Click on Image to switch to Annotated View.
NGC6888 - Crescent Nebula
The Crescent Nebula (also known as NGC 6888, Caldwell 27, Sharpless 105) is an emission nebula in the constellation Cygnus, about 5000 light-years away. It was discovered by Friedrich Wilhelm Herschel in 1792. 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 250,000 to 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.
It is a rather faint object located about 2 degrees SW of Sadr. For most telescopes it requires a UHC or OIII filter to see. Under favorable circumstances a telescope as small as 8cm (with filter) can see its nebulosity. Larger telescopes (20cm or more) reveal the crescent or a Euro sign shape which makes some to call it the "Euro sign nebula".
I`ve been hunting galaxies and other objects, when I completely forgot to put my camera to test which have been modifyed to accept more of Ha light. I probably should start with some nebula that rich in Ha right after modification, but other objects completely stole my attention. Now, I finally decided to go for it and shoot some of those rich red colors.
Due to the narrow field of view of my imaging setup, I couldn't afford to shoot some wide, wast nebulosities. And after some research I found that the Crescent Nebula is just perfect object for this. And as you can see, it fits nicely in the whole view. Only the borders, caused by stacking and dithering, were cropped, keeping the most of the field in tact.
Before imaging this object, I have seen many images of it made by other astrophotographers and observatories which brought to my attention the OIII nebulosity that surrounds this object. Most of those images, with clearly visible OIII, were made with filters and CCD cameras, so my expectations to catch it with just DSLR were very low and telling you the truth, I thought it was impossible.
When in the field, after all preparations, I like to take the test frame with the exposure that I plan to go for the rest of the night and stretch it very aggressively, to make notes of what have been captured by the sensor. To my very surprise, I noticed that I been able to catch some of the OIII and as a bonus, the very faint Ha nebulosity that is visible to the right of the Crescent nebula.
I have imaged for two nights and been able to gather 5 hours of clean and usable data. After stacking and processing, the OIII regions became more visible, but were still laying on the back of the noise, making it hard for me to pull them out during processing. Now, when I`m completely satisfied with Ha, I were after OIII.
I decided to go for another two nights and hopefully gather another 5 hours to improve SNR even more. Eventually, after 4 nights of imaging I did manage to pull out 10 hours (9 hours 45 minutes) of data and it was real pleasure to process it. Honestly, I wish I could afford 20 hours of exposure for this beautiful object, but decided to move on to the next object.
Optics : Meade 10" + CCDT67 @ F8 @ 2018 mm
Camera : Canon T3i (600D) Baader Mod
Mount : NEQ-6 Pro (Self Hypertuned/Belt Mod)
Guiding: Telescope Service OAG9 + SX Lodestar
Acquisition : BackyardEOS 3.0.3
Exposure : 39 x 900 sec @ ISO1600 - 9 Hours 45 Minutes
Stacking : PixInsight 1.8
Processing : PixInsight 1.8