Improve your astrophotography: mastering filters for nebulae

Astrophotography demands precision and the right tools to capture the breathtaking beauty of the night sky. Filters are essential for reducing light pollution and isolating specific wavelengths, allowing for clearer observations and stunning images of celestial objects like nebulae. Whether you’re battling urban light pollution or highlighting the unique characteristics of emission nebulae, mastering filters is key to enhancing your astrophotography.

Vaonis offers two expertly crafted filters: the CLS filter for light pollution reduction and the duo-band filter for advanced nebula imaging. Here’s everything you need to know to get started.

 

How astrophotography filters work

Light, colors, and wavelengths

Light is a combination of radiation of different wavelengths. When passed through a prism or rain droplets, it breaks into a spectrum of colors visible to the human eye.

For our eyes, different wavelengths manifest as the colors of the spectrum. The wavelength of visible light is measured in nanometers (abbreviation: nm, or one billionth of a meter). We can perceive wavelengths ranging from 380 nanometers, corresponding to the color violet, to 780 nanometers, corresponding to the color red.

The visible spectrum spans from the wavelengths 380 nm (violet light) to 780 nm (red light).

Smart telescope sensors detect an even broader range of wavelengths, including infrared, but filtering is often required to produce sharp images.

 

Celestial light vs. Artificial light

Celestial bodies and artificial sources emit light at varying intensities and wavelengths.

For instance, urban sodium vapor lamps emit strong yellow-orange tones.

Conversely, some deep-sky objects like emission nebulae ( such as The North America Nebula) radiate light at specific wavelengths, notably the hydrogen-alpha (656 nanometers, red light) and oxygen-III (500 nanometers, green light) lines.

Filters that block unwanted wavelengths and isolate these emission lines dramatically enhance contrast and clarity in astrophotography.

 

Vaonis filters: a perfect match for astrophotography

Broad-band filters: the CLS filter

Designed for versatility, broad-band filters like the Vaonis CLS filter reduce light pollution while preserving the visibility of most celestial objects. This filter transmits key wavelengths, including hydrogen-beta, oxygen-III, hydrogen-alpha, and sulfur-II, making it ideal for urban astrophotography.

They are an excellent entry-level choice for observing all types of nebulae. While other filters may be more effective at reducing light pollution, they are also more selective about the types of objects they allow you to observe.

The Vaonis CLS filter transmits parts of the spectrum near the hydrogen beta and oxygen III lines, with a 40-nanometer bandwidth, and the hydrogen alpha and sulfur II lines, with a 35-nanometer bandwidth.

A simplified representation of the wavelength ranges filtered by the Vaonis CLS filter.

Narrow-band filters: the duo-band filter

For more precise needs, the Vaonis duo-band filter isolates the hydrogen-alpha and oxygen-III emission lines with a narrow 12-nanometer bandwidth. This makes it particularly effective for observing emission nebulae whether in light-polluted urban environments or under dark skies as it reduces starlight and makes the nebula stand out, especially in dense star fields.

However, its selectivity limits its use to specific objects, such as emission nebulae, certain planetary nebulae, and supernova remnants. For experienced astrophotographers targeting high-contrast images, this filter is invaluable.

A simplified representation of the wavelength ranges filtered by the Vaonis Duo-Band filter.

Which filter should you use?

 

Depending on your target object and light pollution level, the choice of filter is crucial. Use the following table for guidance.

Light pollution levels are measured on the Bortle scale, ranging from 9 (extremely light-polluted skies of large cities) to 1 (a sky completely free of light pollution).

To determine the Bortle scale rating of your observation site, you can check the following map: https://www.lightpollutionmap.info

 

How to identify the type of nebulae

In the Singularity application, nebulae are identified and classified by type.

• Emission nebulae: These are well-suited for observation with the duo-band filter, particularly when located in star-rich regions. This makes them ideal targets for astrophotography.
• Reflection nebulae: These nebulae reflect the light of nearby bright stars and emit across a wide portion of the spectrum. They should not be observed with the duo-band filter, as it would excessively dim their overall brightness and color balance.
• Mixed nebulae: These combine both emission and reflection characteristics. It is also recommended not to use the duo-band filter for these nebulae, as it could excessively reduce visibility of certain parts of the nebulae.

Examples of nebulae to observe with the duo-band filter

In general, nebulae that are predominantly red or green are suitable for this filter. This includes numerous HII regions scattered throughout the Milky Way, which are often the focus of astrophotography.

Examples:

• North America Nebula
• Eagle Nebula
• Rosette Nebula
• Cygnus Loop (Veil Nebula)
• Pacman Nebula
• Tulip Nebula
• Heart and Soul Nebulae
• Bubble Nebula
• Crescent Nebula
• Tarantula Nebula
• Statue of Liberty Nebula
• Dumbbell Nebula (M27)

 

Examples of nebulae not recommended for the duo-band filter

Nebulae with blue, white, or yellow tones are generally not suitable for the duo-band filter.

Examples:

• Lagoon Nebula and Trifid Nebula (mixed)
• Orion Nebula (M42, mixed)
• Horsehead Nebula (mixed)
• Pleiades (reflection)
• Iris Nebula (reflection)
• Carina Nebula (mixed)
• Tarantula Nebula (mixed)
• Ghost of Cassiopeia (reflection)
• Helix Nebula
• Rho Ophiuchi (mixed)

 

About Planetary Nebulae

Certain planetary nebulae emit strongly around the oxygen-III emission line and can be observed with the duo-band filter. For example:

• Dumbbell Nebula (M27) is an excellent target for astrophotography using this filter.

However, other planetary nebulae emit across a broader spectrum and are better observed without the duo-band filter. For instance:

• Helix Nebula is better viewed unfiltered to capture its full range of light.

 

About Star Clusters

While star clusters may emit more intensely in specific visible spectrum colors (sometimes giving them a dominant hue), stars generally radiate across the entire spectrum. Using a filter, even to reduce light pollution, would dim their brightness.

Moreover, stars are highly contrasted against the sky, even in light-polluted areas. Therefore, filters are not necessary for observing star clusters, regardless of the sky quality.

 

About Galaxies

A galaxy contains a variety of celestial objects, primarily stars, and emits light across the entire spectrum. For this reason, it is not recommended to use filters when observing galaxies, except in environments with extreme light pollution.

This detailed guidance ensures that filters are used effectively, especially in astrophotography, to enhance specific celestial features while preserving natural light dynamics.