Telescope filters are small telescope accessories that can make a world of difference to what you see.
If there is one seriously overlooked and underused tool in your astronomical toolbox, it must be the telescope filter. These telescope filters are usually colorful discs, usually 3.175 cm (1.25 ft) or 5.08 cm (2 ft) in size and screw into the eyepiece tube, which is attached to the telescope at one end. Telescope filter works by changing the presentation of the object being observed, they work by blocking (filtering) certain wavelengths of light from passing through the telescope barrel, thus changing what you see through the eyepiece.
This goes against the traditional astronomical perception that every photon is important and that more photons mean a better rendering of the image. But this is exactly what telescope filters are all about: they filter out the light you don’t need and provide only the light you need in a given situation. This is why there are so many different kinds of filters for telescope. Some are colored, some are transparent, but each has a slightly different effect and has been designed with a different purpose in mind.
Sometimes telescope filters can be very effective, for example some filters can enhance the clarity of the polar caps of Mars and can make the otherwise unremarkable disk of Venus show subtle cloud shadows. Neutral density filters reduce glare from bright objects and thus improve contrast. However, it has to be said, that no filter can do magic, and no filter will help you get the sky clear if you have poor visual clarity or experience poor sky transparency.
What Telescope Filters Should I Buy?
Most people, both novice and seasoned astronomers, know about telescope filters and use them for planetary observation, as we’ve covered before. But there are many more kinds of telescope filters, which can reduce the glare of the moon, reduce the dull orange glow of street lamps, and even block all but one specific wavelength of light. They also work wonders if you want to observe objects in deep sky. We describe all of these telescope filters in more detail in the next section. Some telescope filters can even be used together to enhance the effect, but keep in mind that stacking filters in this way will further darken the image.
If after learning about all the different filter types, you think a certain filter might be right for you, how should you start choosing a telescope filter? Many people recommend using a neutral density filter, also known as a moon filter. The moon filters for telescopes darken and weaken the field of view. Therefore, when our close companion, the Moon, is in its dazzling full phase, it is beneficial to use the filter to observe it.
How To Pick The Telescope Filter?
As always, if you can try before you buy, then do it. Astronomical society events and large stargazing gatherings are great opportunities to experience first-hand the variety that different telescope filters can offer. Once you decide to buy, realize that you won’t always be able to purchase individual filters, especially the color filters, which are often sold in sets.
If you use many kinds of telescope filters, you may want to consider buying another accessory, a filter wheel. Some of the filter wheels are motorised and some are manual, but the basic premise is the same, they allow you to be able to switch filters directly without having to take off the eyepiece each time.
Please note that all the telescope filters discussed here are for night time use only. Under no circumstances should they be used to observe the sun, as they do not reduce the dangerous intensity of the sun’s rays. However, as with other objects in the sky, the appearance of the Sun can change when viewed through different (certified) telescope solar filters, but please do not use uncertified solar filters for solar observation.
How Many Types of Telescope Color Filters Are There on The Market?
Telescope color filters are used for observing planets. They are usually referred to by the Wratten number, similar to the style of “#1”. This style of writing is based on the original Eastman Kodak filter series, which came in 100 shades of color. For most astronomical observations, only a few shades are needed, the most useful of which are the following.
- 8 (yellow) – can be used to observe the cloud bands on Jupiter and Saturn.
- 25 (red) – can show details of the surface of Mars and the clouds of Venus.
- The #47 (purple) – is effective on Venus and enhances the Schrott effect.
- 58 (green) – used to improve red features such as Jupiter’s Great Red Spot.
- 80A (blue) – used to show Mars’ sandstorms and clouds, and Jupiter’s ring belt.
1. Ultra High Contrast Filters
Like narrow-band filters, ultra high contrast filters improve contrast, making the background sky darker and allowing deep sky targets to stand out better. Capable of allowing both hydrogen beta and oxygen III lights to pass through, ultra high contrast filters improve your observations of large scale nebulae compared to any other single wavelength narrow band filter.
2. Narrow Band Filters
As the name implies, narrow band filters only retain light of a specific wavelength – usually that emitted by bright emission nebulae or planetary nebulae. By blocking other wavelengths of light, such filters help to improve contrast and thus bring out details. Typical narrow-band filters include the hydrogen beta light filter and the oxygen III light filter.
3. Neutral Density Filters / Polarising Filters
Both filters reduce the glare of bright targets such as the Moon, Venus, Mars, Saturn and Jupiter. Neutral density filters reduce the intensity of all wavelengths and are particularly suitable for use when observing the Moon. Polarising filters generally adjust and control the amount of light.
4. Anti Aliasing Filters
Anti Aliasing filters help you to overcome the chromatic aberration effect often seen with achromatic refracting telescopes, the most common of which is a distinct blue or purple halo around a bright star. For this reason, these filters are sometimes referred to as “violet-cutting” filters. It can be used to observe any target.
5. Light Pollution Filters
Light pollution filters are designed to suppress the specific wavelengths of orange light emitted by sodium streetlights, which darken the background sky. This can help you better observe deep-sky objects, especially nebulae and galaxies, which are more easily obscured by light pollution than planets.