Optolong H-alpha 3nm Narrowband Filter

Optolong H-Alpha 3nm Narrowband Filter

The Optolong H-Alpha 3nm is a high-performance atmospheric suppression tool designed for high-resolution deep-sky imaging. By restricting the passband to a tight 3nm FWHM (Full Width at Half Maximum), this filter maximizes the signal-to-noise ratio (SNR) of the 656.3nm emission line while aggressively rejecting broadband interference from both artificial light pollution and natural airglow.

The H-Alpha (Hα) Emission Line

In the vacuum of space, ionized hydrogen atoms emit photons at specific energy levels. The most critical for astrophotography is the Hydrogen-alpha line at 656.3nm. This wavelength is the primary indicator of HII regions—massive clouds of ionized gas where star formation occurs.

Because H-alpha light penetrates interstellar dust more effectively than shorter wavelengths, using a dedicated 3nm filter allows for the mapping of high-density structural filaments in emission nebulae that are typically obscured in broadband RGB imaging.

Ion-Assisted Deposition (IAD) and Thermal Stability

Optolong utilizes Ion-Assisted Deposition (IAD) to apply the interference coatings. This vacuum-deposition process results in a high-density coating that is superior to standard evaporated films.

• Zero CWL Shift: Precision narrowband filters can suffer from a shift in the Central Wavelength (CWL) due to temperature fluctuations. IAD technology ensures the coating is mechanically stable, keeping the 3nm window locked on 656.3nm throughout extreme temperature drops.
• Durability: The resulting surface is physically hardened, making the filter resistant to the delamination and aging effects caused by humidity and environmental exposure.

Optical Design and Halo Mitigation

A common failure in narrowband filters is the presence of internal reflections around bright stars. The Optolong 3nm series utilizes a non-cemented optical substrate. By manufacturing the filter from a single piece of high-quality optical glass rather than laminating multiple layers, the number of internal reflective surfaces is halved.

When combined with precision anti-reflection (AR) coatings, this design ensures that the H-alpha signal is transmitted with >90% efficiency while maintaining an optical density of >OD4 in the out-of-band regions, virtually eliminating reflections and halos.