Telescope Filters Explained. The Basics to Know

Should you buy telescope filters and if so which ones? Whether you’re just starting or want to advance your backyard telescope experience, filters can make that subtle difference to better see the details of the Moon and planets.

Plossi Eyepiece Set + Barlows With Case
at Amazon (affiliate link)

What does a telescope filter do? Telescope filters add subtle effects by filtering out the transmission of certain light waves to add contrast or boost the appearance of the detail of the Moon and the planets for the observer.

It’s best to start with some basic ones and then add to your observing tool box as you develop your skills in the use of the telescope to see details of the planets or other objects.

Mostly telescope eyepieces screw into the base of eyepieces.

Colored telescope filters – What do the Colors Do?

Eyepiece filters come in a range of colors. The labeling is from the Wratten labeling system for optical filters.

#82A Light Blue — 73% transmission

• All purpose filter

#80A Blue — 29% transmission

What does a blue telescope filter do? A blue telescope filter will improve the detail seen of the Moon and Planets. It adds extra high contrast for Jupiter’s Red Spot and features of the Moon. Best for telescopes with apertures of 6″ or more.

• Mars atmosphere
• Saturn’s belts and polar zones

ND96 Neutral Density – Gray

This is the basic one used in the assortment of telescope filters for observing features on the Moon.

• Moon observation
• Splitting double stars
• Phases of Venus
• Reduces glare – Blocks all visible light evenly

#58 Green — 24% Transmission

• Jupiter – adds contrast

#56 Light Green — 53% Transmission

• Mar’s polar regions

#47 Violet — 3% Transmission

This one is best for telescopes with 10″ apertures or higher.

• Viewing Venus

#38A Dark Blue — 17% Transmission

For 8″ apertures or higher.

• Ideal for contrast – Saturn, Jupiter, Venus.

#25A Red — 14% Transmission

Best for 8″ apertures or higher and one of those best telescope filters for Mars.

• Best for contrast for features of Mars
• Longpass filter – blocks wavelengths short of 580 nm

#23A Light Red — 25% Transmission

Best for 8″ apertures or higher.

• Vivid image of Mar’s Maria
• Longpass filter – blocks visible wavelengths < 550 nm

#21 Orange — 46% Transmission

Best for 6″ aperture or higher.

• Great Red Spot of Jupiter
• Longpass filter – blocks visible wavelengths < 530 nm

#12 Yellow — 74% Transmission

Good for 3″ aperture or higher

• Clouds of Mars
• Longpass filter – blocks visible wavelengths < 500 nm

#11 Yellow-Green — 40% Transmission

• Details of Uranus and Neptune
• Not a longpass filter

#8 Light Yellow — 83% Transmission

• Emphasizes the red and orange tones of Jupiter
• Longpass filter – blocks visible wavelengths < 465 nm

Any cheap red screw-in eyepiece filter

• Will work to counter the blueness of the sky for viewing celestial objects in the daytime

CLS telescope filters

CLS means city light suppression filters. These are filters for reducing the appearance of light pollution. They increase contrast by blocking interfering glow from street lights, for example, to aid viewing. With these, the skies appear blacker. They are useful for urban viewing or anywhere there is light interference.

Do telescope light pollution filters work? A good quality LPF or CLS will work to make the sky appear darker, adding contrast to help you view your target sky object.

Telescope filters for nebula

A nebula filter offers better contrast in viewing of nebulas. They work best on short focal ratio telescopes and offer minimal benefits for anything other than nebulas.

Telescope Filters for Moon

These filters reduce the glow from the Moon so you can see better detail. Neutral gray is the telescope eyepiece moon filter that’s used. If you were to buy one filter type to start with, consider getting a Moon filter. It will help you depict numerous lunar details, including the Apollo spaceship landings, craters and mares of the Moon.

Telescope filters for viewing the Sun

This is a special type of filter. It is not a colored type but rather it looks reflective. See my article on viewing the Sun for sunspots and during eclipse for more information and recommended filters. It’s important to take special care when attempting to view the Sun to avoid permanent eye damage. Never look directly at the Sun.

Telescope filter kits

You can get telescope filter kits, which might be a good option to get acquainted with filters as a beginner.