Aurel Halden UHC nebula filter 36 mm, observatory grade

Specifications

Description

The Halden UHC is an ultra-high contrast nebula filter supplied unmounted at 36 mm diameter, with a glass thickness of 2.0 mm. Its passband is about 24 nm wide and centered near 494 nm, so it transmits the doubly ionized oxygen lines at 496 and 501 nm and the hydrogen-beta line at 486 nm. It blocks most other visible wavelengths, including much of the broadband glow produced by artificial lighting.

Emission nebulae radiate at a small set of specific wavelengths, while the sky background spreads its light across the spectrum. The filter removes most of the background and keeps the nebular emission, which raises contrast between the two. The nebula itself does not become brighter. The surrounding sky becomes darker, so faint outer structure can become visible against it.

Targets that respond well include the Orion Nebula, the Lagoon and Trifid nebulae, the Swan Nebula, and planetary nebulae such as the Ring and the Dumbbell. The effect is most noticeable in telescopes of 100 mm aperture or more, and under moderately dark skies. Galaxies, star clusters, and reflection nebulae emit across a broad range of wavelengths, so on those objects the filter only dims the view. Stars also appear slightly greenish, because the passband lies in the blue-green part of the spectrum.

The 36 mm unmounted glass fits a filter holder or a clip-in mount designed for that diameter, and it is commonly placed in the light path below the eyepiece in a wide-field Dobsonian. Handle the glass by its edges, since the coatings on both faces are easily marked. Store the filter in its sleeve when it is not in use.

About this configuration

This listing covers one configuration of the Aurel Halden UHC nebula filter 36 mm, defined by its grade, coating, case, and edition year. 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 Halden line, which suits imaging with long focal lengths. Hard-sputtered layers keep their passband stable over time, which matters if you compare images taken years apart.

The padded pouch protects the filter from knocks and fits in a jacket pocket at the telescope. The 2023 edition uses the same coating design as later years, with small differences in the cell markings. A filter does not make objects brighter; it darkens the sky background more than the target, which raises contrast.

What's in the box

• Halden UHC filter, 36 mm unmounted
• Protective paper sleeve
• Test report with measured transmission curve
• Padded pouch

Compatibility notes

• Fits 36 mm unmounted filter holders and clip-in mounts designed for that diameter.
• Most effective in telescopes of 100 mm aperture and larger.
• Best results under moderately dark skies.