), alignment and integration.
Adobe Photoshop CS6 for final touch.

Equipment information
Telescope : Sky-Watcher BKP250 10" 1200mm f/4.8 Newtonian
Corrector : Guan Sheng Optical 2" coma corrector and field flattener
Focuser : Moonlite CR newtonian focuser with DC motor and USBnFocus adapter
Mount : Sky-Watcher NEQ6-Pro german equatorial mount controlled with EQMOD via HiTec Astro EQDir
Camera : QSI 683wsg-8 cooled CCD camera with Kodak KAF-8300 sensor and Off Axis Guide (OAG) port
Guidecamera : Starlight Xpress Lodestar autoguider
Filters : Astrodon Tru-Balance E-Series LRGB & Astrodon 3nm H-α, 3nm O-III, 3nm S-II and 3nm N-II
Observatory site : Etten-Leur, The Netherlands