O-III 1.25 inch Narrowband Filter by Baader, 6.5 nm Bandwidth for CMOS Imaging

The Baader O-III 1.25" Narrowband Filter with a 6.5 nm bandwidth is designed for use with CMOS astronomy cameras and 1.25" focusers and adapters. The filter selectively transmits the oxygen-III emission lines near 495.9 nm and 500.7 nm while strongly attenuating other wavelengths, which enhances contrast of planetary nebulae, supernova remnants and other oxygen-rich emission nebulae. Its 1.25" cell makes it compatible with common eyepieces, filter wheels and camera adapters for both visual observation and astrophotography.

Advantages

The narrow 6.5 nm passband increases contrast by reducing background continuum and light pollution without requiring extremely long exposures. Designed for CMOS imaging, the filter provides stable transmission across the intended passband and minimizes out-of-band leakage. The compact 1.25" format allows easy integration into existing optical trains and standard filter holders, enabling quick changes between filters during imaging sessions.

Suitable for

• Enhancing contrast of planetary and diffuse emission nebulae (O-III dominated regions)
• CMOS astrophotography with small to medium aperture telescopes
• Visual observation of faint emission nebulae under light-polluted skies
• Use in 1.25" filter wheels, eyepieces and camera adapters

Key specifications

• Filter type: Narrowband O-III (oxygen-III) transmission
• Diameter / cell size: 1.25 inch (standard 1.25" filter cell)
• Bandwidth (FWHM): 6.5 nm
• Designed for: CMOS imaging and visual use
• Primary transmitted lines: O-III emission near 496 nm and 501 nm

How to use

Install the 1.25" filter into a compatible focuser, eyepiece holder or filter wheel between the telescope and the camera or eyepiece. For astrophotography with CMOS cameras, place the filter as close to the sensor as practical to avoid vignetting and ensure uniform illumination. Use standard calibration routines (darks and flats) when capturing narrowband data to correct for sensor noise and dust shadows. When switching between filters during an imaging session, allow the telescope to refocus if necessary due to slight optical path changes.

Recommendations

For best results, pair the O-III filter with targets that have strong oxygen-III emission such as planetary nebulae and certain supernova remnants. Under heavy light pollution, the narrow 6.5 nm bandwidth will improve contrast, but combining exposures with other narrowband filters or using longer total integration times can further enhance faint structures. Verify mechanical clearance in your imaging train to prevent vignetting when using the 1.25" cell.