Crescent Nebula NGC 6888

The Crescent Nebula, NGC 6888, is a faint emission nebula in the constellation Cygnus, located about 2 degrees SW of γ Cygni (Sadr). It is about 25 light-years across, and about 5000 light-years from Earth.

IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

The star at its centre is near the end of its life, and creating strong stellar winds as it sheds its outer layers. Eventually it will explode in a supernova. But for now, we can see the result of those stellar winds colliding with the surrounding material and forming a shell of shock waves. Undulations in the shell surface give the impression of intertwining filaments, although those details cannot be seen very clearly in this image. The red hue is due to emission from hydrogen atoms, and the very faint blue-green hue comes from emission from oxygen atoms.

Star halos

Although I’ve tried my best to mask the problem in this and other recent photos, star halos have been plaguing me since February when I moved my filter from the OTA-side of the field flattener to be adjacent to the camera (where it should be). The image below shows uncontrolled star halos. I’m led to believe this could be a symptom of reflection between the filter and the flattener glass, caused by the flattener spacing being too close. Apparently my 4/3″ sensor may not be large enough to reveal any coma, so it’s easy to be fooled into thinking the spacing is correct when it’s not quite. I’ve now increased the spacing by 0.5 mm and will see if it has made a difference when it’s next clear…

Uncontrolled star halos


  • 48× 150-s light frames (Gain 900)
  • 2 h total exposure time
  • 48/68 light frames selected for stacking
  • Full use of calibration frames (darks, flats and dark flats)


  • Explore Scientific ED 102 mm Apo f/7 refractor
  • Sky-Watcher EQ6-R PRO SynScan GOTO equatorial mount
  • Altair Hypercam 294C PRO colour fan-cooled camera
  • Altair quad-band one-shot colour (OSC) 2″ filter
  • Revelation Adjustable Field Flattener
  • Altair 60mm guide scope
  • Altair GPCAM2 AR0130 mono guide camera


  • Sharpcap
  • PHD2
  • DeepSkyStacker
  • Photoshop
  • StarNet
  • Topaz DeNoise AI
  • Gradient XTerminator

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