OIII Filter

Specifications

Observing parameters

Overview

An OIII filter passes a narrow band around 496 and 501 nm, the two lines of doubly-ionised oxygen, and blocks nearly everything else. It is the specialist of the family: on objects that are genuinely oxygen-rich — the Veil Nebula, the Ring Nebula, M27, and most planetary nebulae — the improvement over a UHC is dramatic.

On the wrong object it is worse than no filter at all. Galaxies, reflection nebulae and clusters lose almost all their light and simply vanish. The passband is typically 10–12 nm, less than half a UHC's, so the image is also noticeably dimmer and you need aperture to spare.

The practical rule: OIII for planetary nebulae and supernova remnants, UHC for general emission nebulae.

Where it fits

Practical notes

• Try the same object with and without the filter before you conclude it does not work — the Veil is the classic demonstration.
• OIII and H-beta filters are complementary, not competing; H-beta suits the Horsehead and California Nebula.
• Below 150 mm aperture the dimming can outweigh the contrast gain except on the brightest planetaries.
• At high power the dimming compounds; pair with a medium focal length eyepiece.

Pairs with

• Wide-Field Eyepiece
• Filter Wheel
• CMOS Camera
• Star Atlas