Brightwell Nightline UHC nebula filter 2 in, observatory grade

Specifications

Description

The Nightline UHC nebula filter 2 in is an ultra-high contrast nebula filter in a 2 in threaded cell. Its passband is about 25 nm wide and covers the doubly ionized oxygen lines at 496 and 501 nm and the hydrogen-beta line at 486 nm, while the hydrogen-alpha line in the deep red also passes. Most other visible wavelengths are blocked, including much of the glow from artificial lighting. The glass transmits about 90 percent of light within the passband.

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 itself does not get brighter, but the sky around it darkens, so faint outer regions can become visible. Comparing the view with and without the filter at the eyepiece shows how much of the surrounding nebulosity was hidden by sky glow.

Good targets include the Orion Nebula, the Lagoon and Trifid nebulae, the Swan Nebula, the Veil Nebula, and the North America Nebula in a wide-field telescope. Planetary nebulae such as the Ring and the Dumbbell also respond well. The benefit is clearest in telescopes of 100 mm aperture or more under moderately dark skies, where the wide field of a 2 in eyepiece takes in the larger emission regions at once.

A UHC filter does not help with galaxies, star clusters, or reflection nebulae, which shine across a broad range of colors. On those targets it simply dims the view. Because the passband sits in the blue-green part of the spectrum, stars appear greenish through the filter. The filter threads into the 2 in filter cell of a diagonal or the nosepiece of a 2 in eyepiece, and the glass carries an interference coating on one side and an anti-reflection layer on the other.

About this configuration

All configurations of the Brightwell Nightline UHC nebula filter 2 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.

An aluminum case with a foam insert keeps the filter safe during travel to a dark site. This is the 2024 edition, which revised the cell knurling and the case insert of the Nightline line. The transmission figures in the specifications apply to every configuration of the Nightline line.

What's in the box

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

Compatibility notes

• Threads into 2 in eyepieces, diagonals, and filter wheels (M48 x 0.75).
• Most effective in telescopes of 100 mm aperture and larger.