Brightwell Nightline UHC nebula filter 31 mm, observatory grade

Specifications

Description

The Nightline UHC nebula filter 31 mm is an ultra-high contrast nebula filter supplied as an unmounted 31 mm glass disc, for use in a holder or cell sized for 31 mm glass. It passes a band about 25 nm wide that covers the doubly ionized oxygen lines at 496 and 501 nm and the hydrogen-beta line at 486 nm, and it passes the hydrogen-alpha line in the deep red. Most other visible light is blocked, including much of the glow from artificial lighting.

Emission nebulae shine at those specific wavelengths, while the sky background glows across the spectrum. By blocking the background and passing the nebula's light, a UHC filter raises the contrast between the two. The nebula does not get brighter, but the sky around it gets darker, so you may see faint outer regions that were lost in the glow of the sky. Good targets include the Orion Nebula, the Lagoon and Trifid nebulae, the Swan Nebula, and planetary nebulae such as the Ring and the Dumbbell.

A UHC filter does not help with galaxies, star clusters, or reflection nebulae, which shine in a broad range of colors. On those targets it simply dims the view. Stars appear greenish through it, since the passband sits in the blue-green part of the spectrum. Because the glass is unmounted, handle it by its edges, keep the coated face clean, and store it in its sleeve when not in use. The effect is strongest in telescopes of 100 mm aperture or more and under moderately dark skies.

Set the filter in its holder with the coated side facing the eyepiece, as marked on the glass edge, and confirm that it sits flat without pinching. Compare the view with and without the filter at the same magnification to judge its effect on your target, since the change is most apparent on faint extended nebulae. Filters of this type work well at low to moderate magnification, where the nebula fills a larger part of the field.

About this configuration

All configurations of the Brightwell Nightline UHC nebula 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 padded pouch is easy to open with cold fingers, which helps when you swap filters during a session. This is the 2026 edition, the most recent version of the Nightline line in the catalog. Check the size in the specifications against your eyepiece barrels, star diagonal, or filter wheel before you choose.

What's in the box

• UHC filter, unmounted 31 mm glass
• Protective plastic sleeve
• Test report with measured transmission curve
• Padded pouch

Compatibility notes

• Designed for 31 mm unmounted filter holders and cells; it does not thread.
• Most effective in telescopes of 100 mm aperture and larger.