Aurel Clarion broadband filter 31 mm, observatory grade

Specifications

Description

The Clarion broadband filter is an unmounted 31 mm round light-pollution filter for filter wheels and filter drawers. It blocks the main emission lines of sodium and mercury street lighting while passing the blue, green, and red light where stars, galaxies, and reflection nebulae shine.

In imaging, the filter reduces the uneven, orange-tinted background that artificial lighting adds to long exposures. That simplifies gradient removal and color balance in processing, especially for images taken from suburban sites.

A broadband filter keeps star colors close to natural, which makes it suitable for galaxies, star clusters, and reflection nebulae where narrowband filters would block most of the light. It also works with one-shot color cameras behind refractors and Newtonians.

Modern white LED street lighting emits across the visible spectrum, and broadband filters remove less of it. Under LED lighting, expect a smaller improvement than under sodium lighting.

The 31 mm size fits filter wheels designed for 1.25 in equivalent unmounted filters and covers smaller camera sensors. Check the clear aperture of your filter wheel and the size of your sensor before you choose this size.

The glass has a multi-layer interference coating on one side and an anti-reflection coating on the other. Handle it by the edges and store it in its case, since unmounted glass edges can chip.

The Clarion broadband filter suits imagers who capture galaxies and star clusters from suburban sites and want to reduce the effect of older street lighting on their images.

About this configuration

The Aurel Clarion broadband filter 31 mm 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. Anti-reflection coatings on both faces reduce halos around bright stars in the field.

The filter ships in a clear plastic case that keeps dust off the glass between sessions. The passband in the 2024 edition matches the other years; the changes concern the housing only. Check the size in the specifications against your eyepiece barrels, star diagonal, or filter wheel before you choose.

What's in the box

• Broadband filter, unmounted 31 mm round
• Plastic storage case
• Test report with measured transmission curve
• Plastic storage case

Compatibility notes

• Fits filter wheels and holders that accept 31 mm unmounted filters.
• Covers sensors up to about 4/3 in size in typical filter wheel spacing.
• Most effective against sodium and mercury lighting.