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1. Why are CCDs better for recording data than the human eye?

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How does a CCD record an image compared to other methods, such as photographic film (4 marks)

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2. Why would an astronomer want to use a CCD when looking at stars in space? (3 marks)

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Why is it important for the light-sensitive pixels to be illuminated for a pre-set time?
(2 marks)

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Why are CCDs so expensive (2 marks)?

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Why is it more efficient for astronomers to use CCDs when recording images? Refer to the advantages of using CCDs over photographic film and the efficiency of CCDs (6 marks)

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Do you think a CCD pixel is more efficient than a photographic film grain and why?

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Do you think a CCD pixel is more efficient than a photographic film grain and why?

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So that the pixel charges are comparable since if more time passes, more charge will be collected and vice versa. Also you need to have enough time for the photons to liberate enough electrons.

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Although both CCDs and the human eye can pick up fast changing images, such as astronomical phenomena, CCDs can record these changes whereas the human eye is subject to each person.

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larger number of pixels in a set area so more expensive to make

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The light sensitive pixels become charged when exposed to light. They are connected to an electronic circuit which transfers the collected charge from each pixel in order to an output electrode. The electrode is connected to a capacitor. The voltage across the capacitor is recorded electronically, the capacitor is then discharged. The output electrode produces voltage impulses, each impulse is proportional to the light energy received. This allows for a colour to be assigned to each pixel.

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1) A CCD is more efficient than a photographic film grain because there will be a lot of photons illuminating the CCD. About 70% of the photons incident on a pixel liberates an electron, so the quantum efficiency of pixel is 70%. Whereas film grain only has 4% efficacy as 4 in 100 photons will liberate an electron.

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Because astronomers need to record the fast-changing astronomical changes to be further analysed, so CCDs make this easier by capturing these moments. Plus, CCDs can be made to have a larger number of pixels in a smaller area, so astronomers can cool them to very low temperatures to record more data from space.

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2) CCDs are used in many imaging applications, such as surveillance, hand-held and desktop computer video cameras, and document scanners.

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CCD's are able to record fast changing astronomical images, which can't be recorded on photographic film. It is also very sensitive to a wider range of wavelengths than humans. So it is suitable to obtain infrared images. It has a quantum efficiency of 70% from about 400 nm to 800nm.