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 reducing the level of light in one part 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.

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.

Since filters reduce the light that reaches our eyes, the more aperture your telescope has, the more light it gathers, 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.

Note that I am going to focus on the best telescope filters that can be added to a regular telescope. I will not be addressing filters used in specially designed, purpose-built telescopes, such as solar telescopes or H-Alpha telescopes.

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 as shades of white, black, and gray 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 toward 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 gasses in the atmosphere. Certain cloud bands tend to absorb certain colors and reflect other colors more readily, so that we may perceive some colors in the clouds or we may see this as shades of gray. We can take advantage of this by using filters to subtract some colors so that others will be more prominent.

Radiating deep-sky objects, such as stars, nebula, 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 in other light frequencies.

Keep all of this in mind as it will become important to the understanding of filters, what they do and what they don’t do. As we get into the specific types of filters, you will see that some pass all wavelengths and some only pass a limited part of the wavelength spectrum.

Light Pollution 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 vapor and sodium vapor 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 old light pollution filters are fairly ineffective in reducing the sky glow, the light pollution produced by these lights.

I have 3 LP filters that I rarely use as they don’t help anymore. My community has gone to white LEDs.

If you have an LP telescope filter, give it a shot. If you don’t own an LP filter today, take a look at the lights being used in your community before buying one. If they are mercury or sodium vapor lights, the LP filter may help. If they are white LEDs, an LP filter won’t do you much good to reduce light pollution’s effects.

Some people do find these can provide some mild benefits on certain galaxies and bright nebula. Again, experimentation is the best way to see what works for you. Speak to other observers in your area and see what they find effective.

If you would like to understand more about light pollution and how to deal with it, you will find this article helpful.

Here are examples of some best light pollution filters.

• Orion Skyglow filter – I have this one.
• GoSky Light Pollution Filter – I own this too.

Full Spectrum Light Limiting Filters

These full-spectrum light limiters are typically solar and lunar filters. The purpose of these filters is similar to that of wearing sunglasses; they reduce the amount of light that reaches your eyes because the targets are very bright. They are also known as “neutral density filters,” meaning that they pass all light wavelengths, just at reduced levels. They are not supposed to add or alter colors.

Solar filters for viewing the sun are widely available. 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.

Solar telescope filters may be made of glass or they may be based on a plastic film such as Baader Solar Film. Note that if the film looks wavy and not pulled tight, this is normal and will not affect its performance. Both types of filters are good and work well. 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.

Solar filters block over 99.99% of the light in order to make it safe to look at the sun. There are several types. Some show the sun as white, such as the Baader Solar Film mentioned above. Others give a yellow/orange image, such as the film material produced by Thousand Oaks.

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.

When selecting a solar filter, remember that they are sized to fit over the aperture. A solar filter for an 80-mm telescope will likely be larger than 80 mm. And it is not uncommon for the solar filters to be taped on rather than screwed on or clamped on.

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

Here are some examples of the best solar filters for various sized telescopes. I have provided both glass and film-based solar filters. Note that your solar filter does not have to be the same brand as your telescope.

• Celestron solar filter matched to a specific Celestron telescope.
• Orion Solar that fits a variety of telescopes

Lunar filters, or Moon filters, are also light-limiting filters. These are normally attached to the bottom of your eyepiece or, in some cases, the front of the diagonal.

Unlike solar filters, Moon filters are not required in order to view the Moon. However, the Moon is very bright and many people can find it uncomfortable to view the Moon at full intensity, especially if you are using a telescope with a large aperture.

Moon filters are available in a variety of light-blocking levels, which are usually indicated by a percentage rating on the filter. For example, a 25% filter would block 75% of the light and only pass 25%. Common types are 13%, 25%, and 40%. There are also variable moon filters that have two disks. You can rotate the filter disks to change the amount of light that is allowed through.

Which percentage is right for your telescope? There are no strict rules. I use 25% on all of my telescopes, which range from 80 mm to 300 mm. Some people would say you use 25% for telescope apertures of 150 mm or less. A 13.3% would be suggested for telescopes with an aperture greater than 150 mm. But it is up to you. Again, Moon filters are not required.

When the moon is a crescent up to about the first quarter, I may not use a filter. From the lunar first quarter through full moon and then through third quarter, I am more likely to find the filter helpful. Again, for that last quarter, I may not use the filter. When I do use a filter, I feel it enhances contrast and is more comfortable for my eyes.

I also find my moon filter very beneficial for viewing Venus, which is very bright. Venus often appears as a white, bright ball. However, once I apply the Moon filter, the shape of the planet becomes more easily seen, and I can better see the phases of Venus.

Moon filters are not expensive, often available for under $40. They are a good addition to your eyepiece set and many filter sets, and some eyepiece sets will include a moon filter.

Lunar Filters—these attach to the eyepiece

• Orion 25% Moon Filter – I use this one with all of my telescopes.
• Celestron Variable Moon Filter – Goes from 1 to 40% transmission

Planet Filters – Color Filters

Color filters are most often used for viewing planets. Since we see planets based on reflected sunlight, we can consider them illuminated by white light. That means that they have all the colors of the spectrum hitting them. What we see is what they reflect.

By filtering out some colors we make other colors 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.

Color filters are often described by a Wratten number, a system that was developed by British inventor Frederick Wratten. This is a numbering system that comes from the photography world. There are a wide variety of colored 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 marketed as a Jupiter filter, it helps with Jupiter’s cloud bands and can bring up details in the Great Red Spot.

There are many other colors 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 and so are more suited to larger aperture telescopes, say 8” or larger. But I have used them all on scopes large and small. You simply have to experiment.

If funds are limited, my suggestion is to start with a moderately priced set of four or more color filters and experiment with them on the planets and the Moon. Don’t expect explosive results. The benefits are subtle. You might use a color 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.

Here are some examples of starter sets of colored filters. These will let you try out color filters at a modest price. If you find these helpful, then you may wish to consider higher-end colored telescope filters later.

• Six filter set, including 5 color filters and one moon filter. This is similar to my first color filter set.
• Orion Mars Filter – Tuned specifically to Mars
• Orion Jupiter filter – I own this one and find it helpful on Jupiter.

Deep Sky Object Filters – Nebula 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.

There is nothing to stop you from trying color filters on DSOs, but I have not found this particularly useful. However, if you have them, it is always fun to give it a try.

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.

Narrowband filters only let through light of a specific range or set of wavelengths. These are often associated with particular groups of glowing gasses. These are very popular and are often the best choice for a first nebula filter. Examples might be the DGM NPB (narrow passband), or the Orion Ultrablock 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 sources.

If a nebula is rich in oxygen 3 and most of its light is coming from the glow of OIII, an OIII 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. We say that it darkens the rest of the sky and brings up the nebula. This works well on the Veil Nebula, for example.

The H-Beta filter works in a similar fashion. Like 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. This works fairly well on the Horse Head Nebula, for example, shown on the left.

• DGM NPB – Narrow Passband Filter – I have this one and recommend it.
• Lumicon H-Beta Filter – A very well-respected brand.

Summary

Filters limit the light that comes through the eyepiece. By doing so, you can bring up details that might otherwise be hidden. Or, in the case of solar filters, you can block dangerous levels of light and radiation, making the sun observable with your telescope.

The best way to get to know filters is to use them. Try the best filters for telescopes and see what works for you.