Aurel Clarion OIII filter 2 in, observatory grade

Specifications

Description

The Clarion OIII filter is a narrowband nebula filter in a 2 in threaded cell. It passes a band about 7 nm wide centered near 500 nm, which covers the doubly ionized oxygen emission lines at 496 nm and 501 nm. Peak transmission within that band is about 88 percent, and the filter blocks most other visible light.

Ionized oxygen in supernova remnants and planetary nebulae emits strongly at these wavelengths, while the sky background and most artificial lighting spread their light across a broad part of the spectrum. Narrowing the passband removes much of that background, so the faint nebula stands out more clearly against a darker field. The filter does not add light to the target; it removes light from the surrounding sky, which increases contrast without changing the nebula's surface brightness.

Good targets include the Veil Nebula, the Cygnus Loop, the Dumbbell Nebula, and the Crescent Nebula. Objects that emit mainly hydrogen-alpha, such as the Rosette Nebula, pass poorly through this filter, so you should use an H-alpha or UHC filter for them. Galaxies, star clusters, and reflection nebulae show little contrast gain, and the filter simply dims the view of them. Under dark skies, you will see the outer loops and filaments of the Veil with more detail in a telescope of 150 mm aperture or larger.

The filter screws into the barrel of a 2 in eyepiece or the nosepiece of a 2 in diagonal. The cell is machined from brass and threads at M48 x 0.75. The glass is coated on one face with a multi-layer interference stack and on the other with an anti-reflection layer.

About this configuration

The Aurel Clarion OIII filter 2 in is offered in several configurations, and the notes below describe what this particular one includes. The observatory grade comes with a measured transmission curve for the individual filter in the box. Observatory grade glass has the tightest flatness tolerance in the Clarion line, which suits imaging with long focal lengths. A standard coating suits visual use, where small reflections between filter and eyepiece rarely show.

A plastic case is light and stacks with other filter cases in an accessory box. The 2025 edition keeps the same transmission figures as the earlier years of the Clarion line. The transmission figures in the specifications apply to every configuration of the Clarion line.

What's in the box

• OIII filter in 2 in threaded cell
• Plastic storage case
• Test report with measured transmission curve
• Plastic storage case

Compatibility notes

• Threads into 2 in eyepieces, diagonals, and filter wheels (M48 x 0.75).
• Most effective under moderately dark skies with telescopes of 150 mm aperture and larger.