Brightwell Nightline H-beta filter 1.25 in, observatory grade

Specifications

Description

The Nightline H-beta is a narrowband nebula filter in a 1.25 in threaded cell. It passes a band about 12 nm wide centered on the hydrogen-beta emission line at 486 nm and blocks most other visible light. Peak transmission is about 85 percent at the center of the passband, so the view is noticeably dimmer than an unfiltered view.

Emission nebulae shine most strongly at specific wavelengths, while the sky background and artificial lighting spread light across the visible spectrum. A narrow passband centered on the hydrogen-beta line blocks much of that background and passes the nebula's light, which increases the contrast between the two. The nebula itself does not become brighter. The sky around it becomes darker, so faint hydrogen-emitting regions near brighter structures become easier to see. The effect is greatest under moderately dark skies, where background glow is weaker.

Hydrogen-beta emission is strong in regions such as the Flame Nebula and the Horsehead Nebula in Orion, where you can view the glowing gas near Alnitak. Many emission regions in the Milky Way, including parts of the Cygnus region, also respond well. The filter passes little of the oxygen-III light at 496 and 501 nm, so you should not expect much from the Veil Nebula or from planetary nebulae that rely on oxygen emission. Galaxies, star clusters, and reflection nebulae shine in a broad range of colors, so on those targets the filter mainly dims the view.

The filter screws into the barrel of any 1.25 in eyepiece or the nosepiece of a 1.25 in diagonal. The glass has a multi-layer interference coating on one side and an anti-reflection layer on the other. Handle the filter by its cell, and store it in its case when not in use.

About this configuration

All configurations of the Brightwell Nightline H-beta filter 1.25 in 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 standard coating is applied to the filter side of the glass, with a single anti-reflection layer on the back.

A padded pouch is easy to open with cold fingers, which helps when you swap filters during a session. This is the 2025 edition, which added a laser-etched serial number to the cell or the glass edge. The transmission figures in the specifications apply to every configuration of the Nightline line.

What's in the box

• H-beta filter in 1.25 in threaded cell
• Plastic storage case
• Test report with measured transmission curve
• Padded pouch

Compatibility notes

• Threads into 1.25 in eyepieces, diagonals, and filter wheels (M28.5 x 0.6).
• Most useful for emission regions that glow in hydrogen-beta light, under moderately dark skies.