Telescope filters: a beginner’s guide

What types of filter you can buy and how each type alters your view of the night sky.

A good selection of coloured filters is a useful addition when observing Mars. Credit: Pete Lawrence

Just as a telescope reveals parts of the Universe that can’t be seen with the naked eye, filters will help you to spot sights that resist detection even with a telescope. Filters are definitely worth splashing out on in order to get the most from your observing sessions.

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A telescope’s job is to grasp as much light as possible, but filters add a further barrier between your eye and the sky.

You’ll lose some valuable light, which makes your view slightly fainter, but the trade-off for seeing something new or in greater detail is always worth it.

There are four main filter types that can enhance your views: solar, lunar, planetary, deep sky.

More equipment advice:

There are two types of solar filter: eyepiece and full aperture. You may encounter the first with cheap telescopes, and they are extremely dangerous.

These filters are made of glass and screw into the bottom of an eyepiece, close to where your telescope focuses all the light from the Sun. There is a risk that the glass will crack or shatter from this heat, delivering a blinding dose of light.

What to look for on the Sun with three types of solar filter. Credit: Pete Lawrence
What to look for on the Sun with three types of solar filter. Credit: Pete Lawrence

Full aperture solar filters, the second type, are a far safer alternative. They are typically made of glass or a special flexible film, and completely cover the end of a telescope’s tube.

Thus, the amount of heat, light and ultraviolet radiation that enters your scope in the first place is dramatically reduced. These filters still have to be used with care and attention, however; solar observing is about as dangerous as stargazing gets.

Lunar filters decrease light across all visible wavelengths. They work in the same way as sunglasses – the Moon can really dazzle through a scope if its light isn’t dimmed.

There are two types, the first of which is the ‘neutral density filter’. These filters are available with varying  transmissions – in other words they darken more or less of the lunar surface.

The other type combines two polarising filters in one unit, which allows you to vary the amount of light that passes through it by manually adjusting one of the polarisers.

Through a blue filter, Jupiter’s Great Red Spot appears much more distinct. Credit: Pete Lawrence
Through a blue filter, Jupiter’s Great Red Spot appears much more distinct. Credit: Pete Lawrence

Colour filters

Planetary filters are a single colour and are marked with the same Kodak-Wratten numbers as photographic colour filters.

They work by filtering all of the colour out of a scene except for that of the filter itself, which improves contrast.

For novice planetary observers, the single-colour view can look quite unusual. You have to try and tell your mind to ignore the colour wash and focus on the detail visible!

Credit: Light pollution from towns and cities can have a detrimental effect on our views of the night sky. Credit: Dneutral Han / Getty Images
The effects of light pollution can be diminished with the use of deep-sky filters. Credit: Dneutral Han / Getty Images

The final group is deep-sky filters. Also known as light-pollution filters, they are used to eliminate the background glow of street lighting while enhancing the detail in faint nebulae.

Deep-sky filters are specially coated to reduce reflections as much as possible. Unlike the Moon and planets, which are quite bright, to get the best views of nebulae you really want to lose as little light as possible. Hence, these filters are the most complex and most expensive.

Lunar, planetary and deep-sky filters can be safely screwed onto the nosepiece of an eyepiece as they won’t be dealing with the intense light typical of solar observing.

You’ll find them in two diameters: 1.25-inch or 2-inch, to fit the standard sizes of eyepieces.

How telescope filters affect your view

Lunar: neutral density filter

The Moon with a filter (left), and without a filter (right). Credit: BBC Sky at Night Magazine.
Left: with a neutral density filter. Right: unfiltered. Credit: BBC Sky at Night Magazine.

Features such as craters, rilles and mountains can be difficult to see due to the dazzling appearance of the Moon. Fitting a neutral density filter will allow you to enjoy the lunar surface in comfort. It will also help with contrast, making the features stand out more.

Deep-sky: ultra-high contrast

The Eagle Nebula with a filter (left), and without a filter (right). Credit: BBC Sky at Night Magazine.
Left: with an ultra-high contrast filter. Right: unfiltered. Credit: BBC Sky at Night Magazine.

This is a great all-round filter for wispy deep-sky objects. It picks out two oxygen lines and one hydrogen line that are produced by emission and planetary nebulae. This will reveal magnificent detail in the Orion, Lagoon Eagle nebulae, the latter pictured above.

Planetary – red filter

Mars with a filter (left), and without a filter (right). Credit: BBC Sky at Night Magazine.
Left: with a red filter. Right: unfiltered. Credit: BBC Sky at Night Magazine.

A red filter clearly brings out the detail in the markings on Mars. Other coloured filters can be used depending on what you want to see more clearly. Green, for example, would darken the planet’s surface, which would improve your view of the polar caps.

3 telescope filters worth investing in

1

Light pollution (deep sky)

Light pollution filter for deep-sky observing. Credit: BBC Sky at Night Magazine

These will drastically improve your view of the heavens from an urban area by cutting out the main types of lighting and the resulting glow.

2

Neutral density, 25% (lunar)

Neutral density filter for lunar observing. Credit: BBC Sky at Night Magazine

This will reduce the reflected glare of the Moon by 25%. This not only allows a more comfortable view but also increases contrast.

3

Colour (planetary)

Colour filter for planetary observing. Credit: BBC Sky at Night Magazine

Colour filters bring out planetary features – in this red, green and blue filtered image, Jupiter’s atmosphere takes on a new dimension.

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This article originally appeared in the May 2012 issue of BBC Sky at Night Magazine.