Telescope Filters

Telescope Filters – Guide to Astronomy Filters

Telescope filters can be an affordable and simple way to boost the quality of observations offered by your telescope. However, choosing the right filter for the right space object can be a daunting task with so many different options available on the market.

What’s the best filter to view the Great Red Spot of Jupiter? What about the Moon? How can I view Venus at daylight without the sunlight disrupting my view?

In this guide, we will try to answer those questions, and all you need to know about telescope filters. By the end of this guide, you’ll understand the concept behind telescope filters, the different types available along with their use cases, and especially, the best telescope filters according to your needs and what space object you’d like to view.

Without further ado, let’s begin from the very basics of what telescope filters are.

What Are Telescope Filters?

A telescope filter, or sometimes called ‘astronomical filter’, is a type of photographic filter that is used to enhance the details and improve the contrasts of an object observed.

Typically a telescope filter is screwed into the bottom of the eyepiece’s barrel, and it functions by cutting (filtering) unwanted wavelengths of light or certain colors.

The telescope filters block a specific part of the color spectrum below and above a certain wavelength threshold, which will significantly increase the signal to noise ratio, giving the observed object more clarity and contrast.

Why Are Telescope Filters Used on Telescopes?

The night sky, wherever you are, has many unwanted light and colors. The most common unwanted colors come from pollution like street lamps, vehicle lights, lights from houses and skyscrapers, and so on. Besides light pollution, air pollution like dust particles can also cause blurred telescope images.

A telescope filter, as the name suggests, filters out these unwanted colors and pollutions so we can get a clearer image quality. Using a telescope filter alone can significantly improve the astronomer’s view.

Factors Determining Telescope Image Clarity

To really understand the reason why telescope filters are beneficial, however, we have to understand how our eyes work at night during astronomic observation.

There are generally three main factors determining the quality of views delivered by your telescope: sharpness, clarity, and resolving power.  Resolving power would ultimately depend on the aperture of your telescope, so there’s nothing much you can do about it.

We can, however, improve the image’s sharpness and clarity with a telescope filter.

Our eyes contain two kinds of photoreceptors (cells in the retina that receive and respond to light): cones and rods. Cone photoreceptor cells are responsible for colors, and so there are three types of cone cells, each responding to the red, green, and blue wavelength of light.

The rod cells, on the other hand, are responsible for monochromatic lights, and we will primarily use these rods at night vision.

The thing is, we have only one type of rod cells instead of three, and our rod cells would respond best to colors between blue and green. This is why when observing an object at night, it seems bluer than it was in the day. This also means that contrasts play a much bigger role in the dark.

Telescope filters essentially control the type and quantity of light that enter your eyes, allowing us to better distinguish differences in darker conditions. By filtering certain colors (wavelengths), transmitted lights would appear much brighter, increasing contrast, and as a result, significantly improving our view.

Different space objects, however, reflect different wavelengths of light (different colors), and this is why there are also various types of filters that allow us to fine-tune our telescope depending on the celestial object we’d want to observe. We will further discuss these different types of telescope filters below.

Different Categories of Telescope Filters

Telescope filters, as with other optical filters, are categorized with a numbering system called Wratten numbers. The numbering system typically uses a number that might be followed by a letter.

The number will determine the color of the filter, for example, 2A to 2C are pale yellow, while 3 is light yellow, 4 is yellow, and so on. When there are letters following the number, it denotes the strength of the color (2C and 2A are both pale yellow, but 2C has stronger intensity).

This numbering system is named for the British inventor Frederick Wratten, which then sold his photography company to Eastman Kodak in 1912. Kodak’s Wratten filters remain in production to this day.

Some filters, however, do not follow the traditional letter-number scheme of the Wratten System like G, K2, and so on.  Wratten number starts from 1A (white/skylight filter) up to number 106 (amber) and you can check the full list here.

However, not all of them are used in astronomy, and here are some of the common numbers used in telescope filters:

8 (Light Yellow) 

Light yellow telescope filters blocks light short of 465mm but will allow 80% of the light through. This filter is typically used in Jupiter observation, to enhance the red and orange colors in Jupiter’s belts.

Also useful to increase the contract on Mars’s surface, and can also aid the visual resolution on Neptune and Uranus for telescopes with large enough apertures (250 mm or larger). Also effective in filtering the glare from the Moon.

