Brightwell Horizon lunar filter 36 mm

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

All configurations of the Brightwell Horizon lunar filter 36 mm share the same filter glass design; the options change the grade, the coating, and the case. The standard grade uses the same coating design as the higher grades, with wider tolerances on flatness. Standard grade glass suits visual observing at low and medium magnification. With anti-reflection layers on both sides, less light bounces between the filter and the eyepiece or camera sensor.

A padded pouch is easy to open with cold fingers, which helps when you swap filters during a session. 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
• Padded pouch

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.