Astrophotography Filters Explained: RGB, Narrowband, SHO & More

In astronomy, filters play a fundamental role: they make it possible to isolate precise portions of the electromagnetic spectrum in order to better reveal what the eye, and even the sensor, does not spontaneously see. Depending on the intended goal, their applications vary considerably.

The most common uses include the reduction of light pollution, which masks faint deep-sky objects from urban areas, as well as the isolation of specific emission lines produced by gaseous nebulae. These narrow lines, notably hydrogen-alpha (Hα), hydrogen-beta (Hβ), doubly ionized oxygen (OIII), and singly ionized sulfur (SII), correspond to precise atomic transitions and reveal structures that broadband filters do not capture. Red, green, and blue (RGB) filters are also used to reconstruct a natural-color image from a monochrome camera, by combining the three acquisitions in post-processing.

These same channels can be freely reassigned to produce so-called "false color" palettes. The best known is the SHO palette, popularized by the Hubble Space Telescope, which assigns SII to red, Hα to green, and OIII to blue. Countless other combinations are possible, each palette highlighting different phenomena within the same stellar field.

Images © 2024 Boris Irissou. SHO vs. RGB comparison featuring the Tulip Nebula and the April 2024 Solar Eclipse. All rights reserved.

Two major families of filters

To understand why certain filters are preferred over others, it's necessary to look at how they work. There are two main categories.

Hannes Grobe -Filter KB20 - License CC BY 3.0 2008 - Wikimedia.org

Absorption filters rely on the property of certain materials (tinted glass, optical gels) to absorb part of the spectrum while letting the rest pass through. Their main advantage is their low manufacturing cost and their insensitivity to the angle of incidence of light: whatever angle the ray arrives at, the spectral response stays the same. On the other hand, their transmittance in the useful wavelengths is often limited, and their transition between passbands and blocked bands is gradual rather than sharp. They therefore inevitably lose a portion of the light they are supposed to let through, and struggle to isolate very narrow bands. For these reasons, they are little used in astrophotography.

Eric Magnan - Dichroïc mirror used in 2014 at JQI, UMD. - License CC BY-SA 3.0 2014 - Wikimedia.org

Dichroic filters, also called interference filters, work on a radically different principle. Rather than absorbing light, they separate it: the desired wavelengths are transmitted, while the others are reflected. No energy is dissipated as heat, which makes them particularly efficient and thermally stable. This behavior is achieved by successively depositing dozens to hundreds of ultra-thin layers of two alternating transparent materials, generally metal oxides, onto a glass substrate. At each interface, a fraction of the light is reflected. These reflections interfere with one another constructively or destructively depending on the wavelength: some cancel out, others reinforce each other. By precisely adjusting the thickness of each layer, the spectral response can be sculpted with great freedom.

The result is remarkable: transmittance that can exceed 95% in the useful band, a very sharp cutoff between transmitted and blocked zones, and the ability to produce extremely narrow-band filters. The sharpest "narrowband" filters let through only 2.5 nm of spectrum, an infinitesimal spectral window capable of isolating a single emission line amid the sky background. Conversely, dichroic broadband filters make it possible to precisely delineate RGB channels or other custom spectral combinations.

Their only notable drawback is their sensitivity to the angle of incidence: when light arrives at an angle, the passband shifts toward shorter wavelengths, a phenomenon known as band shift. This effect becomes significant at fast focal ratios, where the light rays converge at a steep angle. This is why dichroic filters are generally designed to work at a specific angle, either 0°, like standard screw-on filters, or 45°, like those integrated into beam-splitting optical systems. For the relationship between focal ratio and spectral band shift, I recommend looking at this website.