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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Single-shot color CCDs, like those found in almost all general use digital cameras, are
made by placing red, green and blue filters over adjacent pixels in the CCD. The image
processing program then has to separate the three different color images and recombine
them into a single color image.
Single-shot color CCDs use a “Bayer
filter” with alternating red, green and blue
pixels covering adjacent pixels in a
checker board pattern as shown in the
image to the right.
50% of the pixels are covered in a green
filter, 25% are covered in a blue filter and
25% are covered in a red filter. This
arrangement is used because the
human eye is most sensitive to green
ixels correspond to
ead from the
t of
at
ly see
off
For an astronomer, “signal” is the photons c
ideal world, there would be steady stream o
photon striking a pixel would be converted i
e
number of electrons would be precisely cou
photographer exactly how much light struck
converting light to pixel values in a CCD im
laws and other factors that introduce “noise
s in
pixel values that make the image a less tha
Noise in CCD images can manifest itself in mu
ng “graininess” in darker
background areas, “hot” pixels, faint horizontal or vertical lines that become visible in low
signal areas of the image, blotchy gradients between darker and lighter regions in a nebula,
one corner or side of an image to the other, and especially
light. The green p
luminance and record the greatest detail
while the red and blue filters record
chrominance.
After the raw image is r
CCD, a demosaicing algorithm must be applied to the image to produce a complete se
red, green and blue images by interpolating the missing pixel values. This is exactly wh
normal digital cameras do, but it’s all hidden inside the camera’s electronics. You on
the final processed image. With a CCD camera, the raw image is read into the camera
control program and then processed on your computer. This has the advantage that you
can directly manipulate the raw image to, for instance, vary the color balance.
The 520ci and 520sci are the two single-shot color models in the QSI 500 Series. Single-
shot color models offer the easiest way to take color images of the night sky. The trade
is reduced QE and detail because of the demosaicing and pixel interpolation.
Signal versus noise
oming from the stars in the night sky. In an
f photons from every bright object and every
nto exactly one electron in the CCD. Then th
nted and converted to a number telling the
each pixel. Unfortunately, the process of
age is governed by some fundamental physical
” into an image. Noise is unwanted variation
n exact representation of the original scene.
ltiple ways, includi
a gradient from dark to light from
as low contrast images — the result of a reduced signal to noise ratio. Achieving high
dynamic range, low noise images from a cooled CCD camera requires a basic
understanding of how CCDs work and the different sources of noise that can reduce the
quality of your images.
Courtesy Wikipedia