Brightwell Horizon lunar filter 1.25 in, observatory grade

Specifications

Description

The Horizon lunar filter is a neutral-density filter for the Moon in a 1.25 in threaded cell. It passes about 15 percent of incident light across the visible spectrum, which reduces the brightness of the lunar surface to a level that is more comfortable at high magnification. Because the filter is neutral in color, it does not shift the tones of the Moon's surface, so maria and highlands keep their natural gray and tan appearance.

Lunar observing often produces glare near the bright limb and along the terminator, especially when the Moon is near full and the telescope is at high power. The filter reduces that glare and lets your eye adapt less often between the dim and bright parts of the surface. It is most useful when the Moon is above about 60 percent illumination and the disk is large in the eyepiece. Under those conditions it helps you follow surface detail in craters and rilles near the limb.

The filter screws into the barrel of any 1.25 in eyepiece or the nosepiece of a 1.25 in diagonal. Place it on the eyepiece side of the optical train, and check that it is fully seated before use, because a loose cell can shift and allow stray light past the glass. Store the filter in its case when not in use to protect the coatings from dust and handling marks. Do not use this filter for solar observing, since it does not reduce ultraviolet or infrared light to safe levels.

About this configuration

The Brightwell Horizon lunar filter 1.25 in is offered in several configurations, and the notes below describe what this particular one includes. 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. With anti-reflection layers on both sides, less light bounces between the filter and the eyepiece or camera sensor.

An aluminum case with a foam insert keeps the filter safe during travel to a dark site. 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 Horizon line.

What's in the box

• Lunar filter in 1.25 in threaded cell
• Plastic storage case
• Test report with measured transmission curve
• Aluminum storage case

Compatibility notes

• Threads into 1.25 in eyepieces, diagonals, and filter wheels (M28.5 x 0.6).
• Suitable for any telescope aperture when the Moon is the target.