QSI 520c Guía de usuario descargar pdf (Pagina 34)

QSI 500 SERIES USER GUIDE

red, green and blue filters. The resulting images are then combined using computer image

processing programs into a final color image.

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

light. The green pixels correspond to

luminance and record the greatest detail

while the red and blue filters record

hrominance.

te set of

ce, vary the color balance.

After the raw image is read from the

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

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 se

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 instan

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.

Signal versus noise

For an astronomer, “signal” is the photons coming from the stars in the night sky. In an

ideal world, there would be steady stream of photons from every bright object and every

photon striking a pixel would be converted into exactly one electron in the CCD. Then the

number of electrons would be precisely counted and converted to a number telling the

photographer exactly how much light struck each pixel. Unfortunately, the process of

converting light to pixel values in a CCD image is governed by some fundamental physical

laws and other factors that introduce “noise” into an image. Noise is unwanted variations in

pixel values that make the image a less than exact representation of the original scene.

Noise in CCD images can manifest itself in multiple ways, including “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,

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

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

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