Filters and there Uses
One of the most useful, and some would say essential, part of every astronomers toolkit are filters. They can eliminate parts of the spectrum that you do not wish to view or image - infra red (IR), ultra violet (UV) or ever street light!
By eliminating unwanted frequencies you can bring out the detail of the object, be it planetary, lunar of deep space. For every different object in the sky, there is a filter to observe it with.
Monochrome CCD imagers require the use of filters (LRGB) in order to create a colour image. By using filters to capture the Red, Green and Blue frequencies followed by an IR block filter which acts as Luminescence (or brightness), a full colour photograph can be created with mono cameras.
There are two types of filters used for astronomy - observational and imaging. Both are suitable for observations, however the imaging filters have an additional IR-UV blocking layer which can prevent these frequencies being recorded by CCD and CMOS imaging devices which are sensitive to the IR-UV frequencies. Observational filters can be fitted with a separate UV-IR block filter.
Coloured filters of both types are named by their Wratten number.
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Filters come in 1.25" or 2" sizes and will screw into the focuser end of the eyepiece. Filters can also be screwed into other filters. |
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Wratten #11 is a yellowish-green colour. In photography it is used for colour correction, and for astronomy it is used to bring out surface details on Jupiter and Saturn. |
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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 lightening the red/orange surface details on Mars. As the filter blocks blue and slightly green, it will increase contrast of Martian surface details. |
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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 surface details, including the Martian Ice Caps, as well as enhancing the rings of Jupiter. |
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Wratten #21 is orange and behaves very similar to #15, but with more contrast. Can also be used to bring out the Great Red Spot on Jupiter. |
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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. |
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Wratten #56 is light green is also good for Mars as it will enhance the polar ice caps and the yellow of dust storms on the surface. |
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Wratten #80A is blue and probably the most often used filter. It is excellent for separating the belts of Jupiter and surface detail of Saturn's polar regions as well as lunar observation. |
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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. |
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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 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 greeny hue. |
Canon 350d, StarTravel 102 @ Prime 30s @ ISO1600 - No Filter. |
Canon 350d, Skywatcher StarTravel 102 @ Prime 30s @ ISO1600 - Antares ALP Filter. |
Specialist and Miscellaneous Filters |
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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. This Thousand Oaks filter eliminates 99.9% of the suns energy, making it suitable for solar observations. The filter will retain a Yellow-Orange appearance of the suns surface. They are full aperture filters which fit over the primary lens or telescope opening. These filters are good for "white-light" observation - sunspots and limited granulation. Baader film can also be used, however this will cause the image to appear monochrome as all colour is removed. DO NOT look at the Sun with magnifying glasses, cameras, binoculars or telescopes, or any optical instrument in fact, without the use of a properly designed, approved and tested filter or specialised instrument. Use only materials and instruments designed for the specific purpose for viewing the Sun. DO NOT LOOK DIRECTLY AT THE SUN. Instant blindness will be the result. |
| 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 band pass 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 at their full extent. The Comet Filter also helps you to better distinguish gaseous comets from dusty comets which normally show little contrast gain. |
