NGC 6888, the Crescent nebula is an emission nebula in Cygnus that was discovered by Friedrich Herschel in 1792. Photons from the Crescent travel about 5000 years before arriving at Earth. The nebula is made from clouds of interstellar gas that has been formed by radiation pressure from a red giant Wolf Rayet star (HD 192163). At the core of the nebula is interstellar hydrogen (shown here in red) with an envelope of doubly ionized oxygen (OIII) generated by intense ultraviolet light (shown here in blue.) The Crescent is located in a deep sea of background hydrogen emission nebula common to this area of the Milky Way.

I've had my narrowband filters for too long without putting them to good use so this is my inaugural bi-color narrowband image. I had planned to go for three colors but I liked this bi-color result so much that I stopped at only two colors (for now anyway.) I finally got most of the bugs worked out of my scope hardware/software system and this data was acquired fairly efficiently over a period of about 4 nights (which are pretty short in the summertime here in Oregon.) Of interest is the fact that I used the IFI/Optec FocusLock system to actively control focus in real time while the shutter was open. The FWHM of most of the data in the subs is at or below 1.5 arc-sec, which is a testament to how well the system held focus--for hours on end through significant temperature changes. This image is also a testament to how well the Celestron C14 Edge optics can perform.

C&C is always welcome so let me know what you think.

- John

PS. For the colors, I mapped Ha in to the R channel and the OIII into both the G and B channels. Then I mixed the RGB channels using 55% red, 80% green and 100% blue. These numbers came from fiddling with the mixture until the result produced a "mostly neutral background" and reasonably white stars. There are clearly other ways to do this using the BackgroundNeutralization tool in PI, but after fiddling with a few combinations, this one looked the most pleasing with my data--and it worked pretty well.