11 (Yellow-Green) 

A great filter if you want to observe the surface details on Saturn and Jupiter. Can be used on Mars’ surface, but you need a larger telescope aperture (250 mm and above). 75% light transmission, can be used to darken the surface features of the Moon.

12 (Yellow)

 This filter can also help in enhancing the red and orange features on Jupiter and Saturn but works on a different principle than the no.8 above by filtering the wavelength between blue and green spectra.

As a result, it produces a deeper color than the no.8, so it is the most popular filter for observation on Jupiter and Saturn. Allows 70% of light while blocking visible wavelength shorter than 500nm.

15 (Deep Yellow)

This filter can be used to enhance the features of the Mars surface, especially on Mars’ poles. Also effective to enhance the red features of Jupiter and Saturn. You can also use this to add more contrast when observing Venus. 65% light transmission and blocks light short of 500nm.

21 (Orange)

Orange filters block blue and green wavelengths while enhancing contrasts of red, orange, and yellow areas on Mars, Saturn, and Jupiter.

It is very useful in enhancing the Great Red Spot on Jupiter with the right telescope and conditions, and can also pretty well-rounded for most planetary observation. 50% light transmission while blocking wavelengths shorter than 520nm.

23A (Light Red)

Another well-rounded filter for use on Mars, Jupiter, and Saturn. Only 25% light transmission, so it’s also useful for daylight Venus observations. Light red is effective in blocking blue, so it’s generally effective in darkening the daylight sky. Blocks wavelengths short of 550nm.

25 or 25A (Red) 

The no.24 filter is effective in blocking blue and green colors, making it useful in observing cloud formations and the lighter surface of Jupiter and Saturn. However, it only has a light transmission of 15%, so it’s not very versatile and would require bright enough conditions. Blocks light short of 580nm.

56 (Light Green) 

This filter allows most wavelengths through, but have a peak around 500nm with 50% transmission. These features make this filter a favorite in observing the moon as it’s very effective in enhancing contrast while reducing lunar glare. It will affect all wavelengths more or less equally, so it is effective as a color correction filter.

58 (Green) 

Green filters block red and blue wavelengths, which can be effective in enhancing contrast on some parts of Jupiter and Venus. 25% light transmission, and can be categorized as a color correction filter, changing the color temperature of light while enhancing contrasts.

82A (Light Blue)

 A well-rounded, multipurpose filter: works very well in lunar observation and can enhance the moon’s features effectively, but can also help in Jupiter, Mars, and Saturn observations. It adds more warmth to the color by allowing red wavelengths through with a light transmission of 75%.

80A (Blue) 

Just as versatile as the 82A in enhancing the red colors on Jupiter, Mars, and Saturn while also effective in aiding lunar observations. 30% light transmission while enhancing wavelengths peaking at 500nm.

38A (Dark Blue)

 Another good planetary filter since it can block red and orange wavelengths, useful in Jupiter and Saturn observation. Can also increase the contrasts during Venus observations with 15% light transmission by blocking red, green, and UV light.

47 (Violet) 

A very dark filter that strongly filters green, red, and yellow wavelengths. Great for Venus observations due to its very low, 5% light transmission, allowing great contrasts.

Great for lunar observations to decrease glare, especially if you are using telescopes with a larger aperture. Enhances the blue wavelengths of the spectrum at 450nm.

Non-Wratten Telescope Filters

There are also telescope filters that aren’t categorized via Wratten numbers, for example:

Polarizing filters

Polarizing telescope filters do not block any specific wavelengths. They essentially allow any wavelength through but will filter random scattering light patterns, which as a result will improve the contrast of the object.

Polarizing filters would only allow ‘flat’ waves through, which will also reduce glare and improve the color saturation of the object. We will discuss more of polarizing filters further below.

Neutral Density Filters 

Neutral density, or ND filter, transmits light in a uniformed and consistent manner across the entire visible spectrum.

This feature makes it an excellent filter to reduce glare in brighter objects like the Moon and several planets. It is a must-have filter for lunar observation and is also a favorite in planetary observations.

ND filters come in a variety of densities, essentially the higher the density, the better it is in reducing the glare in the image. The density is based on the amount of light transmission allowed by the ND filter.

They tend to have a number in its product name (i.e. 50) that signifies the amount of light it can transmit (50 being 50%, 10 being 10%, and so on).

