(c) Skyhound

This image was created via color synthesis of Red & Blue Digital Sky Survey images. North is down and east is to the right, as typically seen in a telescope.

For many years the Crescent nebula was on my list of objects that I wanted to see, but it took some time to finally detect it. Catalogs list it as 10th magnitude, but I found it to be much more difficult than this would imply. I tried and tried one year to see the Crescent in my 6-inch to no avail. When I finished my 18-inch Dob, this was  my first serious target. But I was rather disappointed when I failed to see the nebula with my big new telescope on several different nights. 

One night I was very careful with my dark adaptation and the conditions were quite good. Only then was I finally able to discern the nebula, which makes an arc among several bright stars. I was able to detect the nebulosity on the northeast side first. Eventually I found the curving haze to the southwest. With time and averted vision I was able to make out the entire arc of the nebula. But why was it so difficult?

It turned out that I was not using the optimum eyepiece. Later on I obtained an eyepiece with a wider field and it made a big difference. The elongated white area near the center-bottom of the image at the top of the page was clearly visible and much as it appears in the image. 

Filters made an even bigger difference. With a UHC filter the rest of the nebula appeared, looking very much like the image (without the colors). The best view was with an OIII filter which greatly increased the contrast. With the OIII in place the faint wisps in the center appeared, filling the nebula into a complete "pear."

 

The circle represents the simulated eyepiece view in a 12-inch (30 cm) LX200 at 76x.

The Crescent nebula (aka The Euro Sign Nebula and  NGC 6888) is a faint, arcing diffuse nebula that appears in one of the most beautiful star fields of the summer Milky Way, about 2/5th of the way from Gamma Cygni to Eta Cygni. 

It was discovered by William Herschel in 1792 using his 18.7-inch telescope, when he noted it as H IV-72. It was designated NGC 6888 when the NGC catalog was compiled. The Dreyer summary reads, "Faint, very large, very much extended, double star attached." 

The double star referred to is HD 192182, which is the bright star at the bottom-middle of the picture above. This is the double star pair STT 401, which was also discovered by Herschel. It consists of a 7th magnitude star with a 10th magnitude companion 12.9 arc seconds away.

Observing the Crescent Nebula for Yourself


In smaller instruments this is a challenging object to detect. A wide-field eyepiece is a must, and a UHC or OIII filter  may be necessary to view this nebula in scopes smaller than 8-10 inches. AJ Crayon writes, "In my 8 inch f6 at 60X the nebula was about 15'X5' in a northeasterly PA with a larger northeastern side.  On this particular night the nebula was of uniform brightness and the famous crescent shape appeared pear like with several stars involved.  Yes, pear like!"

In 16-inch (41 cm) or larger telescopes the Crescent becomes a showpiece, particularly when an OIII filter is employed. 

Look for the Crescent in Cygnus, which is well placed in the evening in September. The diagram in the right shows Telrad circles centered at the location of the nebula. There is a link to a printable finder chart below.

If you are not going to use a filter, a dark site and a dark night are essential. You also need to be fully dark adapted. I often use an eye patch over my observing eye so that the light from my red-covered tablet or red flashlight don't affect my night vision. Red filters help, but they don't fully protect your eyes. Remember, it takes about 30 minutes for your eye to become fully adapted to the dark. 

If not immediately obvious, the trick to finding the nebula is to identify the four brighter stars that form a parallelogram near the northern end (bottom in the pictures here) of the nebula. Having good charts that match the orientation, scale, and magnitude limit of your telescope can really help. The southern-most star of the parallelogram is the Wolf-Rayet star that is responsible for the nebula, and the northern-most is the double discovered by Herschel. When you find the double, look to the northeast for the brightest portion of the nebula. Once you can spot the bright portion, try tracing it to the southwest of the pair, and finally, look for the fainter parts of the C-shaped nebula.

Finder Chart (pdf)

The Science of The Crescent Nebula


At the heart of this nebula is the Wolf-Rayet star WR 136 (HD 192163). It was once a swollen Red Giant star, gently puffing its outer layers into space. Now having evolved into a Wolf-Rayet star, it has a fast and strong stellar wind, which is overcoming the older layers. As the gasses collide they are heated by shock waves, ultimately causing the gas to glow. 

T.A. Rector (NRAO/AUI/NSF and NOAO/AURA/NSF)

The above image is a combination of three filters, isolating the emission lines of Hydrogen H-alpha (red), Oxygen OIII (blue) and Sulfur SII (yellow). So the red traces hydrogen gas, blue traces oxygen, and yellow traces the (mostly) background sulfur. 

When we use an OIII filter visually, we are isolating the oxygen emission line. The emission line is a single color, so we can make a filter that passes only the OIII color of light. The background skyglow covers a wide range of colors, so we only pass a small portion of it through the filter. As a result, the contrast between the nebula and the sky is greatly improved. Sadly, the H-alpha line is too far into the infrared for our eyes to easily see, or it would be another good choice for improving the visibility of emission nebulae in the eyepiece. 

SkyTools 3 was used in the preparation of this article. 

 Greg Crinklaw — Developer of SkyTools 

SkyTools 3, because the astronomy matters.

Read more about SkyTools 3

Download and try the Starter Edition