Techniques

The Rosette Nebula

by Chris White | March 16, 2015

Rosette nebula © Chris WhiteAlthough I’m not an astrophotography expert, I recently posted this image under the landscapes image forum on NatureScapes.Net and was asked to describe how the image was created. As I sat down to write, I realized a little more depth on my experience would best convey how I stumbled through making this image. As of this writing, I have only 11 night-time shoots under my belt, of which, only four produced images I like. All have been taken from my backyard in the outskirts of light-polluted Burlington, Vermont.

The entry into this discipline of photography has been driven by life circumstances. I use photography to get out and enjoy the natural world. As a small business owner, my real job is extremely demanding at times and leaves me little time to photograph during daylight. Also, my wife has been very ill for the last year. She is recovering steadily, but I am unwilling and unable to travel any real distance or for any extended period of time. Caring for her and our household has largely consumed any free daylight hours I might otherwise enjoy.

I have always loved the night, spending much time in the woods strolling and camping after dark. The night sky amazes me, and I’ve often peered longingly at it, thinking someday I would attempt to photograph it.

That someday has finally arrived. It’s the right time for me to dive into astrophotography. I can do it from my home, at night outside of business hours, and still be close to my family.

Rosette Nebula final image © Chris White

The Rosette Nebula (Final Image)

About the Nebula

The Rosette Nebula (NGC 2237) is one of the largest objects to photograph in the winter night sky (with an apparent diameter of twice the full moon), making it an ideal target for the beginner astrophotographer. It is estimated to be about 5,000 light years from earth, and a daunting 150 light years across. These objects are so massive, making it difficult to put the numbers into perspective. If light travels at 186,000 miles per second, 150 light years is 882,880,000,000,000 miles. Put another way, the Rosette Nebula Complex is 10 million times wider than the distance from the earth to our sun. Will somebody please check my math? I can hardly believe it is this massive! This complex is composed primarily of emission nebulae, which are clouds of high-temperature gas in space. They frequently glow red due to a dominance of hydrogen gas. Within this red rose are what appear to be dark fractures. These are dark nebulae, which are simply large cold clouds of interstellar space dust. These dust clouds would be invisible, if not silhouetted against the illumination of the emission nebulae.

Generating Images

The most important step to creating an image such as this is to generate high-quality raw files, and a lot of them. When photographing in the pitch dark at high ISO, noise is a detail destroyer. To mitigate this noise (at least the random read noise) one can stack multiple images of the same scene on top of one another to increase the pixel signal (scene elements that are desired, in this case nebulae, stars, etc.) and reduce the random noise (which varies from one shot to the next and is mitigated by stacking multiple exposures of the same scene). I am not a physicist, but I do know this technique is effective at increasing the signal (desired data) to noise (undesired data) ratio. Essentially, the noise in a scene is reduced by the multiple of the square root of the number of images stacked. For example, to cut noise in half, four images must be stacked. If 64 images are stacked, noise is reduced by eight times. There are diminishing returns to this equation, and finding a balance of exposure quantity and noise reduction is essential to make files and processing manageable. In my limited experience, I have found a minimum of 60 to 70 images is essential for the objects I have photographed.

Edited single exposure © Chris White

Edited Single Exposure

Edited combination of 72 exposures © Chris White

Edited Combination of 72 Exposures

This is the list of equipment I used to generate my RAW files:

  1. Canon 7D Mark II (Unmodified)
  2. Canon 300mm f2.8 lens
  3. Astronomik CLS Clip In Light Pollution Filter
  4. Canon Timer Remote Controller TC 80-N3
  5. Sky-Watcher Equatorial Tracking Mount (Astro Package, Equatorial Wedge, and Counter Weight)
  6. Really Right Stuff BH-55 Ballhead
  7. Gitzo Systematic Carbon Tripod

Being primarily a wildlife and landscape photographer, I already owned most of this gear. I needed two more pieces—the Equatorial Mount (used to allow the camera to track the sky for long exposures) and the Light Pollution Filter (used to mitigate the undesirable wavelengths of light that emanate from nearby Burlington, Vermont). There are many high-quality equatorial tracking mounts available, and at some point I intend to upgrade. The entire kit came to around $400, which did not seem like too much of a risk should my interest fizzle out. It’s far from top quality, but gets the job done, and for the most part supports my 300 f2.8 lens adequately. For light pollution filtering, I opted for the Astronomic Clip-In filter. It fits into the camera body behind the lens, so you can use it with any lens (with the exception of EF-S lenses which protrude into the body too far), eliminating the need to have a filter for every lens you intend to use.

