Aurel Ashgrove lunar filter 1.25 in, observatory grade

Specifications

Description

The Ashgrove lunar filter is a neutral-density filter in a 1.25 in threaded cell, designed to reduce the brightness of the Moon at the eyepiece. It transmits about 18 percent of incident light across the visible spectrum, which lowers the glare of the lunar surface without shifting its color. Because the attenuation is broadly even across wavelengths, the filter keeps the natural tones of maria and highlands, so you can judge albedo differences and crater floors as they appear.

Full moon and near-full phases put a large amount of light into a telescope, and at high magnification the glare can wash out fine detail along the terminator and on bright ejecta rays. A neutral-density filter reduces the total light entering your eye, which helps to preserve contrast and to ease eye strain during longer sessions. Many observers find it most useful in telescopes of 100 mm aperture and larger, where the lunar image is brightest at high magnification.

The filter threads onto the barrel of any 1.25 in eyepiece or the nosepiece of a 1.25 in diagonal. Handle the glass by its cell, avoid touching the optical faces, and store it in its case when not in use. Because the filter reduces light, the view may appear darker at the eyepiece, so focus at a lower power first and then add the filter if the image is too bright.

Lunar filters do not improve planetary or deep-sky observing, since they remove a significant share of light from those targets. Inserting it into the optical path can shift focus slightly, so refocus after installing it. Sharp focus is easiest to achieve by focusing on the bright lunar limb with the filter already in place, then making fine adjustments as the Moon moves through the field.

About this configuration

All configurations of the Aurel Ashgrove lunar filter 1.25 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 Ashgrove line, which suits imaging with long focal lengths. Anti-reflection coatings on both faces reduce halos around bright stars in the field.

An aluminum case with a foam insert keeps the filter safe during travel to a dark site. The 2023 edition uses the same coating design as later years, with small differences in the cell markings. Check the size in the specifications against your eyepiece barrels, star diagonal, or filter wheel before you choose.

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).
• Most useful in telescopes of 100 mm aperture and larger at higher magnifications.