CCD Cameras
Taking Images
CCD cameras use a photosensitive device known as a charged coupled device or
"CCD" chip. Integrated with the proper software and attached to a telescope this
tiny device has much higher sensitivity and resolving power than film.
Unlike film cameras, CCD cameras and equipment need power to operate and most
require a laptop computer. Dark sky locations are preferred for city-bound
amateur astronomers where these demands will shorten imaging sessions away from
the electricity supply. Planning for single or multiple imaging sessions and the
power requirements they need is as important as the clarity of the skies.
The following is an outline of a standard beginning "first light" session
with CCD cameras and a robotic telescope. Besides taking pictures, a primary
goal of any early CCD imaging session is a better understanding of the
equipment.
The process of CCD imaging involves all of the following
steps:
- Set up the telescope in equatorial mode and do as good a polar alignment as
possible. With the GPS equipped Celestrons, this process is greatly simplified.
- Set up your personal computer and attach it to the CCD cameras and telescope
(if using a planetarium package).
- Turn on the CCD cameras to begin the cooling process. Set the temperature so
that the camera's cooler should never operate above about 90%.
- Choose the object to be imaged and decide what imaging method and exposure
times will be used. For example: If you intend to image a bright object, then
shorter exposures stacked together will reduce blooming. (Blooming is a trait of
CCD cameras that is similar to over-exposure in film. Some CCD cameras support
anti-blooming that reduces blooming, but does not eliminate it.) If you're using
a Track and Accumulate(r), guiding is not necessary.
- If appropriate, attach a focal reducer or eyepiece and holder for eyepiece
projection.
- Attach the CCD cameras to the telescope.
- Point the telescope at a moderately bright star near the object to be imaged
and focus the CCD cameras. Once the bright star is focused, choose a fainter
star and fine-tune the focus.
- Make sure the position of the star and the position reading on the NexStar
hand control (or planetarium software) match. This will assure that after moving
to the object to be imaged it will be in the field of view of the CCD cameras.
- Calibrate the guider if applicable.
- If you use auto-dark-frame subtract, make sure the CCD cameras are setup for
that. If you intend to manage your own dark frames, take them and store them for
use later. If you intend to use one, take a flat frame.
- Take the image(s). Note: if you're using a color wheel, this will involve
taking at least one image per color.
- After closing down the telescope and CCD cameras, retire to your computer to
process the images. This may involve applying dark frames (if you did not have
the camera software do it automatically) and, if you took one, the flat frame.
Apply any histogram changes to adjust contrast.
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