Accessary Instruments for Telescopes
RADIATION DETECTORS
Before discussing the accessory instruments used with optical telescopes, let us consider briefly the most important component of these instruments: the radiation detector. The telescope is capable of collecting light over a very wide range of wavelengths, but it is the radiation detector that determines what the telescope sees. One radiation detector with which we are all familiar is the human eye. It possesses most of the properties of radiation detectors in general and is thus illustrative of the points we wish to make about them.
The properties of interest are the wavelength regions to which the detector is sensitive, the differing response of the detector over that wavelength region, and the natu re and range of detector response. Using the human eye, we can briefly illustrate each of these properties.
PROPERTIES OF RADIATION DETECTORS
The eye is sensitive to the narrow wavelength region between about 3500 and 7000 angstroms. However, the eye does not respond equally to all colors in the visible spectrum. It is most sensitive to the middle of the wavelength region, the green wavelengths, and the sensitivity drops to zero toward either the violet (short wavelengths) or the red (long wavelengths).
The nature and the range of detector response are expressed by the ways in which the eye responds to one photon and to a tremendous flood of photons. Common experience tells us that the eye does not respond in the same way for both. There is some threshold number of photons, depending upon their wavelength, necessary to make the eye respond. In other words, there is a limit to how faint a light source we can see, and that visibility limit depends upon whether we are looking at violet, green, or red light.
All of us have experienced the loss of response of the eye when we try to look at too bright a light. That is, the eye saturates-it no longer responds-and no scene is visible to us, just an intense and painful brilliance. To be useful, the radiation detector's dynamic range between threshold and saturation of visibility should be quite large, say, a factor of 100 or 1000. Now we may ask, "What is the response of the eye to doubling the number of photons in between the lower and upper limits of threshold and saturation?" If we double the number of photons, do we observe that the light is twice as bright? The answer in general is no. By and large, over the dynamic range of response of the eye, doubling the stimulus does not double the response; in other words, we say that the response is nonlinear. This concept of linearity is important because, in seeking the amount of radiant energy emitted by an astronomical source, astronomers usually compare the unknown light source against one of known energy output. Thus they have to know how their radiation detector responds to increasing or decreasing numbers of photons.
Now we look at two other radiation detectors, the photographic emulsion and the photoelectric device.
PHOTOGRAPHIC EMULSION
The photographic emulsion records photons by undergoing a chemical change (a photochemical effect) that will ultimately deposit silver on a glass plate or acetate film. The photographic plate can be made to respond to different wavelength regions within and beyond either end of the visible spectrum, which makes it much more versatile than the eye. Also, its response over a wavelength interval can be made much more uniform than that of the eye. The photographic plate, like the eye, is nonlinear in its response; it has a rather complicated response depending upon the position in its dynamic range.
The photographic plate has a strong advantage over the eye since it will build up a response by storing the image. Thus time exposures allow the astronomer to collect information on a photographic plate about very faint light sources that cannot be detected by the eye through the same telescope. How faint a star can we photograph? The telescope's aperture sets the initial limit. Ultimately, however, the limit is set by the weak illumination in the night sky. This background interference comes from starlight scattered by the earth's atmosphere and from diffuse radiation in the atmosphere (airglow). Unfortunately, the photographic plate's photon-capturing efficiency is low. The emulsion can record only 1 or 2 percent of the incident photons (those that activate the light-sensitive coating). Facing this inefficiency, astronomers have found other types of radiation detectors to improve the telescope's performance.
PHOTOELECTRIC DEVICES
The photoelectric device is an application of the photoelectric effect. The basic principle is to liberate electrons from a metal surface by exposing it to photons in a light beam and then to measure the number of electrons with electronic circuitry. The photoelectric device, like the photographic emulsion, can·be made to respond to different wavelength regions by varying the metals used in making the surface of a device. The biggest advantage of the photoelectric device is that it can be manufactured to have a very large dynamic range of response; in addition its response is linear to the number of incident photons for a fictitious device. With modern electronics it is possible to adapt the photoelectric device to count individual photons or to use a mosaic of devices to form a picture much as a photographic plate does.
As an illustration of the photoelectric device's importance as a radiation detector, only about 15 percent of the nights of observing on the 5.1-meter Hale telescope are devoted to photographic work. On 85 percent of the nights some kind of photoelectric detecting device is being used.
