33
Most pixels on a CCD build up dark current at a constant 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 easy to see in a raw CCD image as a
series of bright dots placed randomly around the image.
6- minute Dark Frame
Above is a 6-minute dark frame taken during the same imaging session with the above image of
M78. Notice the brighter pixels scattered randomly around the image.
Most imaging software will automatically scales the visible range of pixels to match the
underlying data. In the dark frame shown above the average pixel value is just 203 out of a
possible 16-bit dynamic range of 0-65,535. Seeing an automatically scaled dark frame or bias
frame can be a bit disconcerting for a new imager. Fear not, this “noise” will be almost
completely eliminated by subtracting a dark frame from your images.
Dark frames are subtracted from a light frame to remove the dark current from the image. This
subtraction removes the slight differences in dark current build-up from pixel to pixel along with
the larger variations caused by hot or dark pixels.
In general you’ll want to take at least 5 dark frames at each exposure used for your light frames.
You’ll get even better results with 9 or more dark frames. If all your light frames were taken with
5-minute exposures, you’ll need to collect a set of 5-minute dark frames. If you took both 5-
minute and 10-minute light frames, you’ll need a set of 5-minute dark frames and a set of 10
minute dark frames. There is a way to reduce the number of dark frames you collect by using a
set of bias frames but, in general, you’ll achieve the best results taking dark frames with the
same exposure as your light frames.
