Which Astronomy Filters To Use For Astrophotography and Observation

Astronomy filters are used to eliminate unwanted light and bring out fine details of an object, be it planetary, lunar of deep space.

Monochrome CCD cameras require the use of astronomy filters to create a colour image. When using filters with CCD cameras you would normally take exposures to capture the red, green and blue frequencies separately, followed by an IR block filter which acts as Luminescence (or brightness), a full-colour photograph can be then created with mono cameras by combining the images.

There are two types of filters used for astronomy - observational and imaging. Both are suitable for observations, however, the imaging astronomy filters have an additional IR-UV blocking layer which can prevent these frequencies from being recorded by CCD and CMOS imaging devices which are sensitive to the IR-UV frequencies. Observational astronomy filters can be fitted with a separate UV-IR block filter.

Astronomy Filters are catalogued by their Wrattan number, after Frederick Wrattan who invented this way of indexing them.

Filters come in 1.25" or 2" sizes and will screw into the focuser end of the eyepiece. Astronomy filters can also be screwed into other filters.

Wrattan Astronomy Filters

Wratten #11 is a yellowish-green colour. In photography, it is used for colour correction, and in astronomy it is used to bring out surface details on Jupiter and Saturn.

Wratten #12 is a deep yellow and acts as a "minus blue" filter. It will enhance the red/orange details of Jupiter and Saturn as well as lighten the red/orange surface details on Mars. As the filter blocks blue and slightly green, it will increase the contrast of Martian surface details.

Wratten #15 is a very deep yellow or amber colour. It will darken the sky in black and white outdoor photography, and further bring out Mars's surface details, including the Martian Ice Caps, as well as enhance the rings of Jupiter.

Wratten #21 is orange and behaves very similar to #15, but with more contrast. When used on Venus it will show clouds in Venus's lower atmosphere. It can also be used to bring out the Great Red Spot on Jupiter and should improve the contrast of the dark Martian albedo features.

Wratten #23A is a light red filter good for use on Mars, Jupiter, and Saturn. It can also be used for daylight observation of Mercury and Venus as it will increase the contrast between the planet and the blue sky. It does have a low light transmission though (25%) so it isn't good for smaller aperture telescopes.

Wrattan #47 is missing from my collection, but its main use is to bring out the clouds in Venus's upper atmosphere and to enhance the contrast in Saturn's rings. It only allows a small amount of the light gathered to pass through, so it is not suitable for small aperture telescopes (below 6 inches).

Wratten #56 is light green and is also good for Mars as it will enhance the polar ice caps and the yellow of dust storms on the surface.

Wrattan #58 is also missing from my collection. It is a green filter, and its main use is to enhance the contrast in the polar regions of Mars. You can also use W#47 if your telescope has a large enough aperture.

Wratten #80A is blue and probably the most often used filter. It is excellent for separating the belts of Jupiter and the surface detail of Saturn's polar regions as well as lunar observation.

Speciality Astronomy Filters

Variable Polariser / Moon Filter

Variable polarisers are for visual observation and used to adjust the brightness of a bright object such as the Moon, which can be dazzling through a telescope when full. The two sections swivel and vary transmission between 90% and 10%. Neutral Density filters also serve the same purpose but have fixed transmission.

Light Pollution Filter / LPR

Light Pollution Filter is my most used filter. This filter is fantastic for reducing sky glow caused by street lighting and greatly increases the length of exposure before the photograph is washed out. Compare the "before and after" images below of the Great Orion Nebula with and without a light pollution filter.

The filters have a strange mirror finish, in some lights it is silver and mirror-like, while in other lights it is blue/purple/red "flip" and translucent. Looking through the filter by eye and everything takes on a green hue.

The comparison samples below were both taken with a Canon 350d, Skywatcher StarTravel 102 @ Prime, 30s @ ISO1600.

Solar Filters

Solar Filter is ESSENTIAL for viewing the sun. You should not even consider using any other filter, other than one specifically designed for the sun. These filters are good for "white-light" observation - sunspots and limited granulation.

Baadar Film for Solar Viewing

Baader film can be used for solar observations, however, this will cause the image to appear monochrome as all colour is removed. This type of filter is usually supplied as a sheet or film which you can use to make solar glasses or build a solar filter.

Thousand Oaks Solar Filter

This Thousand Oaks filter eliminates 99.9% of the Sun's energy, making it suitable for solar observations. The filter will retain a Yellow-Orange appearance of the Sun's surface. They are full-aperture filters which fit over the primary lens or telescope opening.

Both types of filter will reveal sunspot detail of the sun's photosphere and photographs will start to show more granulation. Solar continuum filters improve solar granulation by transmitting light only in the 540nm portion of the sun's light spectrum, which is free from emission and absorption lines. If you are going to take images, an infrared blocking filter can be used in combination with the Baader solar continuum filter.

Hydrogen Alpha
A hydrogen-alpha filter is an optical filter designed to transmit a narrow bandwidth of light generally centred on the H-alpha wavelength. They are characterized by a bandpass width that measures the width of the wavelength band that is transmitted. These filters are VERY expensive, but if you want to observe and photograph solar prominences, this filter is required.

OxygenIII (OIII)
Filters out all wavelengths apart from those in the doubly ionised oxygen wavelengths. Suited only for emission nebulae where the predominant emission is OIII such as the Veil, Ring and Dumbbell nebulas. Can also be used to resolve double stars where one is much brighter than the other such as Antares.

Hydrogen Beta
Also known as the Horsehead Nebula filter, H-Beta filters isolate the hydrogen-beta line of the spectrum (486nm) in a narrow pass-band just 9 nm wide. The result is extreme contrast between the black background of space and the delicate Hydrogen - Beta emission of extended nebulae. Particularly effective when used on the Horsehead, Cocoon and California Nebulae.

Comet Filter
Comet Filters are a narrow bandpass system (25nm) which isolates the 501nm OIII line and both Cyanogen lines at 511nm and 514nm. The high contrast gain of the filter allows you to see comets to their full extent. The Comet Filter also helps you to better distinguish gaseous comets from dusty comets which normally show little contrast gain.