User guide

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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.
Note: The pixel values in this image have been stretched significantly to show the
variations in the dark frame. In reality this image is almost completely black with
perhaps a few hundred “hot” pixels. This is completely normal and a natural
consequence of how CCDs are manufactured.
MaxIm LE 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.