Because different objects give out electromagnetic radiation at different frequencies. Some objects are very easily viewed in the visible spectrum, others are essentially invisible in that spectrum. Also, some types of electromagnetic radiation can more easily penetrate dust and other debris between the source and Earth. Visible light is very easily blocked by dust, but x-rays for example can pass through them pretty much intact.
Microwave telescopes can allow us to see the sky in microwave frequencies. There are microwave sources that are invisible to optical telescopes, and there may be gas or dust clouds that block visible light but allow some microwaves to penetrate. A breathtaking example of the use of a microwave telescope is seen in the microwave mapping of the sky.
As we understand it, the universe began with the Big Bang, which was that massive outrush of energy at the beginning of time. Almost everywhere in space we see microwave radiation, and that microwave radiation is the "echo" of the Big Bang. It's called cosmic microwave background radiation (CMB), and its discovery won the Nobel Prize for a pair of investigators.
Very shortly after the first radio waves from space were detected, astronomers from
learned institutions all over the world met in convention to decide what to call the
equipment that would henceforth be used to work with this mysterious, unearthly
radiation. After the early, selfish, nationalistic suggestions were defeated, the
convention was split almost 50/50 into two competing groups, aligned in favor of
the two finalists. One side of the convention was convinced that the new instrument
should be referred to as the Penzias, while the other half were just as firmly insistent
that it should be called the Jansky-scope. After weeks of wrangling, bargaining, deadlock,
and bar tabs that the home institutions would no longer subsidize, the convention
collapsed in upon itself out of sheer exhaustion. Neither side had managed to budge
the other side so much as a single Ångstrøm in its direction in weeks, and the realization
began to dawn on them that they never would. Like unto Sir Isaac's gravitation itself,
the forces were precisely equal in both directions, and if the factions continued to strive,
they could expect nothing other than to circle their mutual center into eternity. It was
at that point that the convention came to its senses, the several representatives of
the distant and unrelated nations decided to surrender their non-negotiables, and to
compromise a little for heaven's sakes. They all gathered at the bar one last time, to
raise a toast to each other and to their unanimous agreement on the one designation
for the new radio-wave device to which not one of them had any objection, and it was
from that moment until our day that the telescope used to detect radio waves from the
stars and from the space between the stars has been known as: the Radio Telescope.
Most types of telescope focus radiation. All but one do not actually detect it. The detection of radiation is undertaken by a sensor (a camera or the human eye) at the focus of the telescope.
The exception is a gamma ray detector which, as high energy gamma rays can not be focused, captures the effect that the gamma ray produces when it hits things (it causes a cascade in the atmosphere). A gamma ray detector may be able to tell you in which direction a gamma ray came from and how energetic it was but it can not actually form a image of the gamma ray source.
Radio astronomy is a general term applied to the study of the radio emissions of stars and other objects in the heavens. Stars often give off visible light, which is electromagnetic energy. But they also emit a lot of electromagnetic energy below the optical band. These lower bands are given the general term radio bands, and a huge amount of work has been done (and continues) regarding imaging with radio waves. Radio astronomy actually outstrips optical astronomy in terms of how much it is used, though it almost certainly has a "lower profile" than observations made with the big optical telescopes. Different types of phenomenon or "reactions" occur in stars and other cosmological objects or structures, and these goings on result in the emission of energy (in different electromagnetic wavelengths). There are many that are only visible as radio sources, so we use radio astronomy to "see" them. We also use it to supplement imaging in optical wavelengths to give us better data as to what is out there.
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Radio Astronomy is an academic, scientific, investigative discipline wherein stars, planets, and the interstellar and intergalactic media are studied at electromagnetic wavelengths of millimeters and centimeters, rather than nanometers as in optical, x-ray, UV, and gamma-ray detecting instruments.
infra-red telescope is a telescope in which you can look at everything in the waves of infra-red.
Hubble
GALEX detects ultraviolet (UV) radiation from celestial objects in space. It is specifically designed to study the UV emission from stars, galaxies, and other astronomical sources.
infrared radiation
infrared radiation
detects microwave radiation, which we can't see ourselves or photograph.
A radio telescope detects light in the form of radio waves and a refracting telescope detects light in the visible wavelengths
infra-red telescope is a telescope in which you can look at everything in the waves of infra-red.
Radio Waves
The Compton gamma-ray telescope detects high-energy gamma rays, which are a type of electromagnetic radiation with the shortest wavelengths and highest frequencies in the electromagnetic spectrum. These gamma rays are produced by astrophysical processes such as supernovae, pulsars, and black holes.
Arecibo, VLA
radio telescope detects radio waves and a light telescope views light waves.
The answer depends on what "IT" is and what sort of radiation it is meant to detect!
Electromagnetic radiation in the frequency range between roughly 0.5 - 100 GHz.Loosely known as "radio".
Hubble
A Geiger counter is a device that detects radiation by producing clicking sounds when radiation is present. These clicking sounds are generated as the radiation interacts with the detector inside the Geiger counter.
Radio telescopes are designed to detect sources of electromagnetic radiation such as x-rays or other invisible emissions. They detect electromagnetic radiation from distant galaxies, including stars and planets and other astronomical radio sources. They are usually in the form of large parabolic antennas, sometimes with hundreds linked together in enormous arrays, connected to extremely sensitive radio frequency receivers.