Chroma OIII Filter - 5nm

The 5nm bandwidth offers a better balance for a wider range of telescopes. While a 3nm filter provides higher contrast, it's more sensitive to the wavelength shift in fast systems (f/4 or faster). The 5nm filter is more forgiving, maintaining higher signal throughput at faster focal ratios while still delivering excellent contrast against light pollution.

On an f/2 system the 5nm filter remains a reasonable choice, since 5 nm or wider is recommended for beams faster than f/3, though performance is impacted by focal ratio shift. Measured data shows that at f/2, the filter's center wavelength shifts to 499.2nm and peak transmission is 95.3%. Peak transmission at f/4 is 99.8%, and at f/2 the 500.7nm OIII line still sits inside the half-maximum passband (496.6 to 501.8 nm).

Yes. M27 is a planetary nebula rich in OIII emission. At f/7 your telescope is within the filter's design range of f/4.0 or slower; measured at f/5, the passband centre is 500.9 nm with 99.8% peak transmission, so the passband centre sits close to the 500.7nm line.

Yes. All Chroma astronomy filters (with the exception of the LoGlow filter) are manufactured on a 3.0mm substrate. This ensures they are parfocal, meaning you should not have to adjust focus when switching between your Chroma OIII, LRGB, H-alpha, or SII filters.

The coatings are applied by reactive magnetron sputtering, building thin layers of metal oxides and silicon oxides on a single substrate with no laminations. These coatings withstand humidity changes and extreme temperature fluxes and remain spectrally accurate in centre wavelength and bandpass.

Chroma minimizes reflections by applying an anti-reflection coating to the rear surface of the filter. This prevents light from bright stars from reflecting off the sensor, back to the filter, and then back to the sensor again, a common cause of halos and ghost images in narrowband imaging.

The correct size depends on your camera's sensor and filter wheel. Here are some general guidelines:

• 1.25" Mounted: Suitable for smaller sensors, typically up to 15-17mm diagonal.
• 31mm Unmounted: Typically used with sensors up to about the Four Thirds class.
• 36mm Unmounted: Typically used for APS-C class sensors in a filter wheel.
• 2" Mounted / 50mm Round: Typically needed for full-frame sensors (36x24mm) to ensure the entire sensor is illuminated without corner darkening.

Chroma offers a lifetime warranty against defects on their sputtered catalog filters under normal use. All Chroma catalog filters also carry a one-year full satisfaction warranty.