Aurel Umbral light pollution filter 36 mm unmounted, premium 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

This listing covers one configuration of the Aurel Umbral light pollution filter 36 mm unmounted, defined by its grade, coating, case, and edition year. The premium grade uses polished glass with tighter flatness tolerances, which helps at high magnification. Premium grade filters are checked for the position of the passband before they leave the factory. With anti-reflection layers on both sides, less light bounces between the filter and the eyepiece or camera sensor.

The filter ships in a clear plastic case that keeps dust off the glass between sessions. This is the 2024 edition, which revised the cell knurling and the case insert of the Umbral line. 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
• Plastic storage case

Compatibility notes

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