Neodymium filter

 The Neodymium filter blocks the yellow light of the spectrum, so it will render all objects in a faint bluish color. Useful for planetary observations especially for Venus, but also for Jupiter and Saturn. Also useful for blocking light pollution, although not as effective as a dedicated light pollution filter.

Light Pollution Filter 

While there are many different types of light pollution filters available in the market, typically it is designed to block lights from Sodium-Vapor street lamps, which have a wavelength of 589nm.

This is to ensure that only the wavelength of the light pollution that is blocked instead of blocking out all wavelength as in the case of using a narrowband or long-pass filter.

What Is a Polarizing Filter Used For?

Above, we have discussed the basics of how a polarizing filter works, but what is it used for?

Polarizing filters are typically useful on brighter planets and space objects, especially Moon and Venus. Yet, it is very important in observing the moon.

The moon is often the most desirable object in the night sky for many astronomers, but lunar observation can be very challenging due to the simple fact that the Moon is very bright compared to all other objects in the celestial sphere.

This brightness would produce glare, which makes it very hard to observe the Moon’s surface details. This is where the polarizing filters come in.

The name ‘polarizing’ filters come from the fact that it was made from two polarized surfaces, which can reduce 60% to 99% of the incoming light by varying the angle. A polarizing filter technically allows all wavelengths of light to pass in a uniformed manner, which is a unique feature only available in polarizing filters.

Polarizing Filter

This feature allows a much enhanced, detailed view of the dimmer light without affecting the contrast of the image. While you can also get the same function from a neutral density filter (as discussed above, which is also called ‘moon filter’), polarizing filters provide more versatility by being adjustable.

To summarize, a polarizing filter can help improve color saturation in your images by reducing reflections. In daylight observations, a polarizing filter can also help you create deep blue skies by preventing scattering blue light from coming into your lens, giving you the most focused, clearest blue light possible.

When using a polarization filter, it’s important to note that polarization greatly depends on the celestial position of the sun, so the time of the day and the time of the year can affect the performance of the polarization filter.

If you are looking for a Moon filter, then we’d recommend the Celestron Telescope Moon Filter that would your standard 1.25” eyepiece, or the Orion 1.25-Inch 13 Percent Transmission Moon Filter that offers a 13 % transmission moon filter.

How to Use Filters To View Planets?

Filters play a very important role in observing planets because each of our galaxy’s planets has their own color characteristic, and the space objects might also add unique light pollutants to the planet observed.

The right filter, for example, can help block light wavelengths from a nebula that might disrupt the detail of Jupiter. By doing this, you can get more contrast and a much sharper and cleaner image.

Different planets (and space objects) require different types of filters to get optimal viewing of each planet. You can check the chart below to find the appropriate filters for the various objects.

Telescope Filters for Viewing Planets

Here are our recommendations for the best filters to get for each color:

  • #8 Yellow – great neutral color that won’t change the natural color of the Moon while effectively improving contrasts. Great for moon viewing.
  • #12 Yellow – great for Mars, and might also help with certain conditions of moon viewing.
  • #21 Orange – number 21 is great for Moon and Mercury, very effective in improving contrasts, and would really shine in larger aperture telescopes.
  • #23A Light Red – the number 23, light red filter is especially great for Great at enhancing the contrasts of the red planet, and can also help lunar viewing.
  • #25 Red – great for Venus and Mars, the number 25 or red filter is more versatile than 23A especially for Venus viewing.
  • #38A Dark Blue – great for Venus and Jupiter especially in improving the contrasts of the gas giant.
  • #47 Violet – for Moon and Venus, especially for the clouds on Venus and Mars’ polar caps. Also great to view the rings of Saturn.

What are Chromatic Telescope Filters?

Chromatic aberration filters, as the name suggests,  are used to counter chromatic aberration.

Chromatic aberration, or optical aberration, is a common problem in refracting telescopes. The different wavelengths of lights passing through the lens have a different refractive index, and the refractive index of blue light is greater than that of the red light.

As a result, this phenomenon causes color fringing, especially in the form of blue halo around stars, while the planets and Moon can show blue and/or yellow halo around it.

Chromatic aberration filters can help tackle this issue by filtering the color fringes around the brightest stars and planets.

We’d recommend the Baader Planetarium Semi-APO Filter 2″ FSAPO-2 if you want to get a chromatic aberration filter.

What Are Narrowband Filters Used For?

Narrowband filters only allow a small amount of light pollution in, much more aggressive than the standard telescope filters.

