Want an easy, bright target to check out with your telescope that isn’t the Moon or a planet? Try the Orion Nebula. Located in the middle of the “sword” of the hunter constellation Orion (just under the famous three “belt stars”), the Orion Nebula is arguably one of the most, if not the most, well-studied star-forming regions in the night sky. A mere 1,500 light-years away, the nebula is just a small part of the enormous Orion molecular cloud complex that is still churning out stars to this day. Its distance to Earth and very bright central stars make the Orion Nebula an easy target for small backyard telescopes even under moderately bright skies. It can even be picked up with the naked eye under very dark skies.
In January 2025, the Orion Nebula was captured with the same telescope used to obtain the above featured image but used a different camera known as a “one-shot color” camera. This camera is similar to the one you might have in your smartphone but with a much larger sensor. It can take a full-color picture in a single shot. The camera used to obtain the more recent image is almost a twin to the color camera except that it’s a “monochrome camera”. This means it only captures black and white images because it does not have tiny color filters built into it. To get the colorful result, three separate long-exposure images were taken. Each was shot using a different filter that only lets through a very narrow section (or “band”) of the rainbow, hence the name “narrowband filter.” Nearly nine hours of exposure time (about three hours per filter) went into making the final image.
The narrowband filters were made to let in light produced by specific atoms, so the colors seen in the final images indicate where these atoms lie. During final processing, each of the three images was assigned a color of red, green, or blue and then combined to achieve the colorful result. In this case, red denotes sulfur atoms, green for hydrogen, and blue for oxygen. This color scheme is often referred to as the “Hubble Palette” thanks to many of the beautiful science images released by astronomers using the Hubble Space Telescope that use the same filters and color assignment. The filters used (SII: singly ionized sulfur, Hα: hydrogen-alpha, and OIII: doubly ionized oxygen) are just three of the many filters available to astronomers in scientific research, and artistic license is granted to the image processor in choosing how colors are assigned.
Though researchers often use narrowband filters for scientific work, amateur astronomers also make use of them when imaging deep-sky objects, particularly gaseous nebulae. They are especially useful for imaging when there are bright city lights nearby. Light pollution produces a glow composed of practically all colors of the rainbow, while nebulae only emit in very narrow bands of color. A one-shot color camera will pick up the light of both the city and nebulae, but the bright city lights can completely overwhelm that of a nebula, making it almost impossible to see. Narrowband filters solve this problem by blocking majority of the light pollution while letting the nebula’s special colors pass through. Suddenly, the mere whisper of a nebula seen in regular color images now explodes into view when using narrowband filters, even in heavily light-polluted areas.
While this is a major improvement for astrophotography, nothing beats extremely dark skies. Going back a decade or two, many streetlights gave off an orange glow from sodium bulbs. The light emitted by these bulbs encompassed a very small portion of the rainbow that typically didn’t coincide with what astronomers and astrophotographers were interested in observing, so narrowband filters could easily block their glow.
With so many lights now converted to LED bulbs, which don’t emit in very narrow bands, it has become significantly more difficult to overcome light pollution with narrowband filters. As we move forward, limiting light pollution by choosing better light designs, keeping lights pointed down versus into the sky and only using as much light as we really need can help bring back the dark nighttime skies, which are beneficial not only to astronomers but other lifeforms as well!