Astronomy compels the soul to look upwards and leads us from this world to another.

Astronomy Filters

Using different astronomy filters to enhance views of planets

By , 1st May 2009 in Astrophotography

Astronomy filters are used to eliminate unwanted light and bring out fine details of an object, be it planetary, lunar of deep space. Every object has a filter to best enhance observations.
 

Monochrome CCD cameras require the use of astronomy filters in order 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 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 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.

Most filters screw into an eyepiece
Most filters screw into an eyepiece

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 Filters

Wratten #11

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.

Wratten #12

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.

Wrattan #15

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.

Wrattan #21

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

Wrattan #23a

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 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).

Wrattan #56

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.

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.

Wrattan #80a

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.

Speciality Astronomy Filters

Antares Variable Polariser

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

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.

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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.

No Filter
With LPR Filter
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.

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.

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.

Further Reading
Comments
  1. Rick @ Leupold Binoculars
    Rick @ Leupold Binoculars

    I really enjoy reading your blog! I always learn something new.

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