Meade Image Processing Bedienungsanleitung PDF herunterladen (Seite 10)

The first issue you will face it is the contrast settings of an image. Each image is an

array of 2184

1472 pixels, each pixel is represented by a number 0, 1, 2,

…

, 65535,

corresponding to the amount of light recorded on that pixel. So a pixel with count 0 on it is

completely “black”, and a pixel with count 65,535 is completely “white”. When you open

an image, however, you can tell the software to make every pixel counting less than, say,

200 (you choose this number) to be “black”, and every pixel counting more than, say, 2000

(you choose this number) to be “white”. Pixels with counts between 200 and 2000 will be

various shades of gray. Because all pixels that are brighter than the white point, 2000, will

be the same shade of pure white, all detail vanishes on the bright part of the image. We say

that the bright part has been clipped. However, the contrast on the part of the image with

counts between 200 and 2000 has been greatly enhanced, and the noise below count 200 has

been cut off. This sort of a change in the black and white points is called a linear scaling.

Note and remember that in CCDOPS this linear scaling is not permanent, any such change

you make will only affect the way the image is presented on the screen, but will not really

change the image. You

can always reset the

scale and see again the

clipped part of your

image!

The images that the

CCD takes include dark

current. Each pixel

leaks electrons a little,

and you’ll notice that

the chip records light

even in total darkness.

This leak is due to temperature, and we battle it by cooling the chip as much as we can. The

longer the exposure, the more “false” light is recorded. The warmer the chip, the stronger

this dark current. On top of that, the dark current varies from pixel to pixel! The raw image

is very grainy and has a background haze. The way to get rid of this dark current is simple:

in addition to the regular light image, we take another, with the same device and the same

exposure time, same temperature, but the shutter shut – no light. This is called a dark frame.

The first step of any image processing is to take the light image, and subtract the

corresponding dark frame from it. CCDOPS will do this upon pressing of a few buttons.

Fig. 6: Clipping. The pixels in the middle of the nebula are around 3000.

In image on the left, the white point is set at 2000, which causes much clipping.

In the image on the right, the white point is set at 4000.

Fig. 7: Subtracting a dark removes the noisy dark current.

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Meade Image Processing Bedienungsanleitung Seite 10