Chroma H-alpha Filter - 3nm

Isolating H-alpha at 656.3 nm with a 3 nm Bandpass

This filter targets the primary wavelength emitted by ionized hydrogen, the most abundant element in emission nebulae like the North American Nebula (NGC 7000) and the Heart Nebula (IC 1805). The extremely narrow 3 nm bandpass is the key to its function, aggressively rejecting moonlight, skyglow, and continuum emission from stars to produce a near-black background. The narrow passband separates H-alpha from the SII doublet at 671.6 nm and 673.1 nm.

Measured Transmission from f/2 to f/5

Interference filters experience a "blue shift" in faster optical systems, where the passband moves to shorter wavelengths. The Chroma 3nm H-alpha is designed for systems f/4.0 or slower, where it delivers near-perfect transmission. Measured data confirms its real-world capabilities: it achieves 99.9% peak transmission at f/5 and 99.8% at f/4. At f/3 the peak is still 99.4%, but the passband centre has moved to 655.1 nm, leaving the 656.3 nm line near the red half-maximum edge (656.6 nm); transmission at the line is 72% at f/3. For imagers with very fast systems, it's critical to understand the trade-off: peak transmission drops to 91.4% at f/2.5 and 65.7% at f/2 as the passband shifts away from the 656.3 nm line. For beams faster than f/3, use 5 nm or wider filters; there is also an f/3-optimized 3 nm H-alpha for f/2.8 to f/3.6.

3.0mm Parfocal Substrate and <0.25 Wave/Inch Precision

Chroma builds its astronomy filters (except LoGlow, on 1.1 mm glass) on a single 3.0 mm thick glass substrate, so they are parfocal with one another. Transmitted wavefront is better than 0.25 waves per inch. The coatings are applied via a durable magnetron sputtering process in Chroma's Vermont, USA facility. This multi-layer process applies the primary interference coating on the front surface and an anti-reflection coating on the rear to maximize transmission and prevent internal reflections or "ghosting" from bright stars.

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

The choice between a 3nm and a wider 5nm or 8nm filter is a balance of contrast against system forgiveness. Of the three, the 3nm filter rejects the most background and gives the highest contrast. However, this precision comes with a trade-off:

• 3nm Bandpass: Darkens the sky background most effectively of the three bandwidths, for the highest contrast. It is, however, more sensitive to the blue shift that occurs in fast focal ratio systems.
• 5nm or 8nm Bandpass: These wider filters are more tolerant of the blue shift in fast (sub-f/4) systems, losing less transmission at the H-alpha line. They capture more signal from objects with high radial velocity but will also pass more background light, resulting in lower contrast.

Sizing for Your Sensor: From 1.25" to 50mm Unmounted

To prevent vignetting, the filter must be large enough to fully illuminate your camera's sensor. Chroma offers the 3nm H-alpha filter in 1.25" and 2" mounted, 31 mm, 36 mm and 50 mm round unmounted, and 50 x 50 mm square unmounted sizes. The guidance below is typical; actual coverage depends on filter-to-sensor spacing and focal ratio.

• 1.25" & 31mm: Typically used with smaller sensors; the 31 mm unmounted size gives more room than the 1.25" cell.
• 36mm: Typically used for APS-C class sensors (diagonals up to about 28 mm) in a filter wheel.
• 2" & 50mm Round/Square: Typically needed for full-frame sensors (36 x 24 mm, about 43 mm diagonal) to avoid darkening the corners of your images.