Sensors and Optics

Understanding how the image sensor and the optical system interact allows you to use them more effectively and creatively. The sensor characteristics—the size of the array, the size of a single pixel, and its spectral sensitivity—cannot be changed; but you can control the angular field of view, the angular resolution, and the spectral range captured in the image through the use of various optical systems and filters.

4.1.1 Sensor Size and Field of View

Field of view is the most important single factor in digital imaging—it determines how much sky your camera covers. The field of view depends on the focal length of the optical system and the physical size of the imaging area on the CCD chip. By putting different optics in front of your CCD camera, you can capture a wide-angle view or the tiny disk of a planet.

Calculate the field of view, $Ccd > produced by a CCD camera used on an optical system from the width or height dimension of the CCD, dCCD , and F, the focal length of the optical system using:

Figure 4.1 The angular field of view of an imaging system depends on both the focal length of the optical system and the physical size of the detector. The angular size of a single pixel likewise depends on both the focal length of the optical system and the physical size of the pixel.

This formula is an approximation (that is what above the equal sign means) that is accurate for long-focus systems such as telescopes and telephoto lenses. Be sure to use the same units—inches, millimeters, whichever you prefer—for the chip size and the focal length.

Here are some examples.

To capture deep-sky objects. All but a few of the Messier objects will fit comfortably into a 15-minutes-of-arc field of view, so that figure should be your minimum acceptable field size. Will a sensor that is 6.4 mm wide by 4.8 mm high be right for Messier deep-sky objects if you use it on a telescope with a focal length of 1,000 mm? From the equations, the field of view is 22 minutes of arc by 16.5 minutes of arc, so the combination has a field size large enough for making your own Messier Gallery.

To capture a wide field of view. Use optics with a short focal length. Suppose you want to make a CCD movie of a comet with a 10° long tail using the same imaging sensor—the 6.4 mm by 4.8 mm array; what lens do you need? Substituting 10° and the 6.4 mm dimension of the detector into the equation gives a focal length of 36.7 mm. A standard 35-mm focal length lens for a 35-mm camera should serve admirably.

To capture the planets. At the opposite extreme, imaging Jupiter—which

Table 4.1 Angular Field of View for CCDs

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