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A Guide to Choosing the Right Telescope Filters

A Guide to Choosing the Right Telescope Filters

AstroTelescopium Team |

As you begin to expand your visual and imaging astronomical observations, you will find that certain telescope filters can enhance your views. In this guide, we'll discuss different telescope filter types along with their applications in order to help you decide which filter is right for you.


Why are telescope filters important for astronomy observations?

Telescope filters play a crucial role in enhancing the quality of astronomy observations. They help astronomers to isolate specific wavelengths of light, allowing for a clearer and more detailed view of celestial objects.

By selectively filtering out unwanted light, astronomers can improve contrast, reduce glare, and reveal hidden details that would otherwise be difficult to observe.


Electromagnetic Spectrum and Visible Light

Before discussing the various filter types, it is important to understand the electromagnetic spectrum and visible light. The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies.

This spectrum range is classified by wavelength:

  • Radio waves
  • Microwaves
  • Infrared
  • Visible
  • Ultraviolet
  • X-rays
  • Gamma rays

Light Spectrum

Visible light is comprised of a narrow section of the electromagnetic spectrum that can be perceived by the human eye. It is generally defined as wavelengths in the range of 400–700 nanometers (nm), between the longer infrared wavelengths and the shorter ultraviolet wavelengths.

For reference, a nanometer is a unit of length equal to one billionth of a meter (expressed as 1×10−9 m).

Now that you have an overall concept of the light spectrum, let's get back to telescope filters.


What are the different types of telescope filters?

There are several types of telescope filters available, each designed to filter specific wavelengths of light to your eye or camera. Most commonly, the different filter types fall into one of these four categories:

  • Broadband Filters: These filters transmit a wide range of wavelengths and are useful for general observations.
  • Narrowband Filters: These filters isolate specific emission lines, such as hydrogen-alpha or oxygen-III, allowing astronomers to observe specific objects or phenomena.
  • Color Filters: These filters enhance the visibility of certain features on planets, such as cloud bands or polar ice caps.
  • Polarizing Filters: These filters reduce glare from the Moon or bright planets, improving contrast and visibility.


How to choose the right telescope filter?

When selecting a telescope filter, it is important to consider the specific observation goals and the characteristics of the target object. Here are some factors to keep in mind:

  • Object Type: Different objects require different filters. For example, planetary observations may benefit from color filters, while nebulae observations may require narrowband filters.
  • Light Pollution: If observing from a light-polluted area, consider using a light pollution filter to minimize the impact of artificial light.
  • Telescope Aperture: Larger telescopes may require larger filters to avoid vignetting or blocking the light path.
  • Budget: Telescope filters come in various price ranges. Consider your budget and prioritize filters that align with your observation goals.


Popular Telescope Filters

CLS Filters

CLS Filter

CLS or City Light Suppression filters reduce the appearance of light pollution and are excellent contrast boosters providing a darker sky background. CLS filters are designed for observing the broad range of deep sky objects including nebulae and galaxies.

These filters block the light produced by Mercury and sodium-vapor lights, while allowing most of the visual spectrum and H-alpha lines to pass through. They are perfect for telescopes of all apertures and optimized for telescopes with focal ratios between f/3 ad f/15.

Range from 436nm to 536nm.

CLS filters are a must have for suburban or city backyard astronomers.

Neutral Density Filters

Neutral Density Filter

Neutral density filters are designed to provide an even reduction of light intensity across the entire spectrum without affecting color fidelity of the object you are observing.

They are particularly well-suited for observing or photographing the Moon with any telescope 4" or larger, and can also be used for splitting close double stars where one of the binary pair far exceeds the other in brightness.

H-Beta Filters

H-Beta Filter

H-Beta filters only allow the emission line of hydrogen beta through and block all wavelengths.

Hydrogen is the most abundant element in the universe and many nebulae glow in the light of H-beta. The extreme boost in contrast can make the difference in being able to see an object in visual astronomy and can enhance fine detail in astrophotography.

Range from 478nm to 496nm.

Best used with 8" aperture telescopes or larger.

O-III Filters

O-III Filter

O-III or Oxygen III filters are best for visual observations of planetary and gaseous nebulae. They only allow the two emission lines of oxygen through and block all other colors.

Oxygen III filters have a narrow-band design for difficult city conditions to absorb almost all of the artificial light and provide more contrast and details for urban observers and astrophotographers.

In a slightly brightened sky, for example, the Cirrus Nebula NGC 6992 is practically invisible with a 200mm telescope. However, when using this filter, you can see the fog and its structures without any problems.

Best used with 8" aperture telescopes or larger.

S-II Filters

SII Filter

S-II filters only let through the emission lines of the ionized sulfur at 672nm and blocks all other colors for observing faint planetary nebulae, emission nebulae and supernova remnants.

This filter absorbs almost all of the artificial light and all other emission lines to deliver more contrast and details for urban observers and astrophotographers.

UHC Filters

UHC Filter

UHC or Ultra High Contrast filters increase viewing contrast by blocking a majority of the wavelengths of light pollution including orange/yellow wavelengths of light in order to gain increased detail in nebulae.

Range from 458nm to 508nm. The bandpass is tighter than the CLS Filter, thereby blocking even more light from artificial lights and airglow.

This filter passes the H-Alpha and H-Beta emission lines at 486nm and 656nm, as well as oxygen emission lines at 496nm and 501nm, in order to brighten nebulae while darkening the sky background for increased contrast and subject definition.

Variable Polarizing Filters

Variable Polarizing Filter

Variable polarizing filters serve as adjustable dimmers by allowing continuous adjustment of image brightness to a comfortable level. These filters are very helpful for strong contrast differences in various observation objects. They are especially suited for bright lunar and planetary observation.

Control range of 40 to 1% light transmission.

H-Alpha Filters

H-Alpha Filter

These H-Alpha filters are designed to enhance details found in nebulae that are rich in ionized Hydrogen. They are not meant for solar use (below we will cover solar filters) and are only intended for astrophotography. Many of the red glowing emission nebulae are only visible to their full extent on photos with these filters.

The H-alpha nebula filters make astrophotography near cities possible. Their use facilitates surprisingly good image results despite the existing light pollution. They block almost all light from mercury and sodium vapor lamps. A must for every urban astrophotographer.


Solar Telescope Filters

If you want to take a better look at the Sun's features without having to purchase a specialized solar telescope, simply find the proper solar filter that fits the telescope you already have.

Please note: It is extremely important to not look directly at the sun through your telescope without a quality solar filter.

For optimized safety and viewing, make sure to position the solar filter snugly over the front end of the telescope's optical tube to block the sun’s intense light.

Solar Filter

The Sun Catcher Variable Large Aperture Solar Filter fits on most telescopes from 70mm up to 12-inch SCTs. It is highly tear and puncture-resistant and produces a consistent yellow-orange solar image.

This particular solar filter uses advanced ‘Solarlite’ film manufactured by world-famous Thousand Oaks Optical through years of research and development. Solarlite has the optical quality of glass with the advantages of impregnated polymer making this perfect for the solar astrophotographer.



Telescope filters are essential tools for astronomers looking to enhance their observations. By carefully selecting the right filters based on the target object, observation goals, and environmental conditions, astronomers can unlock a wealth of details and improve the overall quality of their astronomy experiences.



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