Chroma H-beta Filter - 3nm

Isolating the 486.1 nm H-beta Line with a 3 nm Bandpass

While H-alpha is the dominant signal in most emission nebulae, the H-beta line at 486.1 nm is critical for revealing structure and color in specific targets. This filter's extremely narrow 3 nm bandpass is essential for separating the H-beta signal from adjacent blue and green spectral lines, dramatically increasing contrast on objects like the Horsehead Nebula (B33) and the California Nebula (NGC 1499).

The tight bandwidth darkens the sky background significantly more than wider filters, allowing you to pull faint, wispy details out of the noise. For imagers in light-polluted areas, this rejection helps record H-beta-rich regions.

Focal Ratio Performance: 99.6% Peak Transmission at f/4, 90.0% at f/2

Interference filters are sensitive to the angle of incoming light, and a faster focal ratio creates a wider cone of light that shifts the filter's passband towards blue. The Chroma 3nm H-beta is specified for systems f/4.0 and slower, where it delivers optimal performance. At f/4, it maintains a peak transmission of 99.6% with a center wavelength of 485.7 nm, keeping the 486.1 nm H-beta line squarely in its passband.

As the system gets faster, this blue shift becomes more pronounced. At f/3, peak transmission is still an excellent 99.4%, but at f/2 the center shifts to 484.6 nm and peak transmission drops to 90.0%, with the 486.1 nm line close to the red half-maximum edge (486.3 nm). For beams faster than f/3, use 5 nm or wider filters.

Chroma 3 nm vs. 5 nm H-beta: Choosing Your Bandwidth

Chroma offers H-beta filters in multiple bandwidths, and the choice between a 3 nm and 5 nm filter depends entirely on your imaging goals and equipment. The 3 nm filter gives the higher contrast, but it is more sensitive to focal ratio.

• 3 nm H-beta: Rejects more off-band light than the 5 nm, for higher contrast. It suits faint targets from light-polluted skies, provided you are using an f/4 or slower telescope where the blue shift is minimal.
• 5 nm H-beta: Provides a more forgiving option for faster optical systems; for beams faster than f/3, 5 nm or wider is recommended. Its wider passband is less affected by blue shift, ensuring higher on-band transmission even with a steep light cone. The trade-off is lower contrast than the 3 nm version.

Research-Grade Build: A 3.0 mm Parfocal Substrate with Sputtered Coatings

This filter is made in Vermont, USA, on a single 3.0 mm thick glass substrate with a transmitted wavefront better than 0.25 waves/inch. Chroma's astronomy filters (except LoGlow) share this 3.0 mm thickness and are parfocal, so you can swap filters without significant refocusing.

The coatings are applied via an advanced sputtering process, creating a durable filter on a single substrate with no laminations. The primary interference coating is on the front surface, while an anti-reflection coating on the rear surface prevents ghosting and internal reflections from bright stars.