Best Telescope Filters – Beginner’s Guide

In simple terms, the filters we use for visual astronomy reduce the amount of light that comes through the eyepiece. Filters never increase the amount of light. However, by selectively allowing or blocking certain wavelengths of the light spectrum, the filter may help to bring out another part of the spectrum, which will show us details we might not have otherwise noticed.

Filters cannot magically reveal what’s not present in the first place, but they can accentuate details by controlling light. They operate by transmitting particular wavelengths, changing light intensity, or adjusting its polarisation. For instance, a narrow band pass filter can boost contrast on an object by rejecting unwanted light, yet, if too narrow in band pass, might dim the view or attenuate some detail.

There are also neutral-density and polarising filters explicitly used for dimming. While the former reduces light by being semi-opaque, the latter allows light from specific angles. A variable polarising filter, which combines two polarising filters, can adjust dimness by rotating them.

This is the opposite of the way we normally think when it comes to telescopes. We are normally trying to capture as much light as possible so we can see the most detail or the dimmest objects. This is done in a variety of ways to suit the specific use case. Telescope filters are typically full-aperture, covering the front of the telescope, or are eyepiece-attached filters that screw onto the eyepiece. However, the vast array of filters and their varied applications can be daunting for beginners.

Since filters reduce the light that reaches our eyes, the more aperture your telescope has, the more light it gathers, and the more useful filters can be. If your aperture is small, a filter may remove so much light that you see no benefit. What are large and small apertures in this context? There are no clear guidelines, but I would say any scope with 80 mm of aperture or more will let you experiment with filters. If you have more than 150 mm of aperture, you should definitely experiment with filters.

Newcomers to astronomy often hear about the transformative power of filters for visual observation. However, filters are not miracle workers; they necessitate careful selection and usage for optimal results.

Be wary of low-priced filters and filter kits. These are usually low-quality offerings that may compromise on optical quality or bandpass, leading to blurred images or underperforming nebula filters. Pricier filters are usually tested by the manufacturer to conform to optical quality and transmission standards.

Solar filters are outside the scope of this article as they are a different sort of device than the screw-on filters discussed here.

Top Filter Picks

Our top 5 must-have filters

• Best UHC Filter: Astronomik UHC
• Best Oxygen III Filter: Baader Visual OIII Filter
• Best Broadband/Sky Glow Filter: Baader Neodymium Moon & Skyglow
• Best Jupiter Filter: Astro Essentials #80A
• Best Moon/Polarising Filter: Baader Double Polarising Filter

Other picks

• Best Venus Filter – Astro Essentials #47
• Best Mars Filter – Astro Essentials #23A
• Best Mercury Filter – Astro Essentials #21
• Best Uranus Filter – Astro Essentials #11
• Best Saturn Filter – Astro Essentials #12
• Best Daytime Moon Filter – Astro Essentials #21
• Best Minus-Violet Filter – Baader Contrast Booster
• Best H-Beta Filter – Astronomik H-Beta Visual Filter

Do I need a 2” or 1.25” filter for my telescope?

Telescope filters, barring solar filters, are usually designed to attach to your eyepiece via universal 2” or 1.25” threads at the bottom, ensuring easy interchangeability. When using accessories like diagonals, extension tubes, or coma correctors for focal adjustments, attaching the filter to these components rather than the eyepiece can help mitigate potential glare or refocusing concerns.

While investing in 2” filters might seem costly initially, it’s a prudent choice in the long run, eliminating the need to repurchase filters when transitioning to 2” eyepieces. However, in situations where your telescope exclusively accommodates 1.25” eyepieces or the desired filter isn’t available in the 2” variant, a 1.25” filter is appropriate.

For telescopes equipped with a focuser or diagonal compatible with 2” eyepieces, it’s advisable to opt for a 2” nebula or polarising filter by default, regardless of whether you currently utilise any 2” eyepieces. Future telescope upgrades might lead you to use 2” eyepieces, so sticking with the 2” format ensures your filter remains relevant and will save you money from having to upgrade to a new 2” filter later on.

If your 2” to 1.25” adapter lacks filter threads, consider upgrading to a higher-quality unit with filter threads and a brass compression ring. 2” filters can conveniently attach to a threaded 2” to 1.25” adapter, making them compatible with 1.25” eyepieces. Additionally, for those with 1.25”/2” hybrid eyepieces, 2” filters can be connected to your 1.25” adapter to use 2” filters with these oculars.

