Brightwell Horizon lunar filter 36 mm, observatory grade

Specifications

Description

The Horizon lunar filter is a neutral density filter for observing the Moon. It is supplied as a 36 mm unmounted glass disc, so it is intended for use in a 36 mm filter cell, dust cover, or holder that accepts that diameter. Its peak transmission is about 15 percent, which reduces the brightness of the lunar surface while keeping surface detail visible in the eyepiece.

The Moon is much brighter than most deep-sky objects, and near full phase its glare can wash out fine detail along the terminator and across the crater floors. A neutral density filter lowers the intensity of all wavelengths by roughly the same amount, so the color of the image stays close to neutral. Reduced light also makes the eye more comfortable during longer sessions and limits scattered glare across the field of view.

Under thin crescent or first-quarter phases, the filter is often unnecessary, because the terminator shows good contrast without it. Place the filter in the eyepiece barrel or the diagonal, and check that it sits firmly before you point the telescope at the Moon. Keep the glass clean, handle it by its edges, and store it in its pouch when not in use.

This filter is not a solar filter. Do not use any telescope, finder, or eyepiece to look at the Sun without a dedicated solar filter designed for that purpose, because the lunar filter does not protect the eye from solar radiation. Small telescopes benefit less from the filter, since their lunar views are already modest in brightness.

About this configuration

This listing covers one configuration of the Brightwell Horizon lunar 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 Horizon line, which suits imaging with long focal lengths. A standard coating suits visual use, where small reflections between filter and eyepiece rarely show.

The filter ships in a clear plastic case that keeps dust off the glass between sessions. The 2023 edition uses the same coating design as later years, with small differences in the cell markings. The transmission figures in the specifications apply to every configuration of the Horizon line.

What's in the box

• Lunar filter, 36 mm unmounted
• Soft storage pouch
• Test report with measured transmission curve
• Plastic storage case

Compatibility notes

• Fits 36 mm filter cells and holders that accept unmounted 36 mm glass.
• Reduces lunar brightness in telescopes of any aperture.
• Not intended for solar observation.