IMAGE INTENSIFIERS
Electronic image intensifiers do as their name implies, they intensify, or amplify, the light from weak sources of radiation. I n one such system photons from the telescope are focused onto a photocathode surface, which ejects electrons. The electrons are increased in number, accelerated, and focused by means of electric and magnetic fields onto a phosphorescent screen, which emits a spark of light for each electron that strikes it. Thus the faint light from the astronomical source is amplified by the device into light sufficient to record the image on a photographic plate. Alternatively a computer circuit can be used to count the electrons during the exposure. Still other image-intensifying techniques are in use or in developmental stages; these techniques can reduce exposure times by factors of 50 to 100 over those for photographic systems.
SPECTROGRAPHS
The photographic plate and the photoelectric device enhance our ability to detect light from different astronomical sources, but they are not basically analyzing instruments. We can equip an accessory instrument with either of these detectors and attach it to the telescope to analyze light. The two basic types of analyzing instrument are the spectrograph and the photometer.
The. spectrograph disperses the composite light from the source into its component wavelengths so that we can, for example, determine the elements that compose the light source. Spectroscopy, which is the study of the spectra of light sources, is astronomy's fundamental interpretive tool.
A prism or grating spectrograph receives the concentrated light coming from the telescope's objective on an entrance slit. The light diverging past the slit enters a collimator, which delivers a beam of parallel rays to the dispersing device. Then these rays pass through either a prism or reflect off a grating, which separates the light into its constituent wavelengths. The dispersed light is focused by a camera system onto a radiation detector (a photographic plate or a photoelectric device) as individual color images of the entrance slit. Each wavelength forms a distinct image of the slit. The images of the slit in the different wavelengths are arrayed in an orderly progression of colors from red to violet to create the observed spectrum of the composite light falling on the entrance slit.
PHOTOMETERS
The photometer is an accessory device that the astronomer attaches to the telescope at the focal position of the objective to measure the amount of radiation coming from the astronomical object. Where the spectrograph is used to examine the spectral composition of radiation, the photometer can be made to scan the spectrum formed by the spectrograph. It measures the amount of radiant energy, on either a relative or an absolute scale, at one wavelength or in a band of wavelengths. The photometer is much like an exposure meter on a camera: Incident light is converted into an electrical current. One can use a variety of techniques to define the wavelength region for the photometer, such as a spectrograph or color filters. And the radiation detector is generally today a photoelectric device.
The photoelectric photometer is usually limited to measuring only one light source, such as a star, at a time. But the limitation is compensated for by the photoelectric photometer's very great accuracy. Because of its quick response to changes in amounts of light, the photoelectric photometer is particularly useful in continually monitoring the change in brightness of an object whose emission of radiant energy varies with time (for example, a number of stars are known to be variable light sources).
Infrared Devices
In 1800 William Herschel detected the infrared component of solar radiation by positioning thermometers beyond the red end of the sun's visible spectrum and thus foreshadowed the astronomy of invisible spectral regions. What we have seen over the last 15 years in these regions has revolutionized our concept of the universe.
INFRARED TELESCOPES
We can subdivide the infrared spectrum into three segments. A large part of the infrared spectrum is not visible at ground level because of absorption by water vapor, carbon dioxide, and molecular oxygen, which lie between the ground and about 15 kilometers altitude. Consequently, airplanes, balloons, rockets, and satellites are extensively used to lift the infrared telescope above the veiling atmosphere. Astronomers can also locate infrared facilities on mountaintops, such as
the one in the Hawaiian islands to make ground-based infrared observations.
The liquid-helium-cooled infrared detector can be used with the appropriate analyzing instruments on an ordinary optical telescope to study the cosmos. But because of the longer wavelength of infrared radiation, the image-producing quality of the telescope objective need not be so fine as it must be for the visible region. Thus a number of new telescopes have been designed and built for infrared astronomy only. A national observatory for infrared astronomy is built high on the 4200-meter inactive Hawaiian volcano Mauna Kea. A 3.0-meter infrared telescope constructed by NASA and the University of Hawaii is in operation there along with a 3.B-meter infrared telescope belonging to the United Kingdom. Other major infrared telescope facilities are the MMT in Arizona, the University of Wyoming facility, and Mexico's 2.1-meter reflector.
Now that we have described the optical window, somewhat expanded to include available parts of the infrared, we should shift our focus to the radio spectral window in the atmosphere. Thus the next section is on radio telescopes .
