Astronomik OIII 6 nm 31 mm filter with protective ring (Astronomik, SKU: 10215631) is a narrowband filter for imaging nebulae emitting light from doubly ionised oxygen OIII near 501 nm. Its 6 nm full width at half maximum increases the contribution of the selected emission relative to the sky background, both at dark sites and under light pollution.
The filter suppresses light outside the useful passband, from UV to IR. It can be used for monochrome imaging or to build composites from exposures taken through several emission-line filters. Exposure time and channel balance depend on the brightness of the individual lines, camera sensitivity and sky conditions.
The 6 nm passband suppresses background and starlight more strongly than the 12 nm version. It is useful for faint nebulae, crowded star fields and cameras with low dark current. A narrower passband requires particular attention to the optical system’s focal ratio.
The 31 mm format has a protective ring without a screw thread. The glass is 1 mm thick, while the filter has a mechanical thickness of 3 mm. Before installing it in a filter wheel or drawer, check that the holder accepts this construction.
• passband: OIII, 6 nm full width at half maximum
• suppression of unwanted out-of-band light from UV to IR
• recording of one emission-line channel for later combination
• coating: Astronomik MFR
• protective ring around the glass edge
• durable coatings resistant to scratching and moisture
• filter: Astronomik OIII 6 nm 31 mm with protective ring
• protective filter box
• manufacturer: Astronomik
• manufacturer product code: 10215631
• type: OIII narrowband emission-line filter
• full width at half maximum (FWHM): 6 nm
• guaranteed transmission: over 88% at the main OIII line at 501 nm
• typical transmission: 92% at the main OIII line at 501 nm
• out-of-band blocking: from UV to IR
• coating: Astronomik MFR
• recommended focal ratio range: f/4–f/∞
• nominal diameter: 31 mm
• construction: protective ring, no screw thread
• mechanical thickness: 3 mm
• glass thickness: 1 mm
• material: finely polished optical glass
• optical quality: diffraction-limited
• coating resistance: to scratching, moisture and ageing
• parfocality: with standard Astronomik filters using 1 mm glass
• quality control: individual measurement of spectral response
Selective OIII imaging helps reveal nebular structure with a reduced sky-background contribution.
