Telescope Peripherals Help (page 2)
Introduction to Telescope Peripherals—Photometer
Sometimes it is necessary to measure the brightness of a celestial object rather than merely making a good guess or a subjective comparison. A photometer is an instrument that does this. An astronomical photometer is a sophisticated version of the lightmeter, which is used by photographers to determine camera exposure time. The device consists of a photosensor placed at the focal point of the telescope. An amplification circuit multiplies the sensitivity of the sensor. The output of the amplifier can be connected to a circuit that plots the light intensity as a function of time.
Many celestial objects, such as variable stars and visible pulsars, change in visual magnitude with time. Variable stars fluctuate slowly, but some pulsars blink so fast that they look like ordinary stars until a graph is plotted using a photometer capable of resolving into brief intervals of time. Photometers can be made sensitive in the infrared (IR) and ultraviolet (UV) ranges as well as in the visible spectrum.
Visible light can be broken down into the colors of the rainbow, each hue representing a specific wavelength. This can be done by a prism with a triangular or trapezoidal cross section. It also can be done by passing the light through or reflecting it from a diffraction grating , which is a clear plate or mirror with thousands of tiny parallel opaque bands etched on it. The grating works because of the interference patterns produced by light waves passing through the gaps between the dark bands. This is an entirely different phenomenon from the refraction that occurs in a prism, but the practical effect is similar.
A spectrometer , also known as a spectroscope , is a device intended for analyzing visible light at all its constituent wavelengths. Some spectrometers also work at IR or UV wavelengths. Stars, galaxies, quasars, and some nebulae have spectra that contain dark absorption lines at certain wavelengths. Other nebulae are dark except at specific emission wavelengths that manifest themselves as bright lines in a spectrum. The patterns of lines allow scientists to determine the amounts of various chemicals that comprise celestial objects after corrections are made for the absorption effects of Earth’s atmosphere.
Figure 17-11 A is a functional diagram of a simple spectrometer. The light-sensitive surface can be photographic film or a matrix of optoelectronic sensors . The maximum image resolution obtainable by a spectrometer of this type is limited by the grain of the film or the number of pixels per centimeter in the sensor. Higher resolution can be achieved by a scheme such as that diagrammed in Fig. 17-11 B . The rotating prism causes the spectrum to sweep across the objective of a viewing scope. A light sensor connected to the scope measures the intensity of the rays. The angle of the prism at any given moment in time is fed to a computer along with the sensor output. This produces a graph of the spectrum that is called, not surprisingly, a spectrograph .
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.
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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