M20 - Trifid nebula
M20 - Trifid nebula: The Trifid Nebula is one of several bright nebulae in the southern, summer milky way. It is easily seen with a small telescope or even binoculars if you know where to look. The trifid name comes from the three divisions seen in the reddish emission part of the nebula. Also prominent is the blue reflection nebula and the dusty "dark" nebula.
Other Catalogs: Barnard 85
Right Ascension: 18 hrs 2 m
Declination: -23° 2'
Apparent Magnitude: 6.3
Date: July 2010
Telescope: Meade 16" Schmidt Cassegrain with f6.3 reducer (at f6.0)
Camera: SBIG ST-10XE
L: 58x1 minute, binned 1x1
R:G:B: 12x3 minutes, binned 2x2
The camera was at -30°C to -35°C, over several nights.
Processing Notes: Data acquisition with CCDSoft. Reduced and aligned in CCDStack. Subs combined in Sigma Beta. Arcsine stretch import of L into Photoshop. Adjusted curves and levels. Slight blur on the dim areas and sharpening on non-star bright areas. Faded minimum filter on the brighter stars. RGB combined in AstroArt at 1:1.1:2.0 ratio (increased blue because of atmospheric extinction). Adjusted curves and levels, and reduced noise of RGB. Both L and RGB files required "star repair" on the very bright stars. L was combined with RGB using three layers: Luminance on the bottom; desaturated RGB as a multiply(30%) layer; and RGB as a color layer on top. Final stretch and tweak in Photoshop.
Links to images of this object on other sites:
Additional Comments: A previous version of the trifid taken at the FLC observatory, using a 10" LX-200 temporarily installed in the dome in 2008 when the main circuit board for the 16" needed repair, can be seen here: http://www.fortlewis.edu/observatory/image_detail.asp?ID=90
A note on the colors: To get an emission nebula, you need very hot stars that emit ultraviolet radiation. The ultraviolet photons are energetic enough to "excite" the electron in a hydrogen atom into a higher energy state. When these excited electrons decay from one of the many higher excited states into the first excited state, the photons emitted are part of the Balmer series. The lowest energy transition in this series is going from the second to the first excited state, in which case a red, H-alpha (656.3nm) photon is emitted. Going directly from the third excited state to the first excited state, the atom will emit an H-beta photon (486.1nm). The ratio of the red H-alpha and cyan H-beta give an emission nebula its' magneta color. Note that when an electron decays to the ground state, which is a much larger energy drop, an ultraviolet photon is emitted. For more information, see: http://en.wikipedia.org/wiki/Balmer_series The blue nebular region is simply a reflection from the very hot nearby stars.
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