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6. Learning how to use the camera

● The imaging math

In recent years the price of CCD cameras dropped dramatically. Before that, only the

leading research laboratories could afford them, but now even serious amateur astronomers

can buy and learn to use CCD’s.

SBIG (Santa Barbara Research group, a private company) emerged as the industry

leader in the only-a-few-thousand-dollar category CCD market. Their ST-10 model (the one

we have at Ole Miss, see Fig. 1) presently costs $8,000 and uses one of the most sensitive

chips on the market. The image size is 2184 × 1472 pixels, which is in total 3.2 megapixels.

Most importantly for astronomy, it has a built-in cooler that may cool it more than 30°C

below ambient temperature (the temperature of the air around it).

The biggest enemy of astronomical images is noise. A noisy image appears grainy, and

the noise covers the fine details of faint objects. There are three main sources of noise in

images: (i) light pollution, (ii) dark current, and (iii) readout noise.

A CCD chip will collect some electrons even in the absence of light, because of the

thermal motion of electrons in it. This is called dark current. The image will not be

completely black even if there is no starlight, and the longer the exposure, the more “haze”

accumulates on the image. As we have seen it, this background can be subtracted with

image processing software if we take two images: one with the shutter open (light frame)

and one with the shutter shut (dark frame). The computer is instructed to take the difference

light frame minus dark frame, and we are rid of the dark current! However, the dark current

is not quite steady: it fluctuates a little, and these fluctuations constitute noise, which cannot

be removed at all. In other words, the dark current on the light image is not exactly identical

to the dark current on the dark frame, and the difference stays in the image as noise.

The noise due to this fluctuation of the dark current can only be reduced by cooling the

CCD chip. There is great reduction in noise from cooling: every time the chip is cooled 7

the noise is cut in half. We should cool the camera as much as we can. The ability to cool

the camera 35

C below air temperature cuts the noise 16 times, which is a very great

improvement over regular digital cameras. In practice, noise will seriously hurt images if

the chip is warmer than -25

C (but -30

C to -35

C is preferable when narrowband filters

are used). This is achieved by double cooling: icy water is pumped into the camera to keep

it under 10

C, then the thermoelectric cooling can cool off another 35

C below that.

Each individual pixel on an image is represented by a number, the number of electrons

collected from that pixel. No electron means no light, completely black. Many electrons

mean a lot of light, a bright pixel. A full image is represented by 2184 × 1472 of these

numbers. Each pixel can only hold only so many electrons (full well capacity), and if there

is more light hitting a pixel, it cannot produce a larger number of electrons. Worse, some of

the extra electrons will spill over into neighboring pixels, resulting in a large blob for a star.

In practice any pixel holding more than 40,000 electrons is full, the pixel is said to be

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