For planetary observation, typically conducted at higher magnifications using 1.25” eyepieces, a 2” colour filter might not always be cost-effective or available. It’s crucial to exercise discretion in these cases.

Recommended Filter Manufacturers

• Astronomik: Renowned for its high-quality filters, Astronomik is also the original equipment manufacturer for TeleVue filters.
• Baader Planetarium: Typically stands out for its superior filter production, especially in the color/polarizing category.
• Celestron: Although some cheaper colour/Moon filters may be questionable, their nebula filters are essentially rebranded Baader products.
• DGM – Stellar in terms of quality but hard to find.
• Optolong – Imitation Astronomik filters, these are basically the same but with a few % lower total transmission and occasional quality control defects.
• Orion – Orion’s color/lunar filters are just generic, low-quality stuff, but their exclusive nebula filters boast impressive performance for the price.
• SVBONY – A reseller of generic items from China. While many of their filters aren’t top-notch, a few selections are decent, albeit sometimes not for their as-advertised intended use.
• TeleVue – Rebranded Astronomik products.
• Antares – Generic filters but usually decent quality.
• Astro Essentials – Generic filters sold by First Light Optics, usually decent.
• Astromania – Cheap but acceptable filters.
• Rother Valley Optics

Reflecting Objects vs. Radiating Objects

Within the solar system, other than the Sun itself, the objects we observe, including the Moon, planets, dwarf planets, comets, asteroids, and planetary moons, are all reflecting objects. They are seen based on reflected light that comes from the sun.

The moon appears in shades of white, black, and grey based on the reflections from various materials on the surface. Mars is reddish because its surface absorbs certain wavelengths and more readily reflects those that are more towards the red end of the spectrum. Venus appears mostly bright white because it reflects all of the sun’s wavelengths off the clouds.

When we look at Jupiter and Saturn, we can see cloud bands because of the variability in reflectivity of the gases in the atmosphere. Certain cloud bands tend to absorb certain colours and reflect other colours more readily, so that we may perceive some colours in the clouds or see this as shades of grey. We can take advantage of this by using filters to subtract some colours so that others will be more prominent.

Radiating deep-sky objects, such as stars, nebulae, galaxies, star clusters, globular clusters, and others, are sources of light themselves. The light they transmit may be in a similar range to that of our sun, or they may radiate in narrower bands of the spectrum. We can take advantage of this by using filters to block or remove some light to bring out certain details at other light frequencies. Keep all of this in mind, as it will become important to understand filters, what they do, and what they don’t do.

Lunar and Planetary Filters

Colour Filters

Colour filters are simple filters that transmit only a narrow range of colours and block the rest. They can be useful for enhancing the contrast and detail of some planetary and lunar features, such as cloud bands, craters, and maria. They do this by emphasising some colours and suppressing others, depending on the colour of the filter and the target.

Colour filters are primarily effective when observing planets. Given that planets are significantly brighter than deep-sky objects or stars, leveraging human colour perception enhances contrast. Since we see planets based on reflected sunlight, we can consider them illuminated by white light. That means that they have all the colours of the spectrum hitting them. What we see is what they reflect. By filtering out some colours, we make other colours more noticeable. This can help us notice details on the planets. For example, if a red feature is very subtle, by filtering out blue, the red feature may be easier to see.

Monochrome color filters often bear Wratten numbers, a system that was developed by British inventor Frederick Wratten, tracing back to the era of gelatin-based camera filters. This is a numbering system that comes from the photography world. There are a wide variety of coloured filters available. Here are a few examples and suggestions as to where they might be used.

• #12 yellow enhances red and orange features. It is often used on Mars, Jupiter, and Saturn.
• #21 orange reduces blue and green, which sometimes works well on Mars to improve the contrast between light and dark areas.
• #25 red blocks blue and green, which can bring up cloud details on Jupiter.
• #56 light green, can highlight Mars’ ice caps and areas in Jupiter’s cloud belts.
• #58A green blocks of red and blue, which can be useful on Jupiter and Saturn and may bring up some details in the clouds of Venus.
• #80A blue often gets marketed as a Jupiter filter as it helps with Jupiter’s cloud bands and can bring up details in the Great Red Spot.

There are many other colours, and the uses I listed are only examples of how they might be used. Some block very little light, such as the 80A, and some block a lot of light, so they are more suited to larger aperture telescopes, say 8” or larger.

Colour filters are also frequently sold in sets of four to six filters, each with a different colour and number; don’t buy these, as they tend to be low quality and some of the colours may not end up being of any use to you. If selecting only a couple colour planetary filters, we’d champion a top-tier minus-green filter alongside a blue filter. For those with a penchant for daytime lunar viewing, an orange filter is a worthy addition.

