Pellucid Glasswing lunar filter 31 mm, observatory grade

Specifications

Description

The Glasswing lunar filter is a neutral density filter for observing the Moon and bright planets. It is supplied as an unmounted 31 mm disc of neutral density optical glass, 2.5 mm thick, with a peak transmission of about 12 percent. A neutral density filter reduces light across the visible spectrum with little change in color, so surface detail keeps its natural hue. Near full Moon the surface brightness can overload the eye and wash out contrast, and the filter lowers that brightness so that craters, maria, and rilles remain distinct.

The glass has no cell, so it must be held in a 31 mm mounting ring, cell, or filter holder designed for unmounted filters. Place it in the optical train between the eyepiece and the telescope, following the instructions for your holder, and confirm that the glass sits flat and centered so that the field edge remains even. Clean the surface with a soft brush or a blower before each session. Dust on the glass appears as soft spots at high magnification. Handle the filter by its edges and avoid touching the faces.

Use the filter when the Moon is high in the sky or near full, and when its brightness makes the image uncomfortable to view. Near the terminator, where the low sun angle already produces shadows and relief, many observers remove the filter because the extra contrast comes from the shadows rather than from brightness. The filter also reduces the glare of Venus and Jupiter at high magnification. It does not add detail. It only reduces the light reaching the eye, so the observed brightness of the Moon drops in proportion to the transmission. Store the filter in a dry case when it is not in use.

About this configuration

All configurations of the Pellucid Glasswing lunar filter 31 mm 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 Glasswing line, which suits imaging with long focal lengths. Hard-sputtered layers keep their passband stable over time, which matters if you compare images taken years apart.

The padded pouch protects the filter from knocks and fits in a jacket pocket at the telescope. 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 Glasswing line.

What's in the box

• Unmounted 31 mm lunar filter glass
• Protective paper sleeve
• Storage case
• Test report with measured transmission curve
• Padded pouch

Compatibility notes

• Unmounted 31 mm disc that requires a 31 mm mounting ring, cell, or holder.
• It is not threaded and does not fit 1.25 in or 2 in threads without an adapter holder.