Astro-Observer

City Light Suppression (CLS) Telescope Filters

By Astro-Observer · Last reviewed November 8, 2026

City Light Suppression (CLS) filters are specialized accessories designed to improve stargazing and astrophotography in light-polluted areas. CLS is the name of a broadband filter made by Astronomik, and the term is also used loosely for similar filters from other makers. They work by blocking the light of mercury and sodium streetlights while passing the main emission lines of nebulae. This darkens the background sky and improves the contrast of emission nebulae; galaxies and star clusters gain much less, because their light is spread across the whole spectrum.

Key Features:

  1. Wavelength Blocking: CLS filters primarily block wavelengths associated with mercury and sodium-vapor lights, commonly found in urban lighting.
  2. Improved Contrast: By reducing the brightness of the background sky, these filters improve the contrast of faint objects, making details sharper.
  3. Application: Useful for both visual observation and astrophotography, especially for deep-sky objects.
  4. Compatibility: Available in 1.25" and 2" eyepiece sizes, as well as clip-in and threaded versions for DSLR, mirrorless and dedicated astronomy cameras.

Pros:

  • Reduces skyglow from sodium and mercury lighting.
  • Improves visibility of emission nebulae.
  • Easy to attach and use.

Cons:

  • Not suitable for lunar and planetary viewing as they can diminish overall brightness and alter colors.
  • Effectiveness varies depending on the severity and type of light pollution. White LED lighting emits across the whole spectrum and cannot be filtered out, so CLS filters help much less where LEDs have replaced older streetlights.

Usage Tips:

  • Best used in moderately light-polluted environments.
  • For nebulae under bright or LED-lit skies, switch to a narrowband filter (UHC or O-III) instead; stacking a CLS with them adds nothing but extra light loss.

Overall, CLS filters are a useful tool for amateur astronomers observing emission nebulae from suburban or urban areas with moderate light pollution from older sodium and mercury lighting.