Complete Guide to Telescope Filters

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How astrophotography filters work

White light astrophotography (using a camera such as a DSLR or a colour CMOS sensor) is very popular. This is how most astrophotographers start in their field. However, because the photographer can more precisely control what hits the camera, better results can be gained by using filters.

This article explains what filters are, how they work and how they can be used in astrophotography.

What is a light filter?

Light is made up of photons, which vibrate at a range of wavelengths. The wavelength determines the colour of the light. For example, light consisting of photons with a wavelength of 540 nanometres appear to the human eye as green.

If you shine white light through a prism, it gets split into its component colours. What you see is a rainbow, with red down one end and violet up the other. There are no steps in the colour, it changes smoothly from one to another. Light which is visible to the human eye ranges from 400 nanometers (nm), which appears as a deep red, up to around 700nm, which is a vibrant purple.

How does a filter work?

Filters do their work by blocking some of the light that hits them, while letting other light through. Most filters discriminate on colour, meaning they let some colours through and block other colours.

Some filters block all wavelengths evenly, so what comes through is still white light, but dimmer. These are called “neutral density filters”.

Types of Astrophotography Filters

There are many types of filters that are used in astrophotography. This article is mainly about colour filters (both broadband and narrowband), that are intended for use with monochrome cameras. However, some filters are intended for colour cameras.

Filters for use on colour cameras

It should be noted that normal colour filters (for example a blue filter) can be used on colour cameras. The resulting image will simply appear blue, and will have to be processed slightly differently to the more common monochrome image.

Neutral density filters are also intended for use on colour cameras. Because they block all light evenly, the resulting image is simply darker than without the filter. This is useful for very bright targets such as the Sun or, to a lesser extent, the Moon.

Other types of filter intended for use with colour cameras include light pollution reduction filters and nebula filters. These work by blocking or reducing light of wavelengths (colours) that, for various reasons, the photographer doesn’t want.

Light pollution reduction filters reduce wavelengths produced by street lighting or other prevalent city lighting such as fluorescent lighting.

Nebula filters reduce mall wavelengths except for a number of colours that emission nebulas shine in. When using a nebula filter, the black background should appear blacker while the nebula should appear unchanged.

Filters for use on monochrome cameras

Most filters are intended for use with monochrome cameras. Monochrome cameras are different to colour cameras. Colour cameras have some pixels dedicated to red light, some to green light and some to blue light. Monochrome cameras have every pixel simply recording light – regardless of colour. For this reason, they tend to have higher resolution.

Broadband filters (red, green, blue)

As mentioned above, colour filters have the ability to block light of some colours and allow other colours through. For example, a red filter blocks purple, blue, green and some yellow light, while letting red light through. If you look through a red filter, everything will look reddish through it.

The diagram below shows red, a green, and a blue filters, and what happens when a full spectrum (that is, white light) is shone onto the filter. Each filter blocks some colours, but still lets a lot of wavelengths through.

Because red, green and blue filters allow a lot of light through, these are called "broadband filters".

Narrowband (hydrogen, oxygen, sulphur)

A "narrowband" filter works in the same way, the only difference being that it discriminates much more accurately, letting significantly fewer wavelengths through.

For example, a hydrogen alpha filter only lets light of about 656nm through. This is a deep pinkish colour. The filter blocks everything else.

When you hold a hydrogen alpha filter up and look through it, the filter appears very dark. It lets so little light through that what you see is nearly black. If you look at a bright object such as a strong light, you should see dim shapes.

Similarly, an oxygen filter only allows light of about 500nm (a teal colour) through, and a sulphur filter only lets light of about 672nm (another red) though.

These three are the most popular narrowband filters, and are shown in the diagram below.

These filters are named after elements because they let through light that is emitted by these ionised elements in space. The most common gas in nebulas is hydrogen, but there are always other gases present. Photographing through these filters enables the astronomer to identify and highlight the different gases. In addition, absorption nebulas (where the gas is in front of the light source) and reflection nebulas (where the gas is behind the light source) show up differently in narrowband photos.

Further, because there are clear separations between the colours that these filters allow through, the fine details in one colour end up distinctly different from fine details in the other colours. This gives narrowband photographs of nebulas an additional sharpness.

Finally, there are several other narrowband filters, such as Nitrogen II and Hydrogen beta. This means that by using different filters, the photographer can highlight different aspects of the target, bringing out different swirls of nebulas.

