Chroma H-alpha Filter Optimized for f/3 - 3nm

Isolating the 656.3 nm H-alpha Line with a 3 nm Bandpass

Hydrogen-alpha is the dominant wavelength emitted by the vast emission nebulae that populate our galaxy, such as the Soul Nebula (IC 1848) and IC 434, the glowing backdrop of the Horsehead Nebula. This filter uses an exceptionally narrow 3 nm bandpass to isolate this specific 656.3 nm signal, rejecting light pollution and continuum emissions from stars. The result is a dramatic increase in contrast, revealing faint, intricate structures that would otherwise be lost against the skyglow.

Designed for f/3 Systems: 98.9% Transmission On-Target

Interference filters experience a "blue shift" in fast optical systems as the converging light cone strikes the filter at steeper angles. This Chroma H-alpha filter is specifically designed to counteract this effect. In collimated light, its center wavelength is 657.1 nm, but under the cone of an f/3 system, it shifts onto the H-alpha line, centring at 656.2 nm, achieving a peak transmission of 98.9%. This optimization ensures you capture maximum signal where it counts, avoiding the significant signal loss that uncorrected narrow-band filters suffer at fast focal ratios.

• At f/3: The filter lands at 656.2 nm with 98.9% peak transmission, ideal for its design range.
• At f/2.5: Performance remains high, centering at 655.8 nm with 98.4% peak transmission.
• At f/2: The shift becomes more pronounced, centering at 655.1 nm with a reduced peak transmission of 85.8%.
• At f/4: The filter is centered slightly red at 656.6 nm but maintains a high 99.5% peak transmission.

Professional Build: 3.0 mm Substrate and Sputtered Coatings

Chroma's astronomy filters are manufactured in Vermont, USA. The filter uses a single 3.0 mm thick substrate with no laminations and is parfocal with other Chroma astronomy filters (except the 1.1 mm LoGlow). The coatings are applied via a durable sputtering process, with the primary interference coating on the front and an anti-reflection coating on the back to prevent ghosting. With a transmitted wavefront better than 0.25 waves/inch, the filter preserves the sharpness of your optical system.

Multiple Sizes for Any Modern Imaging Train

To accommodate a wide range of telescopes and cameras, this filter is available in six sizes. Which one avoids vignetting depends on your sensor, filter wheel spacing and focal ratio.

• 1.25" and 2" Mounted: Thread directly into filter wheels, filter drawers, or camera nosepieces that accept standard M28.5x0.6 and M48x0.75 threads.
• 31 mm and 36 mm Unmounted: 36 mm typically covers APS-C class sensors in a filter wheel; 31 mm typically suits smaller sensors.
• 50 mm Round and 50x50 mm Square: Typically chosen for full-frame (36 x 24 mm) class sensors.

Chroma f/3-Optimized vs. Standard 3nm H-alpha Filters

The key advantage of this filter is its pre-compensated design for fast optics. A standard 3 nm H-alpha filter, designed for collimated light, slides blue of the 656.3 nm line in an f/3 beam and loses signal at the line (Chroma, Filters at Fast Focal Ratios). This f/3-optimized filter is placed so the shift lands it on the line, with 98.9% measured peak transmission at f/3.

• vs. Standard 3nm Filter: At f/3, a standard 3 nm filter's passband sits blue of the H-alpha line, while this optimized filter is centred at 656.2 nm with 98.9% peak transmission.
• vs. Wider 5nm/8nm Filters: While wider filters are more tolerant of focal ratio shifts, they sacrifice contrast by allowing more background light to pass. This optimized 3 nm filter keeps the contrast of a 3 nm band while holding its signal in an f/2.8 to f/3.6 beam.