NGC 6888 - The Crescent Nebula

The famous Crescent Nebula is listed as NGC 6888 in the New General Catalog of nebula and clusters of stars. As astronomers currently believe, a hot Wolf-Rayet star in the center of the nebula causes this "bubble" blown of stellar wind. The nebula  is embedded into a large H-II region within the Cygnus star constellation. This H-II region is not a recommended target for beginners in astrophotography, but for those who look for a new challenge. Amateur astronomers recently discovered a new candidate of a very faint planetary nebula close to NGC 6888. The new planetary nebula is named Soap Bubble Nebula or PN G75.5+1.7 (not shown in my picture).

To obtain pretty pictures of NGC 6888 and surrounding interstellar matter, larger exposure times, a stable telescope mount and guiding are requirements. A guiding telescope with auto-guider certainly is the most comfortable solution. Modern auto-guiders don't require large focal length. The main telescope may have larger aperture and focal length compared to the guiding telescope. A compact Cassegrain like the Vixen VMC110 combined with an auto-guider and mounted piggypack onto my photographic VC200L main telescope offers great stability and fun with exposure times beyond 1 hour. The camera should provide a reasonable spectral response in the red light of the hydrogen Hα emission line. Therefore, modified digital single lens reflex cameras (DSLR) or dedicated CCD cameras are preferred photographic devices for this class of hydrogen emission nebula.

Observational Notice

Unfortunately, NGC 6888 is one of those famous objects on the night sky which are left out in the Vixen Starbook database of celestial objects. However, with a careful adjustment of the Sphinx SXD telescope mount and manual input of the celestial coordinates of NGC 6888 at the Simbad database,  it is possible to take photographs of the nebula on the night sky. With an exposure time of only 2 minutes per raw frame the nebula did not appear on the display monitor of the camera. Thus, the pictures have been taken blindfolded. Fortunately, I found the picture of the nebula correctly centered with the data reduction.

Image Processing Hint

The strong background light of the moon was subtracted by software within the R, G, and B color channel of the resulting image. This is done in the last two steps within the display section and before the careful application of a gamma correction (gamma=1.2). A special task performs the automatic background subtraction based on statistical calculations. However, it can be done manually, trial and error and using applications like Photoshop, too. Certainly, use of narrow band transmission filters for Hα, Hß or the oxygen O-III line is required to clearly show the very extended hydrogen background distribution. These structures of low surface brightness are not shown in my picture, however. I prefer the use of modified Canon DSLR with clear glass filter of the type Astronomik MC. The UV/IR block EOS clip filter can be mounted in addition onto the camera. This type of modification offers a wider range of photographic applications and may be extended into the near UV and infra-red light, as the UV/IR filter can be removed easily and exchanged with another type of color filter. For several purposes I purchased a complete set of Astronomik EOS clip filters which may be used with my camera. After background subtraction my preferred color correction setting for the Canon 40D-A is simple: Multiply the red and blue channel by a factor 1.4 and leave the green color channel as it is. This is not motivated by a true color calibration. In fact, I visually compared my sky flatfields with the bright sky on daylight and selected the factor 1.4 as a rough estimate to obtain a blue sky from the sky flatfield.

Telescope:Vixen VC200L, focal reducer f/6.4, Sphinx SXD
Camera:Canon EOS 40D-a, 400 ISO, Astronomik UV/IR block filter
Exposure:25 x 120 s, 6 frames manually rejected due to tracking errors.
Calibration:Dark (50 images), sky flatfield (50 images)
Image Processing:Shift & add with correction of subpixel movement, improved noise reduction
Date of exposure:21 August 2010, 00:15 h MEZ
Software:ArgusPro SE
Remark:

Moon light introduced a certain background brightness (corrected) 

 
Image of the Month

Interstellar Gas in Cygnus