Light Pollution Filter

Specifications

Observing parameters

Overview

A broadband light-pollution filter — often called a CLS or sky-glow filter — notches out the yellow-orange sodium lines and the mercury lines that dominate older street lighting, while passing most of the rest of the visible spectrum. Because it passes a wide band, it keeps stars and galaxies reasonably intact, unlike a narrowband filter.

Its usefulness is falling. As cities replace low-pressure sodium with broad-spectrum white LEDs, there is less and less of the spectrum that can be blocked without blocking the object too. Under heavy LED sky-glow these filters deliver little, and a dark site delivers more.

For imaging they retain value: a broadband filter still suppresses sky-glow gradient and lets you stretch deeper before the background swamps the subject.

Where it fits

Practical notes

• Test before you buy if you can — under LED street lighting the gain may be negligible.
• Never use these on planets or the Moon; they dim and colour-shift the image for no benefit.
• A filter cannot replace a dark sky. Driving an hour out is still the cheapest upgrade available.
• For imaging, pair with flat frames; the filter introduces its own vignetting and dust signature.

Pairs with

• Wide-Field Eyepiece
• CMOS Camera
• Filter Wheel
• Planisphere