HAM Radio -
Taiji Quan -
Video and Webcam -
Digital Cameras (compact or DSLR
Photographic Film -
PrimeFocus and Eyepiece Projection -
Afocal Imaging -
Piggyback and Camera Direct -
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 .
HAM Radio -
Taiji Quan -
Last update 21 January 2006 by Jan Andersen.
Background image : Video image of the Pleiades
Top caption : Lunar Eclipse 21-01-2000