ZWO 2" H-alpha 7nm Narrowband Filter

The ZWO 2" H-alpha 7nm Narrowband Filter is designed to dramatically improve deep-sky image contrast and detail by isolating the 656 nm hydrogen-alpha emission line—the dominant wavelength emitted by many nebulae.

By aggressively rejecting unwanted wavelengths, this filter suppresses light pollution, moonlight, and sky glow, allowing faint nebulosity to stand out cleanly against a dark background. The result is higher signal-to-noise, sharper stars, and significantly improved contrast, even when imaging from light-polluted locations.

Why H-alpha Is So Effective

Hydrogen-alpha emission is the primary signal source in many of the most well-known deep-sky objects, including emission nebulae, diffuse nebulae, and planetary nebulae. A dedicated H-alpha filter allows these structures to be recorded with exceptional clarity while greatly reducing background noise.

Many astrophotographers use an H-alpha filter when imaging under a bright Moon or from urban environments, as it effectively removes most artificial lighting while preserving nebular signal. Longer exposures further enhance nebulosity while stars remain smaller and better defined compared to broadband imaging.

H-alpha data can be used on its own or combined with LRGB data to enhance nebular detail and contrast. For users not ready to commit to full H-S-O narrowband imaging, adding an H-alpha filter to an existing LRGB set is often the most effective first step.

Use with Colour Cameras

Although narrowband filters are traditionally associated with mono cameras, the ZWO H-alpha filter can also be used effectively with colour cameras. While it may seem counterintuitive, it provides a practical compromise when time or processing complexity must be kept to a minimum.

This approach works best with fast optical systems, typically f/8 and faster, where sufficient signal can be captured efficiently.

Technical Specifications

• Filter size: 2"
• Central wavelength: 656 nm (H-alpha)
• FWHM bandwidth: 7 ± 0.5 nm
• Peak transmission: approx. 90% (minimum 80%)
• Fine-optically polished for accurate 1/4 wavefront over both surfaces
• Glass thickness: 1.9 mm ± 0.03 mm
• Total thickness: 7.5 mm (5 mm optical + 2.5 mm thread)
• Optical length: 5 mm
• Thread: M48 × 0.75 (standard 2" filter thread)

The 7 nm bandpass is an excellent balance between contrast and signal strength, making it ideal for high-contrast H-alpha astrophotography and for revealing fine nebular detail even under challenging sky conditions.

Filter Orientation – Which Side Faces the Telescope?

ZWO recommends that, for their latest Mark II narrowband filters, the coated side of the filter faces the telescope. While performance differences may be subtle, following this guidance ensures optimal results.

To identify the coated side, place the filter on a clean surface and hold a small object (such as a pen) close above it without touching the glass:

• If you see a single reflection, that is the coated side — it should face the telescope.
• If you see a double reflection, that side has an anti-reflection coating only and should face the camera.

Image below shows a double reflection; this side should face the camera sensor:

Image below shows a single reflection; this side should face the telescope:

Transmission Curve