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Use of Filters

Many filters are available for mounting into an eyepiece , and the are well suited for prime focus imaging , eyepiece projection and afocal imaging . Only for "Camera Direct" are they not suitable . Astronomical filters for mounting directly on cameras are scarce in supply and therefore will not be further commented on here .

"Nebula Filter" - Hydrogen-Alpha .
I have succesfully used a "nebula filter" i.e. a Hydrogen-Alpha filter for solar video imaging , see the solar imaging gallery page . This filter is NOT a Solar Prominence filter , since in order to bring forth solar prominence detail an extremely narrow filter with a bandwidth in the order of one Angstrom is necessary . The nebula filter has a bandwidth of about 10 nm (about 100 times too wide) .

Infrared (IR) Blocking Filters
Since CCD chips are quite sensitive in a good part or the IR range an IR blocking filter is sometimes necessary in order to reduce the range of wavelengths exposing the CCD . One reason is the different refractive index of the lens (to different wavelengths) in a refractor . The second reason is turbulent "seeing" giving rise to different refractive index for different wavelengths , in turn leading to focussing problems . The same is true to some extent for all filters , although some coloured filters will simply enhance particular details in some types of objects .

Colour Images with Monochrome Cameras
You have only a monochrome camera and want to make colour images ? Use a set of coloured filters , red green and blue . Make one image each colour and combine the images with an image processing program on the computer . Do make sure the filters are parfocal or your images will not fit together . For images of Jupiter make sure to make the 3 images in a few minutes since the planet turns rather quickly . The main disadvantage when using filters is the loss of light , i.e. the exposure time will have to be increased . This is difficult with video imaging and only the brightest of objects can be imaged with a video camera . I would recommend imaging the Sun , Moon and planets including Jupiter , and Saturn if you have a telescope with a larger aperture , e.g. 200mm .

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Last update 21 January 2006 by Jan Andersen.
Background image : Video image of the Pleiades
Top caption : Lunar Eclipse 21-01-2000