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Light Detectors

Light Detectors. Quantum Efficiency and CCDs. Light Detectors. People are limited – not all of us can draw this well:. Left: The 3 rd Earl of Rosse’s drawing of galaxy M51 (The Whirlpool), 1845. Right: A photograph of it this year. Mechanical image collection (from mid-19 th Century).

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Light Detectors

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  1. Light Detectors Quantum Efficiency and CCDs

  2. Light Detectors • People are limited – not all of us can draw this well: Left: The 3rd Earl of Rosse’s drawing of galaxy M51 (The Whirlpool), 1845. Right: A photograph of it this year.

  3. Mechanical image collection (from mid-19th Century) • Fine grain film used – enables as many photons to be captured as possible • Approx 25 photons required to deposit a grain of silver on the film – quantum efficiency of 4% Problems: Size of grain could be a limit to resolution if bigger than Rayleigh Criterion for resolution. Poor quantum efficiency Also need very precise mechanism to track image across sky – films are dumb!

  4. Charge Coupled Device (from late 1970s) • Quantum efficiency closer to 70% (just over 1 photon required for a signal in a single pixel) • Can detect wavelengths beyond visible (especially infrared) • Not dumb - computer can do image processing.

  5. How it works (1) Some photons hit the CCD and a small electric charge is created (2) This electric charge is shifted to a ‘store’ (3) The ‘store’ is emptied and a varying voltage is recorded (the potential barrier drops). (4) This is sampled and converted into a digital signal (a number) for processing one pixel

  6. Why should you know about it? • Q.Eff = 70% c.f. 4% for film, 1% for eye (i.e. very sensitive) • Can detect non-visible λ • Versatile • Hubble has 4 CCDs, each with 640,000 pixels – basically a very expensive 2.5 megapixel camera. How many pixels does your camera have? Note: CMOS chips are cheaper and perform a similar (although less well defined) function – you may find one of these in your camera rather than a CCD

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