This is a relatively simple astrophotography tutorial that will help you create stunning photographs of the Milky Way galaxy using your current photo equipment and image stacking.
About the Milky Way
The Milky Way is a spiral galaxy approximately 120,000 light-years in diameter and containing about 200 billion stars, of which our sun is one. It’s estimated that the observable universe contains two trillion galaxies and is, hypothetically, one of an infinite number of universes (multiverse). It’s as unfathomable and humbling as it is beautiful.
In contrast to my homemade pinhole camera solargraphs, featured here in an earlier post, the above photograph of the Milky Way was taken from the same geographic location using decent photo equipment and completed to the best of my technical abilities at the time.
It’s important (if at all possible) to capture the images from a location with minimal light pollution, which is, sadly, increasingly hard to find. See this light pollution map. The quality of the results will be limited in urban areas.
The Bortal Scale was created to measures the darkness of the sky. See this informative post on the Bortal Scale and the impact of light pollution on astrophotography at NightSkyPix. My location for this image was a Bortal Class 4. For contrast, this image was taken in a Bortal Class 2.
Equipment and Capture
For the above image, I used a Nikon D800 camera, Nikkor 14-24mm f/2.8 lens, Gitzo tripod with Ball Head, Adobe Lightroom and Photoshop. A faster lens is better as it lets in more light over the same exposure time. However, great images can still be made using “slower” kit lenses on entry-level or consumer DSLR and mirrorless cameras. The basic techniques should work with any camera that you can control the settings below.
Start with the following:
- Set the camera to “M” manual mode to specifically adjust aperture and exposure time.
- Turn off normal noise reduction. The Photoshop image stacking operation will do a much better job.
- Turn off long exposure noise reduction, or each capture will be twice as long while your camera creates a second, equally long “dark” exposure to subtract noise.
- Turn off lens vignetting and distortion corrections as they can hurt stacking quality.
- 3200 ISO is a good compromise of exposure time versus digital noise on my D800. The higher the ISO, the more noise. Newer cameras may be able to use higher ISO values with less noise.
- The “500 Rule” is often used to determine the number of seconds you can expose without seeing any star trailing as a result of the Earth’s rotation on a full-frame camera. 500 divided by my focal length of 14mm = 35 seconds. However, because of my camera’s high resolution, I found that even 20 seconds showed some star trailing, albeit what I consider to be acceptable. For my needs, a “300 Rule” is more applicable to the D800. Smaller sensor cameras will use an even smaller value to divide into. You’ll have to do some experimenting. Note that an f/2.8 lens (faster) will gather the same amount of light in 20 seconds, and an f/4.0 lens (slower) will gather in 40 seconds. Each full f-stop decrease will halve the exposure time, and every full f-stop increase will double the time needed for the same exposure.
- Set the white balance to 3600K, although it will be likely be modified during post-processing.
- Set focus to infinity. Note that the infinity marks on most contemporary lenses are not accurate so I either use magnified live view manual focusing or autofocus on one of the brightest stars and then disable autofocus if the camera and lens combo can do it.
- Images should be saved in RAW format for maximum post-processing control and the greatest dynamic range.
- If you’re using a DSLR with an exposure delay mode, use it to prevent the vibration caused by mirror slap.
Use a solid tripod to make sure the camera doesn’t move or vibrate in the wind. You may have to weigh your tripod down to make it more stable.
I utilized image stacking to create this photograph. Many photos are combined to increase the signal-to-noise ratio of high ISO images resulting in a cleaner and more detailed photo. Another option for less noisy results would be to decrease the ISO, but that would increase the exposure time, resulting in star trails, which I didn’t want. A third option would be to use a star tracking device like an iOptron Skyguider Pro that moves the camera in perfect sync with the Earth’s rotation allowing for very long exposures at low ISO. Here is a great review of the pros and cons of the current portable star trackers, as of 2020, at AstroBackyard. Since I didn’t have one, I opted for the stacking technique.
I used the D800’s built-in intervalometer to take a series of photos, one after another. If your camera doesn’t include an intervalometer, you will need to get one. See more about intervalometers at NightSkyPix.
The images were imported into Lightroom for basic colour correction work and exported as 16-bit TIFF files. I then switched to Photoshop. Under Scripts in the File menu, I selected Load Files into Stack… and made sure that “Attempt to Automatically Align Source Images,” necessary to counter the Earth’s rotation over the course of the exposures, and “Create Smart Object after Loading Layers” were both checked. Once Photoshop was done processing, I went to Smart Objects in the Layers menu and changed the Stack Mode to Median, which is the magic of this stacking technique. The noise was greatly decreased while the details were increased. I flattened the image and finished with a little more post-processing work.