Pegasus Astro 2inch mounted photographic filter - S2 7nm

Isolating Sulfur-II: The 7nm Bandpass at 672nm

The core of this filter is its ability to reject all light except for a narrow 7nm window centered on the 672nm wavelength. This is the spectral line for singly ionized sulfur (S-II), a key component in many emission nebulae like the Eagle Nebula (M16) and the Rosette Nebula (NGC 2244). By isolating this signal, the filter dramatically increases contrast, revealing delicate structures that are normally lost in the glare of starlight and light pollution.

With a peak transmission of over 80%, this filter efficiently passes the faint S-II signal to your camera sensor. This efficiency is critical for reducing exposure times and improving the signal-to-noise ratio in your final data, allowing you to pull out more detail from the faintest regions of your target.

Optical Precision: 1/4 Wavefront Polish and Parfocal Design

To prevent image degradation, the 2mm thick optical glass is polished to a 1/4 wavefront standard across its surface. This level of precision ensures that the filter will not introduce distortions or soften star images, preserving the resolution of your telescope. The filter's parfocal design means it shares a common focal point with other filters of the same series, eliminating the need for significant refocusing when sequencing different narrowband exposures.

• 1/4 Wavefront Polish: Guarantees that the filter maintains the sharpness and optical quality of your telescope system.
• Parfocal Design: Saves critical imaging time by minimizing focus shifts when you switch between S-II, H-alpha, and O-III filters during an automated session.

Built for Modern Imaging: 2-inch M48 Cell and AR Coatings

The filter is housed in a durable 2-inch cell with standard M48 x 0.75 threads, ensuring compatibility with virtually all 2-inch filter wheels, drawers, and camera nosepieces. The surfaces of the filter are treated with a multilayer anti-reflection coating. This coating suppresses halos and internal reflections that can occur around bright stars, resulting in cleaner data with higher contrast and darker sky backgrounds.

S-II in Context: Completing the Hubble Palette with H-alpha & O-III

While powerful on its own, this S2 7nm filter is most often used as part of a narrowband trio with Hydrogen-alpha (H-alpha) and Oxygen-III (O-III) filters. This combination allows you to create full-color images in the "Hubble Palette," mapping S-II, H-alpha, and O-III data to the red, green, and blue channels, respectively. This technique produces dramatic, scientifically valuable images that reveal the detailed chemical composition and structure of nebulae.