[Previous] [Next] [Contents] [Home]

3. Differences between Optical and Radio Astronomy

There are many differences between optical and radio astronomy. The three main differences are the design of the instruments, the resulting data that is found, and the different sources that are seen.

Both optical and radio astronomy use telescopes; however, there is