How color filters work
A color filter passes light of its own color and blocks much of the rest. Looking through a red filter, for example, red features look brighter relative to blue-green ones, which can make the boundaries between them easier to see. The filter does not add detail. It changes contrast, and it makes the image dimmer, which matters more in small telescopes.
Colored filters are usually labeled with Wratten numbers (for example #21 or #80A). Different brands sometimes use slightly different glass, so treat the descriptions below as general guidance.
Color filter chart
| Color | Common Wratten number | Typically used for | What observers often report |
|---|---|---|---|
| Light yellow | #8 | Jupiter, Saturn | Mild contrast boost, little dimming |
| Yellow | #12 | Jupiter, Saturn, Moon | Can help with cloud belts, sometimes used on the Moon |
| Orange | #21 | Mars, Jupiter | Increases contrast of dark Martian surface markings |
| Red | #23A, #25 | Mars, Jupiter | Darkens blue features, stronger and dimmer than orange |
| Green | #56, #58 | Mars, Jupiter, Saturn | Often used for polar caps and some cloud features |
| Light blue | #80A, #82A | Jupiter, Saturn, Mars | Often used for belt detail and clouds, sometimes the Great Red Spot |
| Violet | #47 | Venus, Mars | Sometimes used to try to see Venus cloud markings; effects are subtle |
These pairings come from long-standing amateur practice and are not guarantees. Many observers find the effects small, and preferences vary.
Filters by planet
- Mars: Orange or red filters are often used for dark surface markings, while blue or green may help with clouds and polar caps. Mars is only favorable for a short window every couple of years.
- Jupiter: Blue, light blue and green filters are often used to pick out belts and the Great Red Spot, and yellow can add a bit of contrast.
- Saturn: Light yellow or similar filters are sometimes used to help with banding.
- Venus: Venus looks nearly featureless in most telescopes. Violet and blue filters are sometimes tried, but results are modest. A neutral density or polarizing filter can also tame its glare.
- Moon: Filters are optional. Some observers like yellow or neutral density, and our moon filter article explains how those compare.
Which color filter to buy first
If you want to try one, cautious choices are a light yellow or a medium blue for Jupiter and Saturn, and an orange or red for Mars. Many beginners benefit more from steady skies, patient observing and good collimation than from filters. A telescope with a larger aperture also makes the filters more useful, since dark filters absorb much of the light. If you do buy, it is often more sensible to buy one or two filters rather than a full set.
Limits of color filters
- The effects are subtle and personal, and vary with sky steadiness.
- They dim the image, which is harder in small apertures.
- Cheap or poorly made filters can reduce image quality.
- They are for the Moon and planets. They do not help with faint nebulae.
Color filters versus other filters
Color filters are different from light pollution and nebula filters. For deep-sky work, UHC and OIII filters are the standard options, and light pollution filters help only certain object types. See the telescope filters guide for the full comparison.
FAQ
What is the best color filter? There is no single best. It depends on the planet. A light blue or green is a popular choice for Jupiter, and orange for Mars.
What is the best light pollution filter for a telescope? It depends on your lighting and target. UHC filters are a general choice for emission nebulae, and OIII suits planetary nebulae. Neither helps much with galaxies.
What is the color of a telescope? The color of the tube is cosmetic and does not affect what you see. People may be asking about filters, whose color determines which wavelengths pass.
What type of telescope lens is best for viewing planets? Good optics, enough aperture and a stable mount matter most. Quality eyepieces in suitable focal lengths help, and filters are an optional extra.