Telescope Peripherals Help (page 2)

By — McGraw-Hill Professional
Updated on Sep 18, 2011

Cameras And Film

Astrophotography is the art of recording the images from a telescope on photographic film. Long exposures make it possible for astronomers to “see” objects much dimmer than they could by looking directly through the telescope.

The sensitivity of an astrophotographic system depends on the light-gathering area of the telescope, on the speed of the film, and on the length of time the film is exposed to the image. The image resolution depends on the size of the emulsion particles in the film, as well as on the telescope’s magnification and light-gathering area. Some cameras and films can record images at near-IR or near-UV wavelengths as well as in the visible range.

Film behaves in strange ways when exposed to visible light over long periods of time. This is one of the reasons digital imaging systems are gradually replacing film cameras in astrophotography.

Charge-coupled Device (ccd)

A charge-coupled device (CCD) is a camera that converts visible-light images into digital signals. Some CCDs also work with IR or UV. Astronomers use CCDs to record and enhance images of all kinds of celestial objects. Common digital cameras work on a principle similar to that of the CCD.

The image focused on the retina of your eye or on the film of a camera is an analog image . It can have infinitely many configurations and infinitely many variations in hue, brightness, contrast, and saturation. However, a digital computer needs a digital image to make sense of and enhance what it “sees.” Binary digital signals have only two possible states: on and off. These are also called high and low or 1 and 0. It is possible to get an excellent approximation of an analog image in the form of high and low digital signals. This allows a computer program to process the image, bringing out details and features that otherwise would be impossible to detect.

A simplified block diagram of a CCD is shown in Fig. 17-12. The image falls on a matrix containing thousands or millions of tiny sensors. Each sensor produces one pixel (picture element). The computer (not shown) can employ all the tricks characteristic of any good graphics program. In addition to rendering high-contrast or false-color images, the CCD and computer together can detect and resolve images much fainter than is possible with conventional camera film.

Optics and Telescopes Telescope
Peripherals Charge-coupled Device

Figure 17-12. Functional diagram of a charge-coupled device (CCD) and basic image-processing system. A computer can provide additional image enhancement.

The Space Telescope—Assets

Once astronomers realized the extent to which Earth’s atmosphere limits the resolution and faintness of objects that can be seen or photographed with telescopes, they began to dream about putting a telescope in space. The Hubble Space Telescope (HST) was the first major optical instrument to be placed in Earth orbit and used for intensive observation within the visible-light range.

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