CCD cameras are electronic cameras that
produce a digital output, much like that of a picture displayed on a computer screen. It
is comprised of pixels, which are the active sensing elements in the camera, much like the
little individual elements of an insect eye. Each pixel will record the quantity of light
that falls on it and deliver the output to a computer connected to the camera. CCD cameras
have thousands of pixels arranged in a grid which each act as an individual light detector
and produce a digital output proportional to the quantity of light that falls upon it.
Displaying these as a grid of points on the computer reveals an image of the area covered
by the pixel grid in the camera. CCD cameras are typically at least ten times more
sensitive than film, thus are capable of really impressive images in short exposure times.
There are a wide range of cameras available. Most of the major telescope manufacturers,
such as Meade and Celestron, produce CCD cameras, sometimes designed for their telescopes,
but they usually work on any telescope. There are also smaller dedicated manufacturers,
such as Starlite Xpress who produce CCD camera for all ‘scopes who have a deservedly
high reputation for their high quality cameras and after sales service.
A CCD camera can be used the same as a film camera as described above,
except that exposures are shorter and it is easy to image stars as points of light without
guiding. One disadvantage of CCD cameras is the small size of the imaging area, which may
be around 5mm by 4mm compared to the 35mmx x 24mm of the film in a 35mm camera. This means
that fields of view are small and apparent magnification is high. The price of CCD cameras
seems closely related to the size of the CCD chip !!
CCD cameras must be connected to a computer that can run the software
that comes with the camera. This will control the camera and take the exposure as well as
download the image for subsequent processing. This is not difficult and most software
nowadays is ‘Windows’ based and easy to use. Having the images electronically
allows lots of options. Even unguided, it is possible to take several short exposures
using a camera lens attached to the camera, which will not show trails. A low f ratio lens
will be needed to get a reasonable field of view. My experience with the Starlite Xpress
MX5 is that a 55mm lens from a 35mm camera gives a field of view around 6 by 4 degrees,
still quite small. Wide angle lenses in the 28mm focal length category or shorter will
give larger fields of view. Short exposures are not a problem and can be electronically
added together easily, often with the camera manufacturers’ software, to produce an
effective exposure of much longer duration. Some software, such as that with the Starlite
Xpress, will do this automatically for you. Thus good images can be taken easily for just
the cost of the CCD camera, but they don’t come cheap, typically running from £500
upwards.
If you now attach the CCD camera to the telescope using the camera
lens, as described above, then any reasonable telescope drive will permit imaging for many
minutes before tracking inaccuracies in the drive system become noticeable as star
trailing on the image (another real advantage of CCD camera is that if you do take a bad
image you just delete it and take another – there is no waiting for film to be
developed or indeed cost of the film !!). I find that a 3 minute exposure like this easily
reveals stars fainter than magnitude 13 !!! as well as showing star clusters and deep sky
objects as fuzzy blobs. The larger deep sky objects such as M31, the Andromeda spiral, and
M42, the Orion nebula, can look really impressive and show much detail.
Finally I must mention the fact that once you have your image
electronically it is possible to process and enhance it to show much detail that may not
be readily apparent. This is a real fun part of the hobby and one that can be done indoors
in the warm !! Discussion of this must wait until another time !!