Typically they work by only allowing spectral emissions of Oxygen III and Hydrogen Beta lines to pass through the filter.

The idea is that this filter will dim the background or darkening the sky surrounding the star or planet, to improve contrast.

However, the downside is that they also block the desirable lights from the stars and planets, dimming them in some cases.

Narrowband filters are best used for observing deep space objects, especially the nebulae and further galaxies. Narrowband filters are especially useful for viewing the Orion Nebula and Swan Nebula, as well as Lagoon Nebula.

Orion Nebula
Orion Nebula, Image source: https://nasaviz.gsfc.nasa.gov/12086

When used right, however, a narrowband filter can also be effective in filtering low to medium level of air and light pollution even with a significant amount of magnification.

In understanding the narrowband filters, it’s important to understand narrowband here refers to the narrowband emission of the nebulae and planetary nebulae, that allows the nebulae to glow.

This phenomenon is caused by electrons absorbing and releasing energy after excitation. The narrowband filters are designed to block the lights caused by this narrowband emission, which are typically caused by Hydrogen-alpha or Oxygen III-based excitation.

So, if you see a narrowband filter product that says “Oxygen III” or “OIII”, it simply means that the filter is designed to block Oxygen III emission.

For Narrowband filters, we’d recommend:

  • Celestron 93623 Narrowband Oxygen III, an Oxygen III narrowband filter, allows you to see sharper details of planets and the Moon, also very effective in improving contrasts in Nebulae viewing.

What Are Some Good Solar Filters for Observing the Sun?

The Sun is obviously a unique object in astronomy viewing, simply because the sun is extremely bright. Thus, solar filters are not only important in improving clarity/sharpness but also to protect both your telescope and your eyes.

It’s very important to always use a solar filter before even pointing your telescope directly to the sun, and don’t look through the eyepiece without a solar filter. It is very dangerous for your eyesight.

The thing is, we can’t do this the traditional way by putting a filter on our telescope’s eyepiece, since it won’t protect the telescope’s tube and objective lens/mirror from the sun rays.

Instead, you’ll need a rather unorthodox method of covering the front of your tube with a solar filter to block the lights before they come in.

However, as we all know, different telescopes have different objective lenses, there might not be a single solar filter that would fit your telescope’s outer diameter.

There are various products available on the market for specific telescope sizes, and here are some we’d recommend:

  • SolarLite Solar Filter: can fit telescopes with 8” diameters (203 mm). The filter itself has a diameter of 9.312” or 236 mm. Many popular manufacturers like Celestron and Meade have popular 8” models, and this can be a great solar filter if you happen to possess an 8” telescope. It’s fairly expensive but does the job pretty well.
  • Threaded Black Polymer Solar Filter: this one will 3” (77mm) telescopes, which is also a popular size. Made by Thousand Oaks Optical, a specialist in the optical filter industry for more than three decades, a testimony for its quality.
  • Astromania Deluxe Solar Filter: this is an adjustable solar filter that is designed to fit an outer diameter of 3.5” to 4.5” (90mm to 112mm) with the outer diameter of the filter being 120mm.
  • Seymour Solar 67mm: this one is for 67mm (2.4”) telescopes, can effectively block 99.99% light, including IR and UV rays.

However, you can quite easily make your own filter with an 8″x8″ Solar Filter Sheet, if you don’t mind the DIY approach. You can just create a custom-sized filter with cardboard and attach this filter sheet.

You can then secure your filter with sticky tape to your telescope. It might not be the best-looking thing but will do the job pretty well, and you can save some money in the process.

End Words

The telescope filters, just like optical filters in photography, are designed to improve contrasts, sharpness, and clarity of the image observed by filtering—or removing—undesirable colors of the wavelength of light.

While the idea of telescope filters is pretty simple, you shouldn’t underestimate their importance if you really want to observe the details of planets, the Moon, or even nebulae that are located millions of light-years away.

As you can see, there are also various different types of telescope filters available in the market, each with their own use case and some of them even have really unique mechanisms that might be confusing for beginners.

However, don’t let the sheer availability of options confuse and overwhelm you, and the basic principle in choosing your telescope filters should remain simple:

  • Decide on what objects or planets you’d like to view – consider these objects.
  • Find the best filter that can enhance this specific viewing – see our table above.
  • Start small, and build your collection of filters as you go.