Chroma Narrowband Filters for Astrophotography in Light-Polluted Skies – AstroGear Today

Chroma Narrowband Filters for Astrophotography in Light-Polluted Skies

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Cresent Nebula taken with Chroma filters + Astronomik LRGB. Credit AstronoMolly

Anyone who images under bright skies knows that even with a light pollution filter, all that background light is hard to beat. The images are never as good as those taken under darker skies. But for some types of astronomical objects, a solution exists: narrowband filters.

I purchased the 3nm-wide Hα and oxygen-III (OIII) filters made by Chroma Technology in April 2020, and have captured quite a few images with them from Bortle 7 skies. The price of the Chroma filters is up there with the more-widely-known Astrodon filters, but the quality is comparable. I tested the Chroma filters over a year and a half of observing and am so happy with the collection that I plan to add an SII filter shortly. Below I will explain where Chroma stands out and where they can improve.

Narrowband filters capture specific wavelengths of light emitted from brightly glowing gases in many nebulas, particularly hydrogen, oxygen, and sulfur. Because they only pass a very narrow slice of the optical spectrum, nearly all light pollution is blocked, allowing for higher-quality imaging from the city. One can image emission nebulae, planetary nebulae, and supplement galaxies with hydrogen-alpha (Hα) data to showcase their star-forming regions.

Astrophotography filters in general come in a wide range of quality and price. I had not previously heard of Chroma, a U.S.-based company founded in 1991 that manufactures filters for a wide range of scientific research applications, but I decided to give them a try after hearing good things in the astronomy community.

Pelican Nebula, captured with the Chroma 3nm Hα filter, 5x600s, ZWO ASI1600MM Pro camera, Celestron C8, Bortle 3.5 sky. Credit: AstronoMolly

A common issue among cheaper narrowband filters is the formation of halos around bright stars. This issue has not been evident with my ZWO ASI 1600mm Pro camera on my Celestron 8-inch Schmidt-Cassegraint at f/6.3.  The halos and spikes on the two bright stars in the OIII image below are from camera saturation.

Pelican Nebula, captured with the Chroma 3nm Hα filter, 5x600s, ZWO ASI1600MM Pro camera, Celestron C8, Bortle 3.5 sky. Credit: AstronoMolly

Reflections from the edge of the filter are another common narrowband filter issue, especially when the coating does not go all the way to the edge. Chroma manufactures their filters by coating large squares of glass first, and then cutting them, to avoid this issue. I have not noticed any reflections from the filters.

Be aware that Chroma filters, like most narrowband filters, are only suitable for imaging systems of focal ratio f/4 or slower; faster systems do not work well with very narrowband filters.  Chroma does offer custom filters for faster systems, but these must be special-ordered.

The filters also include an anti-reflective coating on the back to reduce any reflections between the sensor and the filter. The side that should face the telescope is more reflective than the sensor side, to make things easier for installation.

Chroma 50mm unmounted Hα 3nm (right) and OIII 3nm (left) installed in a filter wheel. Credit : AstronoMolly

As far as transmission goes, Chroma provides measured data on both the spectrum and transmission for each of their filter types.

3nm Hα filter, transmission versus wavelength. Credit: Chroma

 

3nm OIII filter, transmission versus wavelength. Credit: Chroma

Peak transmission of the Hα filter was measured to be 99.48 percent, with nearly 0 percent transmission (<0.0001 percent) at all other relevant wavelengths.  The 5nm and 8nm versions show similar figures, with the 8nm topping out at 96.5 percent transmission. The OIII 3nm filter likewise peaks at 99.6 percent with negligible transmission outside of that band, while the 5nm peaks at 97.7 percent and the 8nm at 94.4 percent. Their SII (sulfur-II) filters show similar values.

M57 Ring Nebula, captured with the Chroma 3nm Hα & OIII filters, 107x600s Hα & 90x600s OIII, ZWO ASI1600MM Pro camera, Celestron C8, Bortle 7 sky. Credit: AstronoMolly

The usual caution applies here, too, that narrowband filters are not a perfect solution for light-polluted skies. That said, the Chroma set has been a constant companion in my observing journey and I do look forward to using more of their filters in the coming months.

Market Price: OIII 3nm 1.25″ Unmounted  $575
Market Price: H-Alpha 3nm 2″ Mounted  $1,300
Website: Chroma.com

About AstronoMolly

I got into astrophotography in July 2015 after receiving my first telescope as a gift, Much trial and error later, I now have three astrophotography rigs set up in my backyard just north of Berkeley, CA, in the San Francisco Bay area, including one dedicated to variable star and exoplanet transit observations. I love doing STEM and astronomy outreach, and I've accrued more than 150 hours of volunteer activities reaching over 20,000 people, both in-person and virtually. I am an AAVSO Ambassador (American Association of Variable Star Observers), an Explore Alliance Ambassador, and a panelist and broadcaster for The Astro Imaging Channel weekly YouTube show. I have a B.S. in Physics from Washington State University, and am currently pursuing my PhD in Physics at University of California, Berkeley, studying neutrinos with my two cats, Orion and Apollo.

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