Chroma H-alpha Filter - 8nm

The 8nm Bandpass: Isolating Hydrogen at 656.3 nm

This filter is engineered to capture the most dominant wavelength of light in emission nebulae: the hydrogen-alpha line at 656.3 nm. The 8 nm full width at half maximum (FWHM) is wide enough to collect strong signal on faint structures while still delivering a dramatic increase in contrast against moonlight and background skyglow.

This makes the 8nm H-alpha an essential tool for imaging vast hydrogen clouds like the North American Nebula (NGC 7000) or the intricate star-forming regions within the Orion Nebula (M42). By blocking light outside its passband, it brings out faint, glowing structures that are washed out in broadband LRGB images, especially from light-polluted locations.

Measured Throughput from f/5 Down to f/2

While designed for systems of f/4.0 or slower, the 8nm H-alpha holds up well in faster telescopes in Chroma's measured data. As an optical system's focal ratio gets faster, the cone of light steepens, causing a "blue shift" in the filter's passband. The 8nm width provides critical headroom against this effect, maintaining high transmission where narrower filters lose signal.

Measured data shows how well it holds up under speed. Even at f/2, its measured peak transmission is 98.8%, with the passband centre shifted to 654.2 nm and half-maximum edges at 650.5-658.0 nm, so the 656.3 nm line stays inside the passband. This makes it a practical choice for fast astrographs, where narrower filters can shift off the emission line and lose signal.

• At f/5: 99.9% peak transmission, centered at 656.2 nm
• At f/4: 99.9% peak transmission, centered at 656.0 nm
• At f/3: 99.8% peak transmission, centered at 655.5 nm
• At f/2: 98.8% peak transmission, centered at 654.2 nm

Built for Imaging: 3.0mm Parfocal Substrate & Sputtered Coatings

Like every Chroma astronomy filter except the LoGlow, it is built on a single 3.0 mm thick substrate, with a transmitted wavefront better than 0.25 waves/inch. This uniform thickness means the 8nm H-alpha filter is parfocal with other Chroma LRGB, SII, and OIII filters, so filter thickness does not change focus when you switch between them during an imaging sequence.

The filter's passband is created with durable, sputtered hard coatings that are resistant to humidity and temperature changes. The primary coating is applied to the front surface, while the rear surface receives an anti-reflection coating to prevent ghosting and internal reflections, preserving the contrast of your final image. Manufactured in Vermont, USA, each filter is backed by Chroma's lifetime warranty.

Chroma 8nm vs. 5nm & 3nm: Throughput vs. Contrast

Chroma offers H-alpha filters in 3nm, 5nm, and 8nm bandpasses, each representing a different trade-off between contrast and versatility. The 8nm is the most forgiving and often the highest throughput option, especially in very fast optical systems.

• 8nm H-alpha (This Filter): The choice for the most signal of the three, especially on fainter targets or with shorter total integration times. Its wider bandpass is tolerant of the blue shift that occurs in fast f/ratio systems, keeping a measured peak transmission of 98.8% at f/2.
• 5nm & 3nm H-alpha: These filters provide higher contrast by rejecting more of the skyglow continuum and starlight. This is ideal for imaging bright nebulae from heavily light-polluted sites. However, they are more sensitive to focal ratio shift, and 5 nm or wider is recommended for beams faster than f/3.