Complete List of Telescope Filters

Filter	Broadband/Narrowband	For Viewing or Astrophotography?	Intended Use	Shop
Moon Filter	Broadband	Viewing	Moon The Moon is very bright, especially a full Moon through a large aperture telescope. A Moon filter is just like a pair of sunglasses for your scope - they dim down the excessive light from the Moon so that you get a better contrast. Very often they introduce a coloured cast over the surface, normally a green colour. However, your brain ignores the colour after a while.
Neutral Density Filter	Broadband	Viewing	General Purpose Similar to a Moon Filter, a Neutral Density Filter darkens all colours equally, meaning there is no colour cast. It is often used interchangeably with a Moon filter.
Solar Filter	Broadband	Viewing	Sun A version of Neutral Density Filter used specifically for viewing the Sun. A Solar Filter must be specifically designed for your telescope (or an experienced user can DIY using solar safety film) and is used to darken the solar disk without reducing or enhancing any specific colour. You must use a solar filter for any solar observation.	saxon Skywatcher Celestron
UV - IR Cut Filter	Broadband	Astrophotography	General Purpose This is a filter that cuts out light both above and below frequencies visible to humans. While you can't see them, these frequencies are very visible to your camera, meaning bright lights in the frame (that is, the stars) are very much brighter than your perception. So to take a photo that reveals a background nebula, especially one that shines in visible wavelengths, you need to overexpose the stars, making them bright, white and bloated. Adding a UV/IR cut filter makes your camera see things in the same way humans do, and so taking a photo through the UV/IR cut filter turns the brightness of the stars down relative to the background nebula. This filter is suitable for use with colour astroimaging cameras.
RGB Filters (Red, Green, Blue)
Red	Broadband	Astrophotography	This is a red coloured filter, allowing a monochrome camera to isolate red (and near-red) frequencies, with the intent to re-combine the colours into a colour image. Suitable for use with monochrome astroimaging cameras.
Green	Broadband	Astrophotography	This is a green coloured filter, allowing a monochrome camera to isolate green (and near-green) frequencies, with the intent to re-combine the colours into a colour image. Suitable for use with monochrome astroimaging cameras.
Blue	Broadband	Astrophotography	This is a blue coloured filter, allowing a monochrome camera to isolate blue (and near-blue) frequencies, with the intent to re-combine the colours into a colour image. Suitable for use with monochrome astroimaging cameras.
Colour Planetary Filters
No. 12 / #12 Filter	Broadband	Viewing	Mars This is a yellow filter, and is designed to increase the contrast of similar colours seen on planets. Yellow is useful for enhancing atmospheric dust clouds on Mars.
No. 21 / #21 Filter	Broadband	Viewing	Moon This is an orange filter, and is considered most useful for the Moon.
No. 23A / #23A Filter	Broadband	Viewing	Mars This is a light red filter, and is designed to increase the contrast of similar colours seen on planets. A light red filter works well for Mars, enhancing contrast on surface features.
No. 25A / #25A Filter	Broadband	Viewing	Mars This is a red filter, and is normally used to enhance surface features of Mars. It can also be useful for separating cloud types on Venus.
No. 56 / #56 Filter	Broadband	Viewing	Mars This is a green filter and useful for enhancing dust clouds on Mars.
No. 80A / #80A Filter	Broadband	Viewing	Jupiter, Saturn This is a blue filter, and is designed to increase the contrast of similar colours seen on planets. Blue filters seem to improve contrast and detail on cloud band on Jupiter and Saturn.
No. 82A / #82A Filter	Broadband	Viewing	This is a light blue filter and is useful for enhancing the polar ice caps on Mars.	No. 82A
Hydrogen Alpha (Ha) Filter	Narrowband	Astrophotography	This is a common type of narrowband filter for astrophotography. It cuts out all light above and below 656 nanometres, just allowing through light from ionised types of Hydrogen in nebulas
Sulphur II (S II) Filter	Narrowband	Astrophotography	This is a common type of narrowband filter for astrophotography. It cuts out all light above and below 672 nanometres, just allowing through light from ionised types of Sulphur in nebulas
Oxygen III	Narrowband	Astrophotography	This is a common type of narrowband filter for astrophotography. It cuts out all light above and below 496 nanometres, just allowing through light from ionised types of Oxygen in nebulas
Oxygen III / Nebula Filter	Narrowband	Viewing	Also known as a nebula filter, this works in a similar way to the Oiii photographic filter, only with a wider frequency tolerance which means it is visually brighter for visual use. It will cut out a lot of extraneous light (both from light pollution and other elements) to darken the background of some emission nebulas.
CH4 (Methane) Filter	Narrrowband	Astrophotography	This is a filter especially useful for photographing Jupiter. It shows the heights (rather than the colours) of the cloud formations on the planet. This filter is suitable for astrophotographers using monochrome cameras.
Semi APO Filter / Fringe Killer Filter	Narrowband	Visual / Astrophotography	This is a filter that is used with doublet refractors to lessen the colour fringing effects of chromatic aberration. It does this by attenuating in the blue part of the spectrum, which limits the blue fringes around the brightest stars in the field of view.
Duo-Band Filter	Narrowband	Astrophotography	This is a filter that allows through light from both Hydrogen alpha and Oxygen III frequencies. Used with a colour camera, it produces a narrowband-like image of a nebula. This filter is suitable for astrophotographers using monochrome and colour cameras.
Tri-Band Filter	Narrowband	Astrophotography	This is a filter that allows through light from Hydrogen alpha, Hydrogen beta and Oxygen III frequencies. Used with a colour camera, it produces a narrowband-like image of a nebula. This filter is suitable for astrophotographers using monochrome cameras.
U-Filter	Narrowband	Astrophotography	Venus This is a filter that centres on 320-380nm – violet and ultraviolet. This allows imaging of cloud structures on Venus. This filter is suitable for monochrome astroimaging cameras.
UHC/LPR Filter	Narrowband	Viewing / Astrophotography	General Purpose and Galaxies This is an “ultra-high contrast – light pollution reduction” filter. It cuts UV and IR, as well as a broad range of frequencies associated with light pollution between 520 and 625 nanometers. It is intended to darken the background sky when viewed from city areas without affecting the light from nebulas.
Neodymium Filter	Narrowband	Viewing / Astrophotography	General Purpose This filter cuts UV and IR light and reduces moonlight and light pollution. It reduces yellows while allowing blue and red.