Setting Up Gear

Winter in Vermont is cold, and this winter has been very cold. I have learned the first step in shooting is to put my gear outside in a garbage bag a couple of hours before shooting. At first I was puzzled when I would have sharp focus on a star or object, return after 20 minutes of automatic shooting, and focus would be off. It does not take much error in focus before stars turn from pinpoints of light to blurry disks. I kept setting focus and returning to find focus had changed. My first solution was to place a piece of tape on my lens barrel across the focus ring to essentially lock it in place. It seemed like this was a little helpful, but still, images were consistently drifting out of focus. After several hours of correcting this issue, I noticed that it went away on its own. I believe that as my equipment cooled to ambient temperature and materials in the lens contracted, the focus changed. Now that I place my equipment outside a few hours before shooting (and use a piece of tape on the focus ring as insurance), the focus drift issue is no longer a problem.

Achieving Alignment and Focus

So I set my gear outside a few hours before shooting, and once the sun went down and the stars were twinkling brightly, I headed out to set it up. For an equatorial mount to accurately track the night sky as the earth rotates, it must be aligned perfectly at the Celestial North Pole. There is a ton of information out there on how to best line up your gear with celestial north, so I won’t go into much detail on polar alignment, however I will add that when using a budget mount and heavy gear, it is best to perform polar alignment with the camera and lens already locked in. This will pre weight and flex the system into the exact position where it will be when shooting. If you align first, then mount your gear, it will push you out of alignment a little bit, and the slightest misalignment will lead to elongated stars due to the poor tracking. I have worked out a system of steps that I go through now that both help achieve the best shooting results, and also minimize the time I need to spend out at -10 degrees F, fiddling with my gear! With every shoot, I continue to learn how to get more out of my gear.

Once polar alignment is achieved, I very carefully manipulate my setup to zero in on my target. Some targets are easy to find, such as The Orion Nebula Complex on Orion’s Sword, while others, like the Rosette Nebula are a little challenging. It is located in a dark area of the sky between the constellations of Monoceros and Orion. As this nebula is invisible to the naked eye (at least to my eyes at my location), my only reference star is a very faint one in a sea of black. It’s hard enough to see a very bright star in a camera viewfinder or live-view screen and almost impossible to do so with a faint star. I used an old trick that helps me acquire birds in flight with my long lenses, and peering over the top of my camera hot shoe, down the barrel of the lens over the knob on the hood, I found my faint star. Nudging my camera setup vertically a small amount I locked the setup in place and crossed my fingers.

Focus must be achieved manually as AF is ineffective at shooting in total darkness. I roughly adjusted focus until I could barely see a couple of stars on live view and then zoomed in on one at 10x. I gently played with the focus until the stars became a tiny pinprick, then taped the focus ring in place to prevent disturbance. Next I took a single 30 second exposure and crossed my fingers, hoping that I would be shooting at the right point in the sky. Reviewing the image, I could barely make out the wisp of what appeared to be nebulosity in the shape of a rosette. Pay dirt. I programmed my controller to shoot 99, sixty-second exposures with an interval spacing of four seconds (to let vibrations settle out between exposures).

With wide angle lenses, the Sky-Watcher is an excellent mount, and every image in a series is tack sharp with nice pinpoint stars and no aberrations due to tracking issues within the scene. Using a relatively heavy 300mm f2.8 lens is another story entirely. I have found that the longer I increase exposure time, the more images I scrap presumably due to gear slippage in the mount and flex in the ball head. With this heavy lens, and 60 second exposure lengths, my keeper rate is about 30-40%. To generate my 60-70 high-quality raw files, with a keeper rate of 30%, I had to plan for a long imaging session. In the end I shot 207 images, and after reviewing each one at 1:1, I ended up with 72 keepers. Now it was time to edit and coax out the scene hidden within these RAW exposures.

Single RAW exposure © Chris White

A Single RAW exposure. One of 72 used to create this image. Capturing the RAW files was only part of making this image. Now that an adequate number of high quality RAW files have been captured, the scene hidden within these RAW files must be coaxed out.

Editing Steps

1. Import files into Lightroom. Only select images where stars are small and round. Discard any image with elongated stars or airplane streaks.

Round stars © Chris White

Nice round stars – keeper!

Elongated stars © Chris White

Elongated stars – discard!

Airplane flying across frame © Chris White

The nerve of some people…airplane flying across my frame.

