Charge-Coupled Devices Astrophysics Lesson 5

Learning Objectives Describe and explain the structure and operation of the charge coupled device State the quantum efficiency of pixel

Light Detectors Once we have got a good image down the telescope, we need to have a way of recording it. In early astronomy the human eye was used, and the results depended on the artistic ability of the astronomer.

Solar Prominence Photographic techniques were used from the middle of the Nineteenth Century.

Quantum Efficiency  An important measure of a detector’s sensitivity to electromagnetic radiation is its quantum efficiency (QE).  This is defined as:-  The eye has a quantum efficiency of only 1 %.

Charged Coupled Device More recently a charged coupled device is used. This is a type of microchip is which light is converted into digital information. They are made out of a silicon wafer divided into small regions called pixels (up to 1 million).

The picture below shows a CCD.

When light strikes CCD...

CCD

CCDs – what you need for exam  A CCD is a silicon chip divided into picture elements (pixels).  Incident photons cause electrons to be released.  The number of electrons liberated is proportional to the intensity of the light.  These electrons are trapped in 'potential wells' in the CCD  An electron pattern is built up which is identical to the image formed on the CCD.

CCDs – what you need for exam  When exposure is complete, the charge is processed to give an image.  Quantum efficiency of pixel > 70%.

Quantum Efficiency of the CCD

Advantages of CCDs  The CCD is about the size of a postage stamp and can have many millions of pixels on it. They work on the principles of quantum physics. They have these advantages over film:  They are sensitive;  Their quantum efficiency is about 70 %. A film has a quantum efficiency of about 4 %

Advantages of CCDs  They are getting cheaper all the time.  The CCD can detect radiations that are beyond the visible spectrum.

Summary The reflecting telescope has fewer drawbacks than the refracting instrument. They use concave mirrors. The telescope can be of the Newtonian or Cassegrain system. The resolution depends on the aperture and the wavelength Detectors can be eye, camera, or CCD. CCD has a quantum efficiency of about 70 % so is more sensitive.

The resolution depends not just on the Rayleigh Criterion, but also on the emulsion of the film. Very fine grain films are used for astronomical observation. The quality of the picture needed to be high and precision mechanisms were essential for tracking individual stars across the sky. If the grains of film are larger than the resolution of the telescope, than that is the limiting factor.