Discovered in 1792 by William Herschel the Crescent Nebula lies in the constellation Cygnus. Cygnus is the large unique cross shaped constellation aligned with our galaxy’s central plane. For those of us in the eastern time zone here in Ontario it is found high over head in the late evening (~10PM) in late August / early September. Cygnus is the swan, visually represented head to tail from Albireo to Deneb with outstretched wings. The Crescent Nebula is comfortably riding along on the swan’s back.

To help set the scale of things, the entire picture above would comfortably fit inside the small green block in the sky map below. The sky map below is about the size of your outstretched hand held up to the sky.

Freestarcharts https://freestarcharts.com/ngc-6888

From a structural perspective, Crescent Nebula is an emission nebula which means we can see it because of the presence of a nearby energetic source making its gases glow. In this case, the source of this fury is a red supergiant (WR 136), in the image it is the bright star nestled in the centre of the crescent. The gases it is illuminating are its own outer shell it ejected into space as it progresses along its life from super giant to super nova.

The description of NGC6888 at https://chandra.harvard.edu/photo/2003/ngc6888/ gives a good scene of the speeds and times involved in this nebula.

The image scale is 0.57arc-sec/pixel and the crescent is 1870pixels long on its major axis, so NGC6888’s angular size from earth is about 18arc-min. From our perspective this makes NCG6888 is a very small object, it would fit between your fingers spaced to hold 2 coins, edge on, at arm’s length. Most sources indicate NGC6888’s distance from earth is approximately ~5000 light years and so I calculate NGC6888 is ~25ly across on its major axis. Way too far to drive with the kids in the back asking if we are there yet.

The image is a 5 hour exposure, 2 hours Luminance and 1 hour each colour channel (RGB) taken on one night in early October 2021.

Location	Old Photons Observatory Ottawa Ontario
Date	October 2021
Conditions
Weather
Mount	Paramount MX
Optical Instrument	Celestron EDGE11 with F7 Focal reducer
Camera Instrument	SBIG STF-8300 CCD Temperature -20C
Focus	Position = 1589 Avg HFD = 3.93 Avg FWHM = 3.19 Temperature: 11.5 Filter: Lum (slot 5)
Guider	ZWO
Focal Ratio	Imaging at f/7, Guiding at f/4.9
Focal Length	Expected = 1960mm Measured = 1954.3 mm
Exposure	5 Hours total Red 1h (6x10min) @ 2×2 Binning Green 1h (6x10min) @ 2×2 Binning Blue 1h (6x10min) @ 2×2 Binning Luminance 2h (6x20min) @ 1×1 Binning
Image scale	Expected = 0.57(arcsec/pixel) Measured = 0.57(arcsec/pixel)
Image Center (J2000)	20° 12’ 47.88” 38° 19’ 01.2”
Image FWHM	Measured = 4.8(arc-sec) 8.5(pixels)
Image Processing	Astrometric Alignment stacking in MaxIM DL Digital Development 3×3 Median Kernel Filter

Online references

NASA Science https://science.nasa.gov/ngc-6888-crescent-nebula

Wikipedia https://en.wikipedia.org/wiki/Crescent_Nebula

Chandra obs https://chandra.harvard.edu/photo/2003/ngc6888/