Brightwell Nightline Skyglow filter 31 mm, observatory grade

Specifications

Description

The Nightline Skyglow filter 31 mm is a light pollution filter in unmounted 31 mm round glass. It passes a broad band about 180 nm wide while blocking the narrow sodium and mercury emission lines that dominate urban skies, along with part of the continuous glow from poorly shielded fixtures. Peak transmission is about 92 percent, so the filter costs little light across the visible passband.

Streetlights, signs, and sodium lamps brighten the sky background across much of the visible spectrum, which lowers contrast for extended objects. The filter reduces that background by absorbing the strongest artificial lines and a share of the broadband glow. Stars keep their colors more closely than they would through a narrowband filter, and the change is a moderate gain in contrast rather than a large jump in brightness.

Good targets under suburban skies include the Andromeda Galaxy, the Pinwheel Galaxy, open clusters such as the Pleiades, and diffuse nebulae, where the background most limits faint detail. Planets and bright double stars gain little, since their contrast is already high, and the filter dims them slightly. Under dark rural skies, a broadband filter offers less benefit, and you may notice a small loss of brightness on faint stars.

The glass is unmounted and must be held in a cell or holder sized for 31 mm optics. Handle it by the edges and keep fingers off the coated faces. Store it in the supplied case, away from humidity and dust, and clean the surfaces only with a soft brush or a lens-safe air blower. Check the glass for dust before each session, since residue on the coated surface scatters light and lowers contrast.

About this configuration

All configurations of the Brightwell Nightline Skyglow filter 31 mm share the same filter glass design; the options change the grade, the coating, and the case. 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 Nightline line, which suits imaging with long focal lengths. The hard-sputtered coating resists scratches and humidity, so the filter tolerates cleaning better over the years.

A plastic case is light and stacks with other filter cases in an accessory box. The passband in the 2024 edition matches the other years; the changes concern the housing only. The transmission figures in the specifications apply to every configuration of the Nightline line.

What's in the box

• Light pollution filter, unmounted 31 mm glass
• Plastic storage case
• Test report with measured transmission curve
• Plastic storage case

Compatibility notes

• Designed for 31 mm unmounted filter holders and cells that accept 31 mm glass.
• Not threaded, so it does not fit 1.25 in or 2 in eyepieces without an adapter cell.
• Most effective under suburban and urban skies.