Q S I 5 0 0 S E R I E S U S E R G U I D E
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Reducing noise in CCD images
CCD imagers have developed a standard set of calibration techniques to reduce or
eliminate different types of noise from CCD images. Calibrating CCD images requires
taking some special kinds of exposures that are then applied to the “light frames” taken of
re called Dark Frames, Flat Fields and Bias Frames.
software help gather these extra frames. After
u to calibrate your images either automatically or
ames should be collected during each imaging session with the CCD at
mes. This will ensure the best possible
imagers plan their night of observing to begin
oaches. That way, you don’t waste precious
the night sky. The calibration frames a
MaxIm LE and other CCD camera control
the frames are gathered, MaxIm allows yo
manually.
All the calibration fr
the same temperature used for the light fra
calibration of the final images. Many CCD
taking the calibration frames as dawn appr
dark time.
The image above is a single raw 6-minute image of the diffuse nebula M78 in Orion. Some bright stars are clearly
visible along with some nebulosity but there are also scattered bright spots around the image caused by “hot” pixels.
ild
easy
e image.
Dark Frames
Dark frames are used to subtract the build up of dark current from a CCD image. Dark
current is caused by heat. Similar to how CCDs convert the energy from a photon into a
stored electron, CCDs also convert the energy from heat into stored electrons. CCDs bu
up “dark current” whether the CCD is being exposed to light or not. The rate that dark
current builds up is dependent on the temperature of the CCD and can be dramatically
reduced by cooling the CCD. Dark current builds up more slowly as the temperature of the
CCD is reduced.
Most pixels on a CCD build up dark current at a fairly consistent rate but that rate will vary
slightly from pixel to pixel. A subset of the pixels in a CCD will build up dark current at a
dramatically different rate from the average. These pixels are called “hot pixels” or “dark
pixels”. Hot pixels and dark pixels are both the result of slight imperfections introduced into
the silicon substrate of the CCD during the manufacturing process. Hot pixels are very
to see in a raw CCD image as a series of bright dots placed randomly around th