Don’t expect explosive results. The benefits are subtle. You might use a colour filter and suddenly notice faint loops in the belts of Jupiter called festoons. Or you might look at Saturn and see two cloud belts with no filter, but notice a third when you add a filter. Give it a try.

Just be sure to leave room in your budget for quality eyepieces and a good UHC filter, which are far more useful devices to have than any colour or polarising filter.

• Minus-Green Filters

These filters effectively eliminate green light, amplifying the vibrancy of Mars and Jupiter, which predominantly exhibit blue, brown, and orange-red hues. Their influence is limited on other celestial objects.

• Minus-Violet Filters

These filters are meant to fix the purple chromatic aberration that comes with refracting telescopes, but they can also bring out certain features of planets.

• Blue & Violet Filters

These filters accentuate Jupiter’s reddish cloud belts and its iconic Great Red Spot. They also amplify details in Venus’ atmosphere and highlight the Martian ice caps and dust storms. Their effectiveness scales with their depth of colour, albeit at the cost of image brightness and hue. Among these, the #80A filter is ubiquitous and particularly adept with Jupiter and Venus, despite its noticeable coloration on Jupiter. The #82A offers a nuanced hue for Jupiter, while the #38A, although producing a dimmer view, might surpass the #80A in revealing Jupiter’s details. The #47 filter emerges as the top choice for Venus, assuming your telescope possesses sufficient aperture.

• Orange & Red Filters

Primarily beneficial for daytime lunar observations, these filters amplify lunar contrast against the daytime sky. The #21 orange and #25 red filters are the leading picks. Though available in other shades, their efficacy on planets is debatable.

• Yellow & Green Filters

The efficacy of yellow and green filters is subtle, often bordering on subjective. The #12 yellow filter is akin to a minus-violet filter for refractors and might accentuate features on Uranus with telescopes of 10” aperture or larger; some claim it helps with contrast in Saturn’s rings. The #11 seems to be slightly more effective in revealing faint Uranian cloud details.

Moon & Polarising Filters

Lunar filters, or Moon filters, are light-limiting filters, which means they reduce the amount of light that reaches your eyes as the moon’s luminosity can seem overwhelming to many observers. However, it’s essential to note that even seeing the full Moon through a large telescope won’t actually harm your eyes. Pupil dilation recovers within 30 minutes post-observation. Using higher magnification can not only dim the image but also enhance detail, provided atmospheric conditions allow. Observing at lower power might limit the detail you can see. This principle also applies to Jupiter and Mars.

Introducing a semi-opaque piece of glass near your telescope’s focal plane while expecting it to transmit all wavelengths unaltered is a tall order. This is why many low-quality, inexpensive “moon filters” often lean green, aiming to minimise sharpness issues, although their effectiveness remains questionable. A cheap, low-quality Moon filter might introduce optical aberrations, compromising the image’s clarity. If you’re adamant about low-power lunar observation with dimming, wearing ordinary sunglasses while you look through the eyepiece might serve better, especially when paired with a low-magnification, extended focal length eyepiece that offers sufficient eye relief.

For those still considering filters for lunar observation with a telescope, a variable-polarizing filter is ideal. Our top recommendation is the Baader Double Polarising Filter. Besides dimming, it offers adjustable dimming levels and is versatile for other unique astronomical ventures. Polarising filters are filters that transmit only light with a certain orientation of its electric field and block the rest. As such, one can also reveal the structure and direction of the tail of a comet by blocking some of the light that is reflected by its dust particles and transmitting the light that is emitted by its gas molecules.

Solar Filters

Solar filters are either etalons (narrowband devices usually built into dedicated solar telescopes) for viewing narrow portions of the spectrum or white light filters affixed to the front of any ordinary telescope. These are called full-aperture filters as they cover the front of the telescope to dramatically reduce the amount of light entering the aperture of the telescope. However, handle them with care. If they get scratched, they must be discarded, as even a small scratch can let in too much UV radiation, which can damage your eyes.

Never use a solar filter that screws onto your eyepiece, as severe eye damage or fire may occur. These are very dangerous and are not typically available anymore, but should you come across one on eBay or at a garage sale, don’t buy it or use it. The risk of damage to your eye is too high.

