M57 Ring Nebula

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2012/06/05 - This is a difficult object, because of the dynamic range. I processed the inner and outer part separately and combined them in the final image. The outer halo's are becoming visible, but for more details I have to add more data.

The famous Ring Nebula appears in the northern constellation of Lyra. It is a prominent example of a planetary nebula. This is a shell of ionized gas expelled into the surrounding interstellar medium by a red giant star, which was passing through the last stage in its evolution before becoming a white dwarf.

This nebula was discovered by the French astronomer Antoine Darquier de Pellepoix in January 1779, who reported that it was "...as large as Jupiter and resembles a planet which is fading." Later the same month, fellow French astronomer Charles Messier independently found the same nebula while searching for comets. It was then entered into his catalogue as the 57th object. Messier and German-born astronomer William Herschel speculated that the nebula was formed by multiple faint stars that were unresolvable with his telescope. In 1800, German Count Friedrich von Hahn announced that he had discovered the faint central star at the heart of the nebula a few years earlier. He also noted that the interior of the ring had undergone changes, and said he could no longer find the central star. In 1864, English amateur astronomer William Huggins examined the spectra of multiple nebulae, discovering that some of these objects, including M57, displayed the spectra of bright emission lines characteristic of fluorescing glowing gases. Huggins concluded that most planetary nebulae were not composed of unresolved stars, as had been previously suspected, but were nebulosities. The nebula was first photographed by the Hungarian astronomer Eugene von Gothard in 1886.

Messier 57 is located south of the bright star Vega, which forms the northwestern vertex of the Summer Triangle asterism. The nebula lies about 40% of the distance from Beta (β) to Gamma (γ) Lyrae, making it an easy target for amateur astronomers to find. The nebula disk has an angular size of 1.5 × 1 arcminutes, making it too small to be resolved with 10×50 binoculars. It is best observed using a telescope with an aperture of at least 20 cm (8 in), but even a 7.5 cm (3 in) telescope will reveal its elliptical ring shape. The interior hole can be resolved by a 10 cm (4 in) instrument at a magnification of 100×. Larger instruments will show a few darker zones on the eastern and western edges of the ring, and some faint nebulosity inside the disk. The central star, at magnitude 14.8, is difficult to spot.

M57 is 0.7 kpc (2,300 light-years) from Earth. It has a visual magnitude of 8.8v and photographic magnitude of 9.7p. Photographs taken over a period of 50 years show the rate of nebula expansion is roughly 1 arcsecond per century, which corresponds to spectroscopic observations as 20–30 km s−1. M57 is illuminated by a central white dwarf or planetary nebula nucleus (PNN) of 15.75v visual magnitude, whose mass is approximately 1.2 M☉ (in solar masses). All the interior parts of this nebula have a blue-green tinge that is caused by the doubly ionized oxygen emission lines at 495.7 and 500.7 nm. These observed so-called "forbidden lines" occur only in conditions of very low density containing a few atoms per cubic centimeter. In the outer region of the ring, part of the reddish hue is caused by hydrogen emission at 656.3 nm, forming part of the Balmer series of lines. Forbidden lines of ionized nitrogen or [N II] contributes to the reddishness at 654.8 and 658.3 nm.

Object information
Name : M57 Ring Nebula
Type : Planetary nebula
Constellation : Lyra
Distance : 2.300 light-year
Apparent dimensions : 230
Apparent magnitude : 8.80

Image information
Image date : 2012/06/05, 2012/05/29
Right ascension : 18:53:35.080
Declination : 33°1' 45.03"
Focal length : 0.00 mm
Focal ratio : 
Image resolution : 0.000 arcsec/pixel
Field of view : 
Sensor temperature : 0°C
Light frames : 8 hours, 30 minutes total exposure time
12x 900 sec. hydrogen-alpha unbinned
11x 900 sec. oxygen-III unbinned
11x 900 sec. sulfur-II unbinned
Image acquisition scripted with Nebulosity v3
Auto-guiding with PHD (medium dither, settle < 0.3)
Bias frames : 100x unbinned
Dark frames : Bap pixel map
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