When you take a daytime photograph with your phone or camera, it generally snaps an image within a few milliseconds. In that short time, you’ve captured a story, a feeling, an event, instantly. Astrophotography isn’t quite as straightforward, as I’m sure you can imagine!
This single frame was captured over a five minute period. An iPhone typically captures a photo in just one-fortieth of a second, but a deep-sky exposure might last 30, 300, or even 600 seconds (that’s 10 minutes). For the Rosette Nebula, I chose a duration of 300 seconds (five minutes) per image. That is 12,000 times longer than the iPhone exposure.
One thing you might notice is that the photo is bad. If you squint, there’s a very faint structure. Light pollution envelops the entire image. Boosting the brightness reveals a lot of noise caused by electronic interference and cosmic rays:
Sometimes satellites fly across the target object, like these during my imaging session of the Rosette Nebula:
It’s possible that those satellites were only in my field of view for ten seconds, yet still ruined the 300 second picture as a whole. The longer your exposure lasts, the more detail you can capture from Earth—and the greater risk of anomalies like space debris, wind, planes, clouds, and more. For these reasons, 300-second exposures are favored over 600 seconds. While 600 seconds would double the amount of light captured, you also double the risk of your image being interrupted.
Similarly, the amount of thermal noise and hot pixels pictured earlier increase the longer an exposure lasts.
So how do we end up with the final image?
Check out the highest quality version of the image and additional information by visiting the author’s Astrobin account. Photo by Evan Gomez-Shwartz.
The Rosette Nebula, after five hours worth of images stacked and post processed in PixInsight. There is also a wonderful, free and open-source astrophotography stacking software called Siril.
The Rosette Nebula could have been featured in last month’s article about star lifecycles, since it is a vast star-forming region about 100 light years wide, located roughly 5,000 light years away in the constellation Monoceros. Inside, a cluster of young stars floods the surrounding gas with intense radiation, causing the entire cloud—containing nearly 10,000 times the mass of our Sun—to glow.
The key to going from a flat, dim picture to a finished image is combining many bad pictures to make one good one. Each individual frame has problems (noise, glow, tracking errors etc.) but they’re random. When we stack dozens or hundreds of these imperfect images, the random flaws average out, while the consistent details of the nebula remain.
The rejected parts of the combined images form a Rejection Map – problem areas detected during stacking which are automatically removed. While the noise and flaws are random, the nebula stays consistent in the frame. By subtracting random incongruences, only the consistent parts of the image remain:
While this series focuses on images taken in Alameda, I want to briefly showcase another Bay Area astrophotographer’s discovery of a potentially new nebula. His name is Imran Badr, and he seemingly discovered a previously unknown ball of gas from his San Jose backyard after stacking one hundred and fourteen hours worth of photos:
You’ve made it through the journey from five minutes, to five hours, to 114 hours. Now that the clouds are rolling in tonight, I think it’s time I got a full eight hours!
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].
