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Long duration exposures (greater than a minute) will be affected by noise, know as dark
current, generated within the CCD camera itself. To remove this noise from your
final image you need to take what is called a dark frame (an image made at the same
ambient temperature and exposure duration of the original image but with the telescope
aperture covered or camera shutter closed). You then subtract this dark frame from your image as the first
step in image processing and the dark current noise
will be removed. The flip mirror finder is useful for
taking dark frames as you can achieve the same effect as covering the telescope aperture
or closing the camera shutter simply by rotating the finder mirror to the 45 degree position. With the mirror in
this position all light is blocked from entering the camera.
These are images taken against a field of even illumination (e.g. the evening or morning
sky when it is light enough not to show stars but dark enough not to saturate the pixels
quickly). These images can be used to correct for variations in
sensitivity across the CCD chip or for specs of dust on the chip, filters
or optics that can appear as dark doughnut
shaped shadows on the image after processing. My first images did not
use flat fields but I now use them to calibrate all my images. I did
not use flat fields initially because it was not always possible or easy to take a
flat field of the evening or morning sky during an imaging session.
What enabled me to take good quality flat fields easily was a lightbox.
This is an device which fits over the end of your telescope and uses a
standard lightbulb to provide the even field of illumination required for
taking flat fields. The advantage of using a lightbox is that you
can take the flat fields at any time during the imaging session and you do
not have to worry about stars appearing in the image and ruining your
flat. I constructed my lightbox myself using one built by Chuck Shaw as the basis for my design.
I made some slight modifications to his plan to ensure my lightbox fitted
over the end of my LX200 10 inch SCT. Take a
look at the the drawings of my lightbox plan. |
