Telescope Peripherals Help
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 .
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