Solar filters that you can add to your telescope are good for seeing sunspots, watching a full or partial solar eclipse, or watching the transition of a planet across the face of the Sun. Eclipses and planetary transits don’t happen very often, so they are a big deal among the amateur astronomy community when they occur.

Nebula & Light Pollution Filters

Where the Moon and planets are reflective, deep-sky objects, or DSOs, generally produce their own light. They are made up of stars or glowing gas clouds. The light coming from them may be white light or it may be focused on a particular part of the light spectrum.

For the most part, filters for observing DSOs are either narrow-band filters or line filters. They may also be called “nebula filters.” Which one you would use depends on what you are observing. Nebula filters make nebulae stand out more by focusing on the specific spectre of light that are released when gases in a nebula are ionized by the strong radiation and heat from a nearby star, making them look like a glowing neon sign. Nebula filters reject light that is outside of these emission lines, which darkens the sky background, attenuates light from stars, and lessens some light pollution.

Narrowband filters only let through light in a specific range or set of wavelengths. These are often associated with particular groups of glowing gases. These are very popular and are often the best choice for a first nebula filter. Examples might be the Baader Neodymium Moon & Skyglow Filter (broadband) or the Astronomik UHC filters. These work extremely well on some nebula but not on others.

Another type is a line or single-band filter. An example would be the popular oxygen 3 or OIII filters and the H-Beta filters available from a variety of manufacturers.

Typically, the narrower and more aggressive the filter’s bandpass, the more efficient it is at its task—given the transmission is adequate. But there’s a trade-off: the resulting image might be dimmer. “Narrowband” filters generally possess a bandpass of less than 30 nanometers. Band passes under 8–10 nanometers are mainly for imaging, as they’re too dim for observational astronomy. On the other hand, “’broadband” filters, with a bandpass exceeding 30–40 nanometers, offer minimal contrast enhancement and are essentially worthless for visual observation.

We highly recommend a UHC filter over all other types of nebula filters, and the vast majority of observers will be completely served by a UHC filter alone and/or an oxygen-III filter to complement it. Most other nebula filters are of questionable worth.

Broadband or Light Pollution Filters (30+ nm band pass)

One of the most common and marketed types of filters for astronomical observation are broadband filters, also known as light pollution filters or skyglow filters.

The idea behind light pollution or sky glow filters is that light pollution comes mainly from street lights, parking lot lights, and similar industrial lights. In the past, these were largely based on mercury vapour and sodium vapour lights. These lights emit light in fairly narrow parts of the light spectrum. That allowed light pollution (LP) filters to filter out those wavelengths while allowing all others to pass. In this way, you could improve the contrast with the sky by removing the glow of the ground lights.

This was probably somewhat effective in the past, but LP filter effectiveness is fading from usefulness today. More and more towns and cities are moving to white LED lights. They are brighter and use a lot less power, making them very cost-effective. Unfortunately for us, they emit full-spectrum white light. As a result, the outdated light pollution filters are not very effective in reducing the sky glow that these lights produce.

But there are some other drawbacks as well:

• Stars and galaxies emit light at many of the blocked wavelengths, so there’s inadvertent attenuation of the brightness of these objects, negating any contrast enhancement.
• Even though regular street lamps only emit light in a few narrow wavelengths, many broadband filters block out a wider range than these emission lines. This makes stars and galaxies less bright without giving you the benefits of a genuine nebula filter.
• Most of these filters have poor transmission to begin with and simply absorb some of the incoming light uniformly across the spectrum.

Beware of misleading labeling: numerous “UHC” filters are actually broadband. Sometimes, this discrepancy is hidden behind suffixed names such as “Deep-Sky”, “CLS”, “LPR”, or “Skyglow”. Don’t fall for manufacturers’ claims that their broadband filters are superior. They might suit imaging needs, but for observational astronomy, they are largely useless, with one notable exception.

UHC Filters (18-25nm band pass)

Ultra-high contrast filters, or UHC filters, are specialised filters that let through a small band of light. This band of light includes bluish-green wavelengths from hydrogen-beta and oxygen-III emission lines, which come from ionized nebula gases, as well as fainter deep-red hues from certain nebulae’s hydrogen-alpha emissions. They are extremely useful tools for viewing most nebulae.

UHC filters can be very effective for enhancing the contrast and detail of emission nebulae, such as the Orion Nebula, the Lagoon Nebula, and supernova remnants like the Veil Nebula. They accomplish this by transmitting the light that the nebulae’s ionised gas emits while blocking the majority of the visible spectrum of light, including background sky glow from stars and light pollution. UHC filters also bring out planetary nebulae by dimming the background sky and most stars, but they mute the natural colours of these objects, and once you’ve found them, they are usually best viewed without any filter at all.

