Pellucid Obelisk light pollution filter 2 in, observatory grade

Specifications

Description

The Obelisk light pollution filter is a broadband filter in a 2 in threaded cell. It is designed to block the sodium-vapor emission line near 589 nm and the mercury emission lines near 546 nm and 436 nm, which account for much of the glow over towns and cities. The passband is about 150 nm wide and covers most of the visible spectrum, so you will see stars and galaxies in close to natural color.

Skyglow is concentrated in the wavelengths where artificial lights put out most of their energy. By removing those lines, the filter lowers the background brightness and raises the contrast of extended objects against the sky. Because the passband is wide, the filter passes more of the light from broadband targets than a narrowband filter would, so the image stays brighter and less colored. The gain in contrast is moderate, and it depends on how much of the local lighting uses those lines.

Good targets include the Andromeda Galaxy, the Pleiades and other open clusters, the Orion Nebula, and the Lagoon Nebula viewed from suburban skies. Galaxies and clusters benefit most, since their light is spread across the spectrum and the background is what limits their visibility. The filter does little for targets under truly dark skies, where the background is already low. It also dims the view slightly, so use it in telescopes where light gathering is not limiting.

The filter screws into the front of any 2 in eyepiece or the nosepiece of a 2 in diagonal. Handle the glass by the cell and keep the coated surfaces clean with a blower and a soft brush. Store it in its case when not in use to prevent scratches.

About this configuration

This listing covers one configuration of the Pellucid Obelisk light pollution filter 2 in, defined by its grade, coating, case, and edition year. 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 Obelisk line, which suits imaging with long focal lengths. The hard-sputtered coating resists scratches and humidity, so the filter tolerates cleaning better over the years.

An aluminum case with a foam insert keeps the filter safe during travel to a dark site. 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 Obelisk line.

What's in the box

• Light pollution filter in 2 in threaded cell
• Plastic storage case
• Test report with measured transmission curve
• Aluminum storage case

Compatibility notes

• Threads into 2 in eyepieces and diagonals (M48 x 0.75).
• Suited to telescopes of any aperture under suburban skies.