Meade Deep Sky Imager, Руководство пользователя, Страница: 34 / 58

In oil painting, it is important to prepare the surface of the canvas to receive the paint. The unfinished surface of
the canvas is not conducive to the fine brushstrokes required for fine art. The surface may have fabric strings or
grainy bumps that will distort the finished picture. Canvas preparation is accomplished by applying oil gesso
with broad regular strokes in one direction. After drying for several days the artist sands the surface until
smooth and repeats the gessoing process as needed to achieve a smooth surface. Only then is the canvas
ready for the artist to begin.
A similar process is needed for the Deep Sky Imager which can be thought of as a canvas for a fine painting,
only in this case the canvas is the silicon surface of the imager. This silicon surface is uneven in a different way.
If the Deep Sky Image sensor is covered to block all light for a period of time and the resulting image examined,
you would expect it to be uniformly black, but it is not! This image, which is called a “dark frame”, is speckled
with a pattern of dots of varying intensities. The brightest dots are called “hot pixels” and all of these are created
by electrons leaking into the pixels due to heat or other means and not from exposure to light. This pattern is
very predictable and changes only in intensity with time. If this “dark noise pattern” is not removed from the
exposure it will add distortion to the picture.
Fortunately, because “dark noise” is predictable, it can be subtracted from an exposure leaving a clear image.
AutoStar Suite software performs these “dark noise” subtractions automatically for Meade imagers because they
transfer complete and uncompressed images to the computer.
Images from webcams can't apply “dark noise” subtraction. Why? Because they compress their data and are
unable to create a “dark frame” for correcting the image.
Some Words about Dark Frames
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