Aurel Umbral light pollution filter 36 mm unmounted, observatory grade

Specifications

Description

The Umbral light pollution filter is a 36 mm unmounted glass filter for telescopes that accept large-format eyepieces or filter holders. It passes a broad band about 180 nm wide while blocking the narrow emission lines of sodium and mercury street lamps, which produce much of the glow over many towns. In a suburban sky, you can use it to recover contrast while keeping most of the light from stars and extended objects.

Sodium lamps emit mainly near 589 nm, and mercury lamps emit strong lines at 436, 546, and 578 nm. The filter reduces these lines while passing much of the rest of the visible spectrum, so the sky background dims relative to the target. Unlike a narrowband nebula filter, it does not concentrate on a few emission lines, which makes it useful for galaxies, open clusters, and reflection nebulae. Stars keep their white color, because the passband is broad and covers most of the visible range. Expect the view to be dimmer overall. The benefit is strongest under suburban skies, where the glow is intense, and it is smaller at dark sites, where the background is already dim.

Galaxies and reflection nebulae keep much of their natural color through this filter, so the change in appearance is small. Hold the filter by its edges, clean the glass with a soft cloth, and keep it in its sleeve when not in use. The coatings sit on the glass surfaces, so avoid touching them. Before each session, check that the filter sits flat in its holder to avoid uneven illumination at the field edge. Allow the filter to reach the eyepiece temperature before use to reduce condensation on the glass in cold conditions. Store it away from heat and humidity, since both can degrade the coatings over time.

About this configuration

All configurations of the Aurel Umbral light pollution filter 36 mm unmounted 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 Umbral line, which suits imaging with long focal lengths. Anti-reflection coatings on both faces reduce halos around bright stars in the field.

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. Check the size in the specifications against your eyepiece barrels, star diagonal, or filter wheel before you choose.

What's in the box

• 36 mm unmounted light pollution filter
• Soft cleaning cloth
• Protective card sleeve
• Test report with measured transmission curve
• Padded pouch

Compatibility notes

• Designed for 36 mm unmounted installation in a compatible filter holder or Dobsonian eyepiece adapter.
• Most effective under suburban skies.