In a perfect astrophotography world, we would live in the modern world, but with one exception: nighttime lighting would be minimal or nonexistent. Although that scenario is not realistic, we can achieve something similar. There are a variety of optical filters that do a fine job of filtering out the wavelengths of common lighting fixtures and allowing only wavelengths through that are typical of astronomical objects. Choosing the right light pollution filter for astrophotography depends on several factors, including your camera type, light pollution level at your observing location, and the celestial objects you want to image. Even in pristine skies, filters can sometimes enhance hidden detail. Keep in mind that all of these filters will require longer exposures. Here are some popular light pollution filter options to consider:
Broadband filters
Broadband filters are exactly what you would think. They do a good job of filtering out the light pollution bandwidths that are common to night illumination lamps but allow through the wavelengths of a variety of astronomical objects. In general, they are suitable for many objects including nebulae, stars (natural colors are preserved), comets, the moon, and galaxies. Here are a few examples.
Optolong L-Pro: A good all-around filter that works well for both modified DSLRs and astronomy cameras. It helps reduce the overall light pollution but allows some natural color to come through, making it suitable for capturing a wider range of deep-sky objects like galaxies, nebulae, and star clusters. It is available in a variety of screw-on sizes as well as clip-in options for select cameras.
Astronomik CLS: This is another quality broadband filter. This one has clip-ins that fit a bigger variety of cameras. The Astronomik CLS filter:
- Blocks specific wavelengths of light pollution, primarily from mercury and sodium vapor lamps.
- Allows a wide range of other wavelengths to pass through, including those emitted by deep-sky objects like nebulae and galaxies. This allows for capturing a broader range of colors compared to narrowband filters.
Narrowband filters
Narrowband filters are useful for more limited targets because they filter out most wavelengths except those that are common to celestial objects, such as H-alpha, H-beta, and OIII. Naturally, the more limited the bandwidths allowed to pass, the smaller the database of objects that can benefit. They are also useful in light polluted skies.
- Optolong L-eXtreme: A highly effective filter for heavily light-polluted areas. It blocks out most light pollution and allows only specific wavelengths of light to pass through, which are typically emitted by emission nebulae like hydrogen alpha (H-alpha) and oxygen III (OIII). This filter is best used with modified DSLRs or astronomy cameras and is not suitable for capturing galaxies or other broadband targets. This filter comes in a variety of screw-on sizes.
ZWO Narrowband Filters: These come in various options like H-alpha, SII, and OIII, allowing you to target specific emission lines in nebulae. They are most effective with astronomy cameras and require longer exposure times compared to broadband filters. They come in screw-on versions that are suitable for dedicated astrophotography cameras as well as one-shot-color DSLR or mirrorless cameras. The nice thing about these filters is that you can zero in on a particular wavelength and create your own color palette by taking images with each and combining them.
Additional factors to consider:
- Filter size: Because these filters come in a variety of sizes and some are made for specific brands of cameras, make sure the filter size matches your camera lens or telescope.
- Filter material: Glass filters are generally considered higher quality but are also more expensive. Plastic filters are a more affordable option but may not be as durable or transmit light as effectively. Personally, I would not waste money on cheap plastic filters. Go with known quality brands.
- Filter transmission curve: This is the most important factor when it comes to choosing the right filter for the right object. It’s important to match the bandwidths that can pass to the bandwidths that are most important in the object you plan to capture.
Remember, choosing the right light pollution filter is just one step in capturing stunning astrophotography images. It’s important to practice, experiment with different settings, and find a location with minimal light pollution for the best results. But if you can’t travel to a dark sky site, these filters will still give very acceptable, if not spectacular results.