RADIATION DETECTORS
Before discussing the accessory instruments used with optical telescopes, let us consider briefly the most important component of these instruments: the radiation detector. The telescope is capable of collecting light over a very wide range of wavelengths, but it is the radiation detector that determines what the telescope sees. One radiation detector with which we are all familiar is the human eye. It possesses most of the properties of radiation detectors in general and is thus illustrative of the points we wish to make about them.
The properties of interest are the wavelength regions to which the detector is sensitive, the differing response of the detector over that wavelength region, and the natu re and range of detector response. Using the human eye, we can briefly illustrate each of these properties.
PROPERTIES OF RADIATION DETECTORS
The eye is sensitive to the narrow wavelength region between about 3500 and 7000 angstroms. However, the eye does not respond equally to all colors in the visible spectrum. It is most sensitive to the middle of the wavelength region, the green wavelengths, and the sensitivity drops to zero toward either the violet (short wavelengths) or the red (long wavelengths).
The nature and the range of detector response are expressed by the ways in which the eye responds to one photon and to a tremendous flood of photons. Common experience tells us that the eye does not respond in the same way for both. There is some threshold number of photons, depending upon their wavelength, necessary to make the eye respond. In other words, there is a limit to how faint a light source we can see, and that visibility limit depends upon whether we are looking at violet, green, or red light.
All of us have experienced the loss of response of the eye when we try to look at too bright a light. That is, the eye saturates-it no longer responds-and no scene is visible to us, just an intense and painful brilliance. To be useful, the radiation detector's dynamic range between threshold and saturation of visibility should be quite large, say, a factor of 100 or 1000. Now we may ask, "What is the response of the eye to doubling the number of photons in between the lower and upper limits of threshold and saturation?" If we double the number of photons, do we observe that the light is twice as bright? The answer in general is no. By and large, over the dynamic range of response of the eye, doubling the stimulus does not double the response; in other words, we say that the response is nonlinear. This concept of linearity is important because, in seeking the amount of radiant energy emitted by an astronomical source, astronomers usually compare the unknown light source against one of known energy output. Thus they have to know how their radiation detector responds to increasing or decreasing numbers of photons.
Now we look at two other radiation detectors, the photographic emulsion and the photoelectric device.
PHOTOGRAPHIC EMULSION
The photographic emulsion records photons by undergoing a chemical change (a photochemical effect) that will ultimately deposit silver on a glass plate or acetate film. The photographic plate can be made to respond to different wavelength regions within and beyond either end of the visible spectrum, which makes it much more versatile than the eye. Also, its response over a wavelength interval can be made much more uniform than that of the eye. The photographic plate, like the eye, is nonlinear in its response; it has a rather complicated response depending upon the position in its dynamic range.
The photographic plate has a strong advantage over the eye since it will build up a response by storing the image. Thus time exposures allow the astronomer to collect information on a photographic plate about very faint light sources that cannot be detected by the eye through the same telescope. How faint a star can we photograph? The telescope's aperture sets the initial limit. Ultimately, however, the limit is set by the weak illumination in the night sky. This background interference comes from starlight scattered by the earth's atmosphere and from diffuse radiation in the atmosphere (airglow). Unfortunately, the photographic plate's photon-capturing efficiency is low. The emulsion can record only 1 or 2 percent of the incident photons (those that activate the light-sensitive coating). Facing this inefficiency, astronomers have found other types of radiation detectors to improve the telescope's performance.
PHOTOELECTRIC DEVICES
The photoelectric device is an application of the photoelectric effect. The basic principle is to liberate electrons from a metal surface by exposing it to photons in a light beam and then to measure the number of electrons with electronic circuitry. The photoelectric device, like the photographic emulsion, can·be made to respond to different wavelength regions by varying the metals used in making the surface of a device. The biggest advantage of the photoelectric device is that it can be manufactured to have a very large dynamic range of response; in addition its response is linear to the number of incident photons for a fictitious device. With modern electronics it is possible to adapt the photoelectric device to count individual photons or to use a mosaic of devices to form a picture much as a photographic plate does.
As an illustration of the photoelectric device's importance as a radiation detector, only about 15 percent of the nights of observing on the 5.1-meter Hale telescope are devoted to photographic work. On 85 percent of the nights some kind of photoelectric detecting device is being used.
