8/28/2011 NGC 6888 The Crescent Nebula

30 August 2011

This is my rendition of NGC 6888, also known as The Crescent Nebula.  It is an emission nebula in the constellation Cygnus. 

While it looks like a supernova remnant, it is actually a planetary nebula.  (from wikipedia) It is formed by the fast stellar wind from the Wolf-Rayet star WR 136 (HD 192163) colliding with and energizing the slower moving wind ejected by the star when it became a red giant around 400,000 years ago. The result of the collision is a shell and two shock waves, one moving outward and one moving inward. The inward moving shock wave heats the stellar wind to X-ray-emitting temperatures.

I had a really hard time processing this one, and I’m still not completely satisfied with the result.  I’m using 8nm Baader filters and I’m imaging from near downtown Seattle, with all the associated light pollution. Not sure if tighter bandpass Astrodon filters might help improve SNR on this subject.

Over 15 hours of exposure time went into creating this image.  That’s actually just counting the images that went into the final product.  Many more hours of images were not used due to various defects.

Here are the details:

NGC 6888 – The Crescent Nebula
LRGB image using OIII + 30% Ha for Luminance, HA for Red, OIII for Blue and OIII + 30% Ha for Green
Baader filters
13 exposures of Ha, with 20 minute subs each
32 exposures of OIII, with 20 minute subs each
Imaging scope: Astro-Tech AT10RC Ritchey Chrétien at f/6.7 (native f/8)
Focal reducer: Astro Physics CCDT67 focal reducer
Imaging camera: QSI 583wsg monochrome
Guide camera: Starlight
Xpress Lodestar
Mount: Celestron CGE hypertuned by Deep Space Products
Capture and stacking in Maxim DL
All other processing in Photoshop