Aurel Clarion lunar filter 2 in, observatory grade

Specifications

Description

The Clarion lunar filter 2 in is a neutral density filter for observing the Moon. It reduces the light reaching the eye evenly across the visible spectrum, so the bright lunar surface can be viewed at high magnification without glare. Peak transmission is about 15 percent, which lowers the apparent brightness of the Moon to roughly one-seventh of its unfiltered value while leaving the colors of the surface largely unchanged.

Lunar filters are most useful near full Moon, when the illuminated surface is brightest and glare can wash out maria and crater rims. At that phase, the filter lowers the intensity of the light entering the eye, which makes fine detail easier to see and reduces eye fatigue during longer sessions. Near the first and last quarter, when the terminator casts long shadows across the surface, many observers omit the filter, since the contrast along the terminator is already high.

The filter threads into the front of a 2 in eyepiece, the nosepiece of a 2 in diagonal, or a 2 in filter adapter. Keep it on the eyepiece side of the light path and do not use it at the objective end of the telescope. This filter is not designed for solar observation and must never be used to view the Sun. Clean the glass only with a soft brush or a lens cloth, and keep it in its case when not in use.

A neutral density filter does not change the apparent size of the Moon or the sharpness of the image, because the optical path is unaltered. Both faces of the glass carry an anti-reflection coating to limit ghost images and maintain contrast. The filter is intended for lunar use and does not replace a dedicated solar filter, so it should not be combined with other filters in the same light path.

About this configuration

All configurations of the Aurel Clarion lunar filter 2 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 Clarion line, which suits imaging with long focal lengths. Anti-reflection coatings on both faces reduce halos around bright stars in the field.

The filter ships in a clear plastic case that keeps dust off the glass between sessions. The passband in the 2024 edition matches the other years; the changes concern the housing only. The transmission figures in the specifications apply to every configuration of the Clarion line.

What's in the box

• Lunar neutral density filter in 2 in threaded cell
• Padded storage case
• Test report with measured transmission curve
• Plastic storage case

Compatibility notes

• Threads into 2 in eyepieces, diagonals, and filter adapters (M48 x 0.75).
• Intended for visual lunar observation and not for solar use.