2. Apply lens correction to all images. Even a small amount of vignette-ing will plague you later after you have stacked your images. After stacking, it is too late to apply lens correction. Doing it beforehand will yield a much more evenly lit scene.

3. Open images as “Layers” in Photoshop. (More detail below on how to better manage this PC memory intensive task). Align layers, assign opacity for stacking and merge together.

4. Perform Flat Field removal to eliminate as much light pollution and color cast as possible.

Before flat field removal © Chris White

Before Flat Field Removal: Heavy blue color cast from CLS filter, residual light pollution glow.

After flat field removal © Chris White

After Flat Field Removal: Color casts and light pollution glow removed, ready for final edits.

5. Import back into Lightroom for final edits.

Editing Steps in More Detail

  1. Import files into Lightroom. Only select files where stars are small and round. Discard any image with elongated stars or airplane streaks. To end up with a high-quality final image, the source files must also be of high quality. Out of 207 images shot in this session, I ended up discarding 135. Below is an example of what I was looking for, compared with a couple of examples of what I was trying to avoid.
  2. Apply lens correction to all images. For my workflow, I can’t stress how important this is. When you take a lot of images over a long period of time, there will be all sorts of possible variations from one image to the next. With my heavy camera setup mounted on my budget equatorial mount, over the four hours I was shooting, drift in the system moved the Rosette Nebula from dead center to almost ¼ of the way across the frame to the right. This meant that the vignette from the lens was not even from start to finish once the images were aligned. Simply applying a lens correction at this point makes life so much easier once the images are stacked and final edits are being applied. I have learned this procedure the hard way, and spent hours on a previous scene trying to dodge and burn a scene to have even illumination.
  3. Open Images as “Layers” in Photoshop… Align layers, assign opacity for stacking and merge together. There are several programs out there you can acquire for stacking images of the night sky. I rushed out and purchased ImagesPlus, and downloaded DeepSkyStacker for free. At this point, I don’t like either one. My frustrations with these programs likely are borne out of my difficulties in learning unfamiliar software. I will continue to experiment with these programs, but for now I get the best results when I manually align and stack images as layers in Photoshop. Opening 72 full-resolution RAW files as layers in Photoshop is an extremely memory intensive task for your PC. Each time I have done this, it brings my computer to a standstill. Every alignment edit and opacity change takes much more time to execute, thus extending the time I was spending editing these files. I’m not a big fan of editing, but will put up with it if the end result is worth it. I devised a workflow system to essentially process my files manually in batches. I can open six images as layers in Photoshop and my computer doesn’t even blink. In fact, it eagerly executes mouse and key-stroke edits, so the solution I came up with is as follows:
    1. Open first six images as layers.
    2. Align manually by nudging and rotating if necessary.
    3. Set opacity starting with bottom layer to take advantage of boosting signal and reducing read noise.
      1. Opacity per layer determined by the following (Top Layer Being Layer Number 1): Layer Opacity = Layer Number/Number of Layers
    4. Merge all layers
    5. Repeat steps 1 through 4 for the remaining images in sets of six. (In this case I ended up with 12 merged images)
    6. Now take the 12 merged images and apply steps 1 through 4 to these.
    7. This is the final merged image that has benefited a boost in signal to noise.
  4. Perform Flat Field removal to eliminate as much light pollution and color cast as possible. Being just a couple of miles from Burlington, Vermont does not provide the best conditions for shooting the night sky, in fact the light pollution that radiates from the southeast is a bit discouraging. To compensate for the light pollution, I am using an Astronomik CLS Light Pollution Filter. It does a great job at filtering out much of the spectrum of streetlamps and other artificial light sources, but unfortunately adds a strong blue cast to the scene. This must be removed to get at the nebula data hidden below. Steps used to do this:
    1. In Photoshop, open the image created through combination method in step 3 above.
    2. Duplicate the Image: Image->Duplicate
    3. Apply Dust and Scratches Filter to the Duplicated: Filter->Noise->Dust & Scratches
      1. Set Radius to 50-70 (Until all star detail is removed and most of the nebula is blurred out)
      2. Set Threshold to 0-5 (Experiment with these settings to find what works best for the scene)
    4. Use Healing Brush to eliminate any detail left within the Nebula or very bright stars.
      1. Essentially, anything left on this image will be removed from the original.
    5. Use Gaussian Blur to smooth out any strange “tiers” in the image.
    6. Click on Original Image and Apply the Duplicate Image:
      1. Image->Apply Image
      2. Change Source to the Duplicate Image
      3. Change Blending to Subtract.
      4. Set Offset to 20. (10-30 offset works well with my workflow), and click OK.
    7. Save File as Tif.
  5. Import back into Lightroom for final edits. As you can clearly see, the image above does not look very impressive. There is a faint pinkish glow, but nothing that really looks like the final product. It is in this final step that I coaxed out the final scene. One of the biggest limitations to creating this image is that I am using an unmodified camera body. While camera sensors are capable of capturing infrared light, most camera manufactures install IR filters to prevent this IR light from reaching the sensor, thus improving image quality for daylight shooting. Unfortunately, for astrophotography, the IR filter prevents much of the hydrogen alpha light emitted by emission nebula from reaching the sensor. So, to overcome this deficiency, I really pushed some of the settings in Lightroom to the limits to create the scene I knew was there. Here are the settings I changed:

Final Edits in Lightroom

Color settings in Lightroom

Saturation settings in Lightroom

Detail settings in Lightroom

Other Images

Orion nebula complex © Chris White

Orion Nebula Complex

The pleiades © Chris White

The Pleiades

Horsehead and flame nebula © Chris White

Horsehead and Flame Nebula

My Setup

Camera equipment set-up

Camera lens set-up © Chris White

Helpful Resources

  1. Jerry Lodriguss’ website includes great basic information on astrophotography: www.astropix.com.
  2. “Astrophotographer’s Guide to the Deep Sky” by Jerry Lodriguss. This CD is absolutely essential for any night sky enthusiast. This guide is a comprehensive listing of objects to be found in the night sky for the Northern Hemisphere. It is broken down by season, and there are hundreds of objects described with photographs. Technicals for each image are shown, along with locations of each in the sky. In addition to the wealth of information, this guide is extremely inspirational.
  3. “Photoshop for Astrophotographers” by Jerry Lodriguss. Another excellent CD that has some great information on how to get the most out of Photoshop for astrophotography editing. Several of the steps I used to create this image were partially derived from this guide.
  4. Roger Clark’s website: www.clarkvision.com. There is a lot of great and inspirational information on Roger’s website. In fact, I was partially inspired to finally make the jump and get into astrophotography by an image Roger posted on NatureScapes.Net of the Horsehead and Flame Nebula. Roger is an excellent astrophotographer and graciously posts a ton of great information on his site about shooting and editing.
  5. “The Deep Sky Imaging Primer” by Charles Bracken. An interesting read that gave me some great things to think about when approaching astrophotography. Bracken discusses the physics of cameras and light and the limitations of cameras for shooting astrophotography.
  6. “Photography Night Sky: A Field Guide For Shooting After Dark” by Jennifer Wu and James Martin. While not a guide to deep sky astrophotography, this is an excellent read on wide shooting of the night sky. It is easy to want to zoom in on the subject, whether shooting birds, mammals or the night sky. Sometimes going wide and incorporating more of the environment yields the best results.
About the Author

Chris White is an amateur nature photographer from northwestern Vermont. He and his wife own a large maple sugaring farm and collect sap from more than 22,000 maple trees. In addition to producing maple syrup, they craft maple products at their retail shop and distribute their products all over the world. Chris has always had a strong connection to and deep love for the natural world, and uses photography as an excuse to get out and enjoy it. For Chris, there is nothing better than drifting in a kayak on a secluded body of quiet water, patiently waiting for a scene to develop before him. To see more of his work, visit his website at www.whitephotogallery.com, and check out his new Facebook page.

7 thoughts on “The Rosette Nebula

  1. Wow great article, super photos and very inspiring. One piece of gear I didn’t see listed was the Really Right Stuff something that attached the equatorial mount to your tripod. What is that, I don’t think I’ve seen one before. Thank you so much for the information and a superbly done explanation of your process.

  2. You are welcome Steve! Thanks Steven! Since this article was written I have made some changes to my process. 1) I purchased a canon rebel and had it modified for astrophotography by Hap Griffin. 2) Upgraded my tracking mount to an Astrotrac, which is giving me upwards of 100% keepers with long exposure and long lenses. 3) Learned how to use ImagesPlus for stacking and dark, bias and flat frame subtraction. This has really streamlined my workflow and removed much of the tedium. Keep in mind that this article is about how I made this specific image, and not necessarily what I would recommend for Astrophotography. At some point, if NSN would like, I will document my updated process, after I have many more months under my belt.

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