UHC filters are ineffective for broad-spectrum objects like galaxies and stars, as well as reflection nebulae, which shine from reflected starlight. Using a UHC filter will merely dim these objects with no benefits. However, a UHC is useful for combating the effects of light pollution on nebulae as well as improving contrast even under a dark sky, as the natural sky background is not jet-black and many low-contrast objects can be hard to see unfiltered even under perfect conditions.

For many astronomers, especially those with telescopes under 10″ in aperture, a quality UHC filter might be the only nebula filter they need. A good UHC largely obviates the need for specialised oxygen-III or hydrogen-beta filters unless you are an extremely devoted observer with demanding requirements (and good skies to support such observations).

Best UHC Filters

• Astronomik UHC: Not to be confused with the near-useless UHC-E broadband, this filter boasts superb transmission across the primary nebula emission lines. For many, this could be the sole filter required.
• SVBONY Oxygen-III: Marketed as an OIII filter, its 18nm band pass essentially categorises it as a UHC. It admits hydrogen-beta light but, similar to the UltraBlock, blocks hydrogen-alpha. Despite its slightly narrower bandpass and lower transmission compared to premium brands, it remains a commendable choice for those mindful of cost. However, steer clear of the SVBONY “UHC” filter—it’s essentially a broadband filter.

OIII Filters (8-15 nm band pass)

If a nebula is rich in oxygen 3 and most of its light is coming from the glow of OIII, an O-III filter, or oxygen-III filter, will let that light through and block out the rest of the spectrum. Without the filter, you might see the nebula, but it might be faint or washed out compared to the rest of the sky. But put in the OIII filter, and only that light comes through. This helps to bring out the oxygen-rich portions of these nebulae and also highlights planetary nebulae at low power, just like a UHC does, but the background sky and stars are dimmed even more.

If you want to see more contrast and detail in oxygen-rich emission and planetary nebulae like the Veil Nebula, the Swan Nebula, and the Ring Nebula, these filters might work even better than UHC filters.

Owning both OIII and UHC filters can greatly enhance your stargazing experience, although the benefits of an OIII may seem marginal, especially for smaller telescopes. An OIII on its own is a little too specialised.

Here are our top three picks for an OIII filter:

• Baader Visual OIII Filter: This is essentially the twin of the Celestron Oxygen-III filter, boasting the slimmest band pass available at a mere 8.2nm. While this specificity might reduce some light on certain celestial objects, for broader band pass requirements, a UHC filter would be more fitting anyway. Among O-III filters, the Baader is our top recommendation due to its pronounced contrast enhancement over even the finest UHC filters.
• Astronomik OIII Visual Filter: 15nm band pass and impressive transmission. Identical to the Tele-Vue Bandmate OIII. However, its expansive band pass overlaps considerably with high-grade UHC filters, like the Astronomik UHC, potentially making it an unnecessary addition for some.

Hydrogen-Beta Filters (8-12 nm band pass)

The H-Beta filter works in a similar fashion to the OIII filter. It is focused on letting only a single light line through. In this case, it is letting through 486 nm (nanometer) wavelengths.

Frequently dubbed “Horsehead filters” due to their ability to enhance views of the famed Horsehead Nebula (IC 434), hydrogen-beta filters might feel superfluous if you already own a top-tier UHC filter. The appeal diminishes further considering their hefty price tag, often exceeding £100, for a tool that might see infrequent use on less than five targets in the night sky.

• Lumicon H-Beta Filter: With an 8nm band pass, this filter delivers a notably dim yet high-contrast image. It arguably stands out as the most adept H-beta filter on the market.
• Astronomik H-Beta Visual Filter: Sharing specs with the Tele Vue Bandmate H-Beta filter, it offers a broader 12nm band pass.

Imaging Filters

“Narrowband” imaging filters commonly have band passes of well under 10nm. They are usually not usable for visual observation at the eyepiece. Many imagers use broadband or “CLS” filters to compensate for the worst effects of light pollution; monochrome cameras can be used with RGB filters as well as narrowband filters. These filters include the common nebula O-III, H-Alpha, and H-Beta like visual filters, but also sulphur, methane, and others that can be used for both deep-sky and planetary work with a filter wheel and monochrome sensor. Shopping for these is a vastly more subjective experience that heavily depends on the gear you are using and what you are shooting; our recommended filter brands are a good place to start.