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How to Use Telescope Filters?

When you buy a telescope, usually you get simple but necessary accessories it cannot function without: eyepieces, a Barlow lens, a wrap-around prism or diagonal mirror and a telescope viewfinder. Typically, most amateur telescopes come with these accessories.

Sometimes, though, not all of the necessary accessories are included. As a rule, expensive telescope models only come with one eyepiece and require purchasing the necessary kit that includes different filters. How to use telescope filters? Let me describe.

Telescope Filters – Why Do We Need Them?

The basic principle of filters is to emphasize light of certain wavelengths (colors) and suppress others. This makes it possible to emphasize characteristic details of the disks of planets or nebulae. Thus, you can safely observe the Moon or the Sun without fearing for your own health. Astronomy filters have been specifically designed for safe observation of the night sky.

What are telescope filters?

A color filter is an important, sometimes even necessary, accessory for telescope observations. Such filters for telescopes can be divided into several types: solar filters, colored planetary filters, and narrowband filters for observing and photographing nebulae.

Solar filters are used to safely observe the disk of the Sun. Under no circumstances do we recommend observing the Sun through a telescope not equipped with a special filter. As 99.999% of visible light is cut off by solar filters, it is absolutely safe to observe the Sun when using it. A solar filter must be worn on the telescope lens. That is, the inner diameter of the solar filter must be equal to the outer diameter of the telescope tube. Observing through an ocular solar filter is not safe because the sun’s rays cause heating and may result in filter cracking! Observing the Sun can cause heating and damage to the filter attached to the eyepiece.

The most budget-friendly version of a solar filter is to make the filter to the diameter of the telescope using special solar film. Such film is also completely safe and gives a rich contrast picture. Depending on the film manufacturer, the color of the solar disk may vary when observing. Also, a distinction is made between visual and photographic film. For safe visual observations, only visual film is suitable.

Another option is off-the-shelf glass solar filters designed for a specific telescope tube diameter.

solar filter for sun observation

How do telescope filters work? Why do we use telescope filters?

Color filters are used mainly for visual observations of the planets. Such filters make the image of the planets more contrasting and highlight details on their surface. Colored filters include a neutral gray or green colored lunar filter that mutes the brightness of the moon, making observations more comfortable.

The red filter is used for daytime observation of Venus, observations of the polar caps on the surface of Mars, and blue clouds on Jupiter. The orange filter is very useful for observing the Moon, for daytime observation of Mercury, for detailing the surface of Mars, belts and festoons on Jupiter. The yellow filter enhances the surface contrast of Venus, increases the visibility of seas and clouds on Mars and belts on Jupiter. The green filter enhances detail contrast on the Moon, improves detail contrast on Venus, and is useful for observing Mars’ shoulder storms and polar caps.

Special narrow band filters are filters that cut off certain wavelength ranges, leaving a narrow bandwidth of radiation. Such filters are used both for visual observations and for astrophotography of deep space objects emitting in a certain spectrum.

Types of telescope filters

There are two main types of filters:

  • Solar, lunar, and color planetary – for observing objects that are relatively close by;
  • Deep sky and the like – for studying objects in certain spectral ranges that are deep in space, such as nebulae.

Varieties of planetary light filters

Filters that make it easier to study the Sun, as well as the Moon and other satellites, stars and planets, vary quite a bit.

Solar filters are designed to observe the surface of the star nearest to us, the spots on it, the flares, and the granulation. They attenuate the blinding light of the Sun by several thousand times, i.e. they act as a protective device and are sometimes called black filters. The construction of glass or synthetic films is covered with a thin metal layer and cuts off more than 99% of the sunlight. That said, in quality filters the brightness is uniform, accuracy is maintained at a high level, and the light balance is just right.

Lunar (gray) light filters improve image quality and clarity. Screwed into the eyepiece, with their help block the bright moonlight (18 to 80%), allow you to discern many of the smallest details on the surface of Earth’s satellite, especially those where the contrast is weak.

In addition to the standard neutral-gray varieties, you can use polarizing composite filters made of Polaroid glasses that allow you to change the level of light transmission and reduce the brightness of the moon in its different phases.

Telescope color filters are usually sold in sets (along with solar and lunar filters). They are designed for studying the planets and solving specific problems. The highest-quality models allow, for example, a good look at certain small features of planetary disks or comet tails, enhance the contrast, which makes it possible to study the topography of the planets. By transmitting light rays of only a certain wave, they serve a variety of purposes.

Also, there is one more filter type, the light pollution filter. These filters are made of special glass that greatly reduces the transmission of certain colors that correspond to the spectrum of mercury and sodium lamps.

These filters can actually be used not only for taking pictures of the starry sky but also for taking pictures of city streets at night, where the light from mercury and sodium lamps is very close and greatly affects the color balance. You should keep in mind that the influence of such filters on familiar objects could be too strong, and colors could creep up more than you want them to.

Many filters of this kind have limitations on angle of view: with wide-angle lenses you may get uneven color balance shift depending on distance to the edge of the frame. Therefore, it is necessary to choose the filter and the lens for it carefully.

How do you pick what’s best for your need?

As written above, you should choose exactly the right filter to observe the planet you want to see in the telescope:

  • Mars, the polar regions on Saturn and the Great Red Spot on Jupiter are clearly visible through the green filters;
  • red filter allows you to observe Mercury and other planets during the day and view the seas of Mars, increases the contrast of the Moon;
  • orange filters that neutralize the blue-green part of the spectrum, suitable for studying Jupiter, Saturn and sunset observations;
  • yellow filters improve visibility of Saturn’s surface and rings;
  • violet can reduce the glow of Venus, etc.

Filters are easy to combine with each other: red, orange, yellow, green, and blue filters are standard in kits.

How to install the filter?

There is a telescope eyepiece filter guide. With oculars it is simple, you need to unscrew the eyepiece of a telescope skirt and screw a filter between it and the eyepiece itself.

As for the solar filter, to say definitely, you should know the outside diameter of the telescope tube and, accordingly, the inside diameter of the filter frame. If it appears to be a little larger, you can wrap some duct tape or duct tape around the filter. But if it is smaller, you will not be able to observe the stars.

telescope with filter on it


Let’s consolidate the information and answer the most popular questions:

Can light pollution filters remove all types of light?

Although light pollution filters can remove most of the unwanted waves, you should not rely on their full effectiveness. You will anyway have to experiment with a combination of many light filters to get the picture you want.

Do all nebulae benefit from the use of a filter?

Special narrowband filters are filters that cut off certain wavelength ranges. Such filters are used for astrophotography of deep space objects such as nebulae.

Are telescope filters worth it?

If you want to observe the night sky, you might not succeed without the use of light filters. They were specially designed for this purpose. You should also understand that different color filters are needed for observing different planets, stars, nebulae and other astronomical objects.

Telescope Filters: The First Thing You Need

As you have already understood, it is impossible to observe the sky, the planets and other objects in our space without telescope filters.

What kind of light filters do you use? Were you able to detect Pluto with the filters? Share your thoughts about the topic in the comments below.

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