In the previous installment of this series, we learned how star maps help astrophotographers in Alameda locate objects millions of light years away. Today you’ll see how these photos get their mesmerizing colors thanks to false color techniques, but it isn’t as fake as it sounds! Finally, you’ll learn how you can take a picture of our own galaxy using any camera—even analog film—and a tripod.
June is the month of color—Pride Month. As seasonal rain subsides, rainbow flags take the place of rainbows in the sky. As you may know, a rainbow is light being split into multiple colors through water. So, if rainbows are H2O plus sunlight, are they even physical objects at all? Or are they “H2O artifacts?” Observing and photographing celestial objects raises a similar question.
Pride Month is a reminder that color is expressive. Astrophotography offers a similar opportunity to color the universe however you see fit.
Rainbows aren’t a perfect metaphor, as H2O doesn’t emit light, it just bends and splits it. Still, the idea of light being broken into wavelengths helps us understand how images like the one featured today, the Tadpole Nebula (NGC 1893), can be captured from Alameda. Nebulae like this emit their own light from the chemicals which make up their atomic composition and temperature. The most common emission lines are hydrogen-alpha (Hα), oxygen III (O III), and sulfur (S II). These chemicals emit light in the visible spectrum, but their signals are typically too faint to detect without long exposures and special filters.
As discussed in the previous article, we combine hours of “stacked” photos to reveal these objects from our city. Because of light pollution, Earth’s atmosphere, and distance, the atoms Hα, O III, and S II are somewhat indistinguishable in color. To help visualize them, we assign specific colors to each one when editing the picture. This is where art influences the science of astrophotography.
The palette you’d probably recognize is from the Hubble Space Telescope which interprets the isotopes as follows: Hydrogen-alpha becomes green, oxygen blue, and sulfur red. There are other palettes common amongst astrophotographers, but you have artistic license to pick any colors you want. In our example of the Tadpole Nebula, the central region contains an overwhelming amount of O III, while the right hand edge is mostly Hα. To reinforce the tadpole theme, I might color the O III channel a deep blue to evoke a pond, and the Hα as dark green to hint at surrounding grass.
Here’s how this customized version looks:
Whatever the colors, these very specific wavelengths are extremely narrow bands of light on the color spectrum. For that reason, we call them “narrowband emission objects.” But galaxies, star clusters, and reflection nebulae emit a much broader spectrum of color. Those are referred to as “broadband objects.”
Before touching on broadband, I want to bring attention to how astrophotographers combat light pollution in cities like Alameda. Even though these objects are nearly impossible to see with the naked eye, special filters allow very narrow wavelengths of light to pass to the camera. These filters are called “narrowband filters“ and reject all but a few nanometers of light.
Some filters are so strong that even a full moon barely impacts the ability to photograph an object. The L-Para filter, pictured above, lets just 14 nanometers of light squeeze through its coated glass. There are almost 25 million nanometers in one inch, and this is a two-inch filter, so it reflects/rejects so much light that it closely resembles a mirror to the human eye.
Broadband objects, on the other hand, get their light from stars. Much like our sun, those stars usually emit the entire spectrum of light. That’s why they are considered broadband. A familiar example is the Pleiades. Its stars shine so bright that their light reflects off the surrounding space dust. For this reason, the wispy areas of the Pleiades is not an emission nebula. It’s actually starlight bouncing off otherwise invisible dust. Thus, the Pleiades is one of many reflection nebulae. Powerful narrowband filters block too much light to be useful on broadband targets, so while it sounds counterintuitive, it’s actually easier to photograph faint nebulae versus bright galaxies when under urban skies.
Another broadband example is the Whirlpool Galaxy from last month’s article. Galaxies, including our own Milky Way, are made of stars and emission gasses clumped together, meaning they contain a combination of narrowband and broadband objects.
You’ve seen these bright nebulae within the core of the Milky Way in many photos like this one I took in Point Reyes. That was a single (not stacked) image taken with an ancient DSLR and generic camera lens on a tripod, with zero filters and minimal post processing. In dark skies you can even capture it with your cell phone. This month I hope to shoot it in Alameda using a $250 DSLR released 17 years ago and a second-hand camera lens. Here’s how:
1. Wait for the right moment.
In the Bay Area, the Milky Way’s core is visible from spring through early fall, especially June through August. Wait for a clear, moonless night and use an app like PhotoPills to find when and where the Milky Way will rise.
2. Chase the darkness.
I won’t find perfect darkness in and around Alameda itself, but I’ll try from Ballena Bay. I want a clear path of dark skies, so I will take the picture when the Milky Way is above San Mateo rather than a bright city like San Francisco.
3. Put my Canon 5D Mk II and a used lens to the test.
A tripod and wide angle lens will get you very far. My lens is F/1.8 at 20mm and my Canon 5D Mark II has a full frame sensor. A lens around 14–24mm on a full-frame camera (or 10–16mm on a crop sensor) is ideal, but I’ve even used a 50mm lens. The wider your lens, the longer your shutter can stay open without stars turning into streaks. Here’s a handy tool to calculate the exposure time for your particular setup.
Or you can try these starter settings:
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- Manual or Bulb mode.
- Shutter speed: 10–35 seconds.
- Aperture: as wide as your lens allows (f/2.8 or faster if possible).
- ISO: 800–3200.
- Focus: set to infinity and fine-tune using Live View on a bright star. If you’re feeling fancy, you can look for a free Bahtinov Mask 3D model that matches your lens. It can be IIID-printed at no cost through the Alameda Free Library.
4. Shoot, adjust, repeat.
Use a two-second timer or a remote shutter to avoid shaking the camera. If you’re using an old Canon like I am, you can temporarily jailbreak it with Magic Lantern to unlock advanced features for astrophotography. Take a test shot and tweak focus or exposure as needed. If the stars are blurry or trailing, or if your picture is blown out, try a shorter shutter, lower ISO, or check focus again. You want the histogram to be pushed to the right, but of course we need to avoid over-exposing the image, which is tricky from our island surrounded by bright cities.
5. Edit
What you see on your camera screen may look gray or may be too faint to see against the city lights, but you may have just captured real structure from our galaxy. Boost contrast, clarity, and shadows in editing and you’ll start to see the Milky Way emerge.
In next month’s article, I’ll share my results—successes, mistakes, and all.
Check out Evan’s AstroBin Before and After images. Click the image for details:
While on vacation at Timber Cove in 2020, Evan Gomez-Shwartz accidentally photographed the Milky Way with his phone. Since then, he’s been taking photos of outer space at every opportunity possible, now with better equipment. The Alameda-based astrophotographer’s favorite subjects to photograph are nebulae and galaxies. Reach him via [email protected]