IMAGE INTENSIFIERS
Electronic image intensifiers do as their name implies, they intensify, or amplify, the light from weak sources of radiation. I n one such system photons from the telescope are focused onto a photocathode surface, which ejects electrons. The electrons are increased in number, accelerated, and focused by means of electric and magnetic fields onto a phosphorescent screen, which emits a spark of light for each electron that strikes it. Thus the faint light from the astronomical source is amplified by the device into light sufficient to record the image on a photographic plate. Alternatively a computer circuit can be used to count the electrons during the exposure. Still other image-intensifying techniques are in use or in developmental stages; these techniques can reduce exposure times by factors of 50 to 100 over those for photographic systems.
SPECTROGRAPHS
The photographic plate and the photoelectric device enhance our ability to detect light from different astronomical sources, but they are not basically analyzing instruments. We can equip an accessory instrument with either of these detectors and attach it to the telescope to analyze light. The two basic types of analyzing instrument are the spectrograph and the photometer.
The. spectrograph disperses the composite light from the source into its component wavelengths so that we can, for example, determine the elements that compose the light source. Spectroscopy, which is the study of the spectra of light sources, is astronomy's fundamental interpretive tool.
A prism or grating spectrograph receives the concentrated light coming from the telescope's objective on an entrance slit. The light diverging past the slit enters a collimator, which delivers a beam of parallel rays to the dispersing device. Then these rays pass through either a prism or reflect off a grating, which separates the light into its constituent wavelengths. The dispersed light is focused by a camera system onto a radiation detector (a photographic plate or a photoelectric device) as individual color images of the entrance slit. Each wavelength forms a distinct image of the slit. The images of the slit in the different wavelengths are arrayed in an orderly progression of colors from red to violet to create the observed spectrum of the composite light falling on the entrance slit.
PHOTOMETERS
The photometer is an accessory device that the astronomer attaches to the telescope at the focal position of the objective to measure the amount of radiation coming from the astronomical object. Where the spectrograph is used to examine the spectral composition of radiation, the photometer can be made to scan the spectrum formed by the spectrograph. It measures the amount of radiant energy, on either a relative or an absolute scale, at one wavelength or in a band of wavelengths. The photometer is much like an exposure meter on a camera: Incident light is converted into an electrical current. One can use a variety of techniques to define the wavelength region for the photometer, such as a spectrograph or color filters. And the radiation detector is generally today a photoelectric device.
The photoelectric photometer is usually limited to measuring only one light source, such as a star, at a time. But the limitation is compensated for by the photoelectric photometer's very great accuracy. Because of its quick response to changes in amounts of light, the photoelectric photometer is particularly useful in continually monitoring the change in brightness of an object whose emission of radiant energy varies with time (for example, a number of stars are known to be variable light sources).
Infrared Devices
In 1800 William Herschel detected the infrared component of solar radiation by positioning thermometers beyond the red end of the sun's visible spectrum and thus foreshadowed the astronomy of invisible spectral regions. What we have seen over the last 15 years in these regions has revolutionized our concept of the universe.
INFRARED TELESCOPES
We can subdivide the infrared spectrum into three segments. A large part of the infrared spectrum is not visible at ground level because of absorption by water vapor, carbon dioxide, and molecular oxygen, which lie between the ground and about 15 kilometers altitude. Consequently, airplanes, balloons, rockets, and satellites are extensively used to lift the infrared telescope above the veiling atmosphere. Astronomers can also locate infrared facilities on mountaintops, such as
the one in the Hawaiian islands to make ground-based infrared observations.
The liquid-helium-cooled infrared detector can be used with the appropriate analyzing instruments on an ordinary optical telescope to study the cosmos. But because of the longer wavelength of infrared radiation, the image-producing quality of the telescope objective need not be so fine as it must be for the visible region. Thus a number of new telescopes have been designed and built for infrared astronomy only. A national observatory for infrared astronomy is built high on the 4200-meter inactive Hawaiian volcano Mauna Kea. A 3.0-meter infrared telescope constructed by NASA and the University of Hawaii is in operation there along with a 3.B-meter infrared telescope belonging to the United Kingdom. Other major infrared telescope facilities are the MMT in Arizona, the University of Wyoming facility, and Mexico's 2.1-meter reflector.
Now that we have described the optical window, somewhat expanded to include available parts of the infrared, we should shift our focus to the radio spectral window in the atmosphere. Thus the next section is on radio telescopes .
