Telescope Filters: How to Choose the Right Type

Observing can be a challenge. Many nebulae and galaxies are extremely faint, and even Jupiter’s Great Red Spot isn’t all that easy to see. Sometimes you just need a little help to see that challenging object.

Filters may be just the ticket – they offer a relatively affordable way to increase the contrast of any telescope and see things you couldn’t see before.

What do telescope filters do, anyway?

Filters work by blocking light. Although astronomers talk a lot about the importance of gathering more light, there are situations where blocking some light can help you see more.

Most types of filters block certain wavelengths while letting other wavelengths through. This can noticeably increase contrast – if you use the right type of filter for the job.

Some filters reduce the overall quantity of light (regardless of wavelength) to allow you to look at bright objects like the moon or sun. This is crucial when looking at the sun, as you can permanently destroy your eyes if you don’t use the correct type of filter.

This video has a quick overview of the most common types of filters:

Check your eyepiece size

With the exception of solar filters, almost all filters are designed to attach to an eyepiece. So it’s important to buy filters that will fit your eyepieces. Most eyepieces have a diameter of either 1.25″ (32mm) or 2″ (51mm).

If your telescope has a 2″ focuser, you may have both 1.25″ and 2″ eyepieces that work with your telescope. In this case, you may want to choose a “favorite” eyepiece and buy filters to fit that eyepiece. For many people, this will often be their eyepiece with the longest-focal-length – or maybe just the most expensive one.

Planetary filters

Planetary filters are simply colored filters that help to bring out the details of a planet’s surface or atmosphere. They are usually inexpensive, and often sold as a set with four or more colors. Many eyepiece sets also include a few planetary filters.

Since each planet has a variety of features, you may want to try different filters to see how they enhance different parts of the planet. For example, a blue or green filter can enhance details in Jupiter’s cloud bands or the Great Red Spot. But a yellow or orange filter may bring out different features in Jupiter.

So there isn’t necessarily one “best” filter for each planet. If you buy a filter set, you’ll be able to experiment with filters to see which ones you like best.

1.25″ filter sets

The Celestron 14-Piece Eyepiece and Filter Kit (shown below) includes five Plossl eyepieces, a 2x Barlow lens, six planetary filters, and a moon filter. If you’re also wanting to expand your eyepiece collection, this can be a very good deal, but if you just need the filters there are cheaper options.

There are also similar kits from other brands, such as the Meade Series 4000 Eyepiece and Filter Set and the Orion Telescope Accessory Kit.

If you just want filters, the Neewer 1.25″ Filter Set is a very affordable option. One nice thing about this set is that it also includes a skyglow filter (also known as a light pollution filter), which most filter sets don’t include. We’ll talk more about light pollution filters later.

2″ filter sets

If you have 2″ eyepieces, the Neewer 4-Piece 2″ Filter Set is one of the cheapest options, although it only includes four filters.

The Celestron 2″ Eyepiece and Filter Kit is another option, but it is fairly pricey. Keep in mind that 2″ eyepieces are more expensive, so most of the cost of this kit is due to the three included eyepieces (40mm, 32mm, and 26mm)

Moon filters

When viewed with the naked eye, the moon is bright.

When viewed through a telescope, the moon is insanely bright.

Looking at the moon through a telescope won’t damage your eyes, but it can be uncomfortable. Moon filters reduce the brightness without altering the color, making it easier to look at the moon. They also reduce glare, allowing you to see more details.

Some of the planetary filter sets mentioned above include moon filters, but you can also buy them separately.

The Orion 13% Transmission Moon Filter is a good option. It blocks 87% of the light (letting 13% through), dramatically reducing the moon’s brightness.

Variable polarizing filters

Variable polarizing filters are made from two pieces of polarized glass. By rotating one piece, you can dial in exactly the brightness you need.

Southstar has a 1.25″ Variable Polarizing Filter and a 2″ Variable Polarizing Filter. Both filters are adjustable from 1% to 40% transmission, so they can be used in a wide range of situations – everything from a full moon to planetary viewing.

Light pollution filters

Look around you. Are there any streetlights, cars, floodlights, or other sources of light? Unless you’re in an extremely secluded area, there is probably light pollution all around you.

Most people don’t notice light pollution – until they try stargazing. Even in rural areas, there is enough light pollution to make astronomy more difficult, especially if you’re trying to see nebulae or galaxies.

Luckily, light pollution reduction filters can help. These are sometimes called skyglow or broadband filters, but the basic idea is the same: to block the wavelengths of light caused by light pollution, while still letting other wavelengths through.

Although these filters can’t make nebulae or galaxies brighter, they can increase contrast by darkening the sky around the nebula or galaxy.

The Orion 1.25″ SkyGlow Broadband Filter is a popular choice (also available in a 2″ version).

Note: Light pollution filters won’t be able to eliminate all light pollution, especially if you’re in an urban area. Even if you’re using a filter, it’s a good idea to find the darkest skies you can.

Narrowband filters

While light pollution filters work by blocking certain wavelengths of light, narrowband filters block most wavelengths and only let a narrow band of light through.

So narrowband filters are sort of the more “extreme” cousin to broadband filters. They are somewhat less versatile than broadband filters – they are used specifically for viewing emission nebulae (such as the Orion Nebula, Lagoon Nebula, and Ring Nebula). However, for those types of nebulae, they can be even more effective than broadband filters.

These filters can start to get a bit more expensive, but the Orion UltraBlock (also available in a 2″ version) is a good midrange option. There are also pricier options like the Lumicon UHC and the Thousand Oaks Nebular Filter.

Emission line filters

Some filters are designed to pass an even narrower band to capture specific emission lines, usually from oxygen or hydrogen contained in the nebula. One of the most popular types is the Oxygen III (or O-III) filter, sold by Baader, Thousand Oaks, and other companies.

Other types of emission line filters include Hydrogen beta (also known as H-beta) and Hydrogen alpha (or H-alpha). However, H-alpha filters are only used for astrophotography – our eyes don’t pick up the H-alpha wavelength very well, so you shouldn’t buy that type of filter for observing.

Solar filters

Solar filters are different from any other type of filter, and it’s extremely important to use them correctly – otherwise you can severely damage your eyes. But when used correctly, they allow you to observe the sun or a solar eclipse through your telescope.

Unlike other filters, solar filters must be placed on the telescope itself. That means you’ll need to use a filter that is sized specifically for your telescope. For example, this filter is sized for telescopes with an outer diameter of 5.56″ to 5.81″. Make sure to accurately measure the outer diameter of your telescope rather than using the aperture measurement.

Another option is to buy a solar filter sheet and securely attach it to the end of your telescope with tape. You can also make a DIY cardboard holder for the solar filter sheet so you can easily remove the filter from the telescope and reuse it later.

If you go the solar filter sheet route, a few words of warning:

  • Check it for holes or gaps: Even a tiny hole or nick in the solar film can potentially damage your eyes. Make sure there is no light leak.
  • Never remove the filter while it’s pointed at the sun: Make sure the filter is in place before aiming the telescope at the sun. Also, make sure to turn the telescope well away from the sun before removing the filter.
  • Use a well-known brand: I recommend using Thousand Oaks or Baader solar film since these are respected brands.