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