NGC 6888 – The Crescent Nebula
Throughout the summer months of 2017, I began a new deep sky project on the striking Crescent Nebula in Cygnus. For backyard astrophotographers, the nights of June and July are short and hot, and this can make a long exposure astrophotography project a challenging task.
Although the Crescent Nebula is bright, it is notoriously hard to capture a detailed portrait of the area. In my image, a combination of Narrowband H-Alpha data and color images were needed to do this emission nebula justice. To create the final composite image in Photoshop, I used the steps outlined in my HaRGB tutorial.
If you plan on shooting the Crescent Nebula with a DSLR camera, consider using a Hydrogen Alpha filter. Aside from capturing the h-alpha wavelength of light found in certain nebulae, a narrowband Ha filter also ignores unwanted city light pollution and renders the stars in your image much smaller.
NGC 6888 – The Crescent Nebula
A large effort was given to reduce noise during the image processing stages of my final image. The complete capture and processing details are outlined further down this page.
For a behind-the-scenes look at capturing the Crescent Nebula with a DSLR camera, have a look at the video below.
At magnitude 7.4, it is possible to view NGC 6888 through a telescope under the right conditions. I have never had the pleasure of enjoying the Crescent Nebula visually, so I’ll have to settle for photographing it for now.
To locate the Crescent Nebula, look no further than the Swan in the sky. The constellation Cygnus is home to many astrophotography treasures including the massive North America Nebula, and the delicate Veil Nebula. (NGC 7000 and NGC 6960, respectively)
NGC 6888 is easy to locate by star-hopping from Deneb in Cygnus.
The Crescent Nebula is an emission nebula about 5000 light years from Earth. A wide field refractor telescope is well-suited to capture NGC 6888 as well as much of the surrounding hydrogen gas. A telescope with a longer focal length will capture the fine details of the Crescent Nebula in detail.
The telescope used for the photo above was an Explore Scientific ED102 CF Refractor.
Latest Version (2018)
In early June 2018, I photographed the Crescent Nebula using a cooled CMOS camera and a large apochromatic refractor. The focal length of the Explore Scientific ED140 (910mm) was a perfect fit for the target. The resolution of the ASI294MC-Pro camera sensor produced a large image of the nebula in the center of the field with room to spare on all sides.
First, I captured 4 and 5-minute subs using a 2″ narrowband filter. The STC Optical duo-narrowband filter collects both Ha and OIII wavelengths of light at the same time. These gases are prominent in the Crescent nebula, so it turned out to be a fantastic filter to use on this DSO.
The hydrogen alpha structure and tendrils of this nebula were dynamic and bold right away, while the subtle OIII shell this object is famous for began to take shape over time. In total, I collected 9.5 hours on the Crescent Nebula in Ha and OIII using the duo-narrowband filter.
A mono astrophotography camera would have captured more detail in the Crescent in a shorter period of time, but using this filter with a color camera meant no filter changes or building channels in post processing. I think this filter has some exciting capabilities with both cooled one-shot-color CMOS cameras, and Astro-modified DSLR’s.
Once the structure of NGC 6888 was captured in narrowband, I decided to capture some true-color RGB images to create a more natural looking final image. My primary goal here was to capture more natural looking star colors that were not too red. I used a 2″ Baader Moon and Skyglow on the ASI294 for these images under a moonless June sky.
The create the composite image using both narrowband and true-color RGB images, I’ll leverage the HaRGB Photoshop technique outlined in this processing tutorial. Basically, the intense narrowband details will be blended into the color image as a luminance layer to get the best of both worlds. This way, I can produce an image that captures the natural beauty of the Crescent Nebula, while getting some help from the isolated wavelengths of light in Ha and OIII.
Deneb in Cygnus, part of the Summer Triangle
The constellation where this nebula resides is absolutely full of deep sky targets.
Cygnus the swan rises high into the Eastern sky in July and contains one of the 3 stars that make up the summer triangle. Deneb is the brightest star in Cygnus, with Vega and Altair in nearby constellations Lyra and Aquila to complete the asterism.
An advantage of shooting deep sky objects in Cygnus rather than further South in the Milky Way is the elevation this region gets. There is less turbulence in the air pointing straight up, so the images are usually sharper overall. As an added benefit to photographers, gradients in the image are less severe due to the distance from the city glow that rises from the horizon.
Deneb proved to be a convenient star for focusing my camera during my Crescent Nebula project. Focusing can be difficult when using a Ha filter with a DSLR, so ultra-bright stars like Deneb come in handy for live-view focusing.
Below you will find my final image of the Crescent Nebula for 2017. With frames captured from early June through to mid-July, it is time to move on to another project. (Hint – Cocoon Nebula!)
For DSLR camera owners
For those of you who plan to image NGC 6888 using an unmodified DSLR, do not expect to see much more than the western outer arm of the structure in a single exposure. A modified camera will start to pick up the fainter eastern arm, but the central tendrils will not likely be visible until adding Ha data.
Like most targets, both of these issues can be overcome by capturing more data. Expect to soak in at least 4+ hours worth of exposure time before attempting to process your image. This value will change depending on your imaging location.
Much of the data for this project was acquired during with varying amounts of moon glow present. The additional imaging time is another huge advantage of owning a 12nm h-alpha filter for your DSLR camera.
This was a surprisingly challenging target from the city. Even after 3 hours worth of combined Ha and RGB data, the resulting HaRGB was still rather unimpressive and noisy. This dilemma was caused not so much by the light pollution, but by the extremely warm nights I was imaging in.
The average ambient temperature throughout this process was roughly about 22-25 degrees Celcius. With the camera capturing long 300-second exposures, the poor sensor in my Canon 600D consistently toppled the 30 degrees Celcius mark.
Despite the heat, I am rather pleased with the amount of detail I was able to pick up. The whispy nebulosity in the surrounding area was a nice surprise to see in the Hydrogen Alpha images.
NGC 6888 in HaRGB (DSLR)
Total Ha Exposure: 5 Hours, 9 Minutes (62 frames)
Total RGB Exposure: 2 Hours, 45 Minutes (55 frames)
Total Integrated Exposure Time: 7 Hours, 54 Minutes
All Ha data was captured using 5-minute image subs at ISO 800, ISO 1600 and ISO 3200. The reason I chose to fluctuate the ISO range on a per-night basis was based on outside temperature. I tried to keep the sensor below 30 Degrees Celcius (unsuccessfully!) during each session.
The RGB data was captured using a SkyTech CLS CCD filter to minimize light pollution. However, this came at the expense of a natural star color in my final RGB image. The contrast in the Crescent Nebula was formidable, but the background sky color left much to be desired.
The imaging sessions were controlled using Astro Photography Tool.
Photographed on: June 12, June 22, July 1, July 8, 2017
Telescope: Explore Scientific ED102 CF
Mount: Skywatcher HEQ-5 Pro
Guide Camera: Altair GPCAM2 AR0130 Mono
Guide Scope: Altair Starwave 50mm
Camera: Canon Rebel 600D (modified)
ISO: 800, 1600, 3200
Filters: SkyTech CLS CCD, Astronomik 12nm Ha
Processing Software: Deep Sky Stacker, Photoshop CC