Chroma OIII Filter - 3nm

Isolating the 500.7 nm OIII Line with a 3 nm Bandpass

This filter targets the 500.7 nm line, the signature wavelength of doubly ionized oxygen and a primary signal in many emission nebulae. The exceptionally tight 3 nm full width at half maximum (FWHM) is designed to reject light outside the band, dramatically increasing the contrast of planetary nebulae like the Dumbbell Nebula (M27) and intricate supernova remnants such as the Veil Nebula (NGC 6960).

• High-Contrast Imaging: By passing only a 3 nm sliver of the spectrum, the filter darkens the sky background more effectively than wider 5 nm or 8 nm filters, making it easier to capture faint, delicate structures from urban or suburban locations.
• Bicolor and Hubble Palette: OIII is a fundamental component of popular false-color imaging palettes. The high signal purity from this 3 nm filter ensures clean data for combination with H-alpha and SII, producing vibrant and detailed images.

Measured Performance Down to f/2: A 1.5 nm Shift & 93.6% Peak Transmission

While designed for systems at f/4.0 and slower, Chroma provides measured data for performance in faster astrographs. At f/4, the filter's center wavelength shifts by only 0.3 nm to 500.4 nm, with peak transmission holding at 99.2%. In an f/2 system, the center wavelength experiences a more significant blue shift of 1.5 nm to 499.2 nm, and peak transmission drops to a measured 93.6%.

This blue shift is a known characteristic of all interference filters in fast optical cones. The trade-off for the 3 nm filter's extreme contrast is reduced on-band transmission in systems faster than its f/4.0 design target. For systems near f/3, there is an OIII 3nm optimized for f/2.8-f/3.6; for beams faster than f/3, use 5 nm or wider bandpasses.

Research-Grade Build: 3.0mm Parfocal Substrate & Better Than 0.25 Waves/Inch Wavefront

Every Chroma filter is manufactured in Vermont, USA, using durable sputtered hard coatings on a single, 3.0 mm thick glass substrate. This construction prevents delamination, while anti-reflection coatings on the rear surface eliminate ghosting and internal reflections. The filter's transmitted wavefront is better than 0.25 waves per inch.

The standardized 3.0 mm thickness makes this filter parfocal with all other Chroma astronomy filters (except the LoGlow). This means you can swap between LRGB, H-alpha, and SII filters in your wheel without having to stop and refocus your camera, a critical time-saver during a short imaging window.

The 3 nm Advantage: Maximum Contrast vs. Wider Bands

Choosing a 3 nm filter over a wider 5 nm or 8 nm version is a decision about contrast. For imagers in light-polluted areas or those working during bright moon phases, the 3 nm bandpass offers a significant advantage by suppressing more unwanted background light. This superior rejection allows for longer sub-exposures and reveals fainter details that would otherwise be lost in the noise.

The primary trade-off is signal strength, especially in very fast optical systems where blue shift can move the passband partially off the emission line. A 3 nm filter is best suited for scopes at f/4 or slower, where measured peak transmission stays at 99.2% (f/4) to 99.3% (f/5).