Chroma H-alpha Filter - 5nm

Isolating the 656.3 nm H-alpha Line with a 5nm Bandpass

This filter targets the primary emission line of ionized hydrogen at 656.3 nm, the most dominant wavelength in most emission nebulae like the North American Nebula (NGC 7000) or the Orion Nebula (M42). The 5nm Full-Width Half-Maximum (FWHM) bandpass is narrow enough to dramatically increase contrast by rejecting light pollution and moonlight, while cleanly separating the H-alpha signal from the nearby Sulfur-II (SII) doublet.

Measured Throughput: 75% at f/2, 99.8% at f/4

Interference filters experience a "blue shift" in faster focal ratio systems, causing the passband to move to shorter wavelengths. The Chroma 5nm H-alpha filter is designed for systems at f/4.0 or slower, and measured performance confirms its stability in this range. At f/4, it delivers a peak transmission of 99.8%, and at f/5, it reaches 99.9%.

For faster systems, the trade-off becomes apparent. At f/3, peak transmission is still 99.7%, falling to 97.1% at f/2.5. At f/2, peak transmission drops to 75.0% and the passband centre moves to 653.0 nm, leaving the 656.3 nm line at the filter's half-maximum edge (649.6-656.4 nm). For these instruments, a wider bandpass or a purpose-built fast-optimized filter may be a better choice.

The 5nm Choice: Balancing Contrast and Focal Ratio Tolerance

The 5nm bandpass represents a strategic balance for most imagers. It offers higher contrast than the wider 8nm version, darkening the sky background for cleaner data. At the same time, it is more tolerant of focal ratio shift than the 3nm version.

• vs. 3nm Filters: A 3nm filter provides higher contrast but is the most sensitive to blue-shift in fast optics. The 5nm filter is more forgiving of focal-ratio shift.
• vs. 8nm Filters: An 8nm filter is more robust against focal ratio shift but passes more background signal, reducing overall contrast. The 5nm filter provides a darker background and better rejection of unwanted signal.

Professional Build: 3.0mm Parfocal Substrate and Better Than 0.25 Waves/Inch Wavefront

Every Chroma filter is manufactured in Vermont, USA, to exacting research-grade standards. The coatings are applied via an advanced sputtered process onto a single 3.0 mm glass substrate, giving extremely durable coatings that withstand humidity changes and extreme temperature fluxes. This single-substrate design avoids the risk of delamination common in laminated filters.

The 3.0 mm substrate thickness is standardized across Chroma's astronomy filter line (except the LoGlow filter), making them parfocal. This means you can swap between LRGB and other narrowband filters without significant refocusing. Transmitted wavefront is better than 0.25 waves/inch.