QSI 520c Guía de usuario descargar pdf (Pagina 32)

QSI 500 SERIES USER GUIDE

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

25% are covered in a red filter. This

ed because the

set of

you

Signal versus noise

For an astronomer, “signal” is the photons coming an

ideal world, there would be steady stream of phot

photon striking a pixel would be converted into ex

number of electrons would be precisely counted a

photographer exactly how much light struck each

converting light to pixel values in a CCD image is ical

laws and other factors that introduce “noise” into a

pixel values that make the image a less than exac

Noise in CCD images can manifest itself in multipl

background areas, “hot” pixels, faint horizontal or

signal areas of the image, blotchy gradients between darker and lighter regions in a nebula,

a gradient from dark to light from one corner or sid of an image to the other, and especially

as low contrast images — the result of a reduced signal to noise ratio. Achieving high

s from a cooled CCD camera requires a basic

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

arrangement is us

human eye is most sensitive to green

light. The green pixels correspond to

luminance and record the greatest detail

while the red and blue filters record

chrominance.

After the raw image is read from the

CCD, a demosaicing algorithm must be applied to the image to produce a complete

red, green and blue images by interpolating the missing pixel values. This is exactly what

normal digital cameras do, but it’s all hidden inside the camera’s electronics. You only see

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

can directly manipulate the raw image to, for instance, vary the color balance.

Single-shot color models offer the easiest way to take color images of the night sky. The

trade off is reduced QE and detail because of the demosaicing and pixel interpolation.

from the stars in the night sky. In

ons from every bright object and every

actly one electron in the CCD. Then the

nd converted to a number telling the

pixel. Unfortunately, the process of

governed by some fundamental phys

n image. Noise is unwanted variations in

t representation of the original scene.

e ways, including “graininess” in darker

vertical lines that become visible in low

dynamic range, low noise image

understanding of how CCDs work and the different sources of noise that can reduce the

quality of your images.

Courtesy Wikipedia

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QSI 520c Guía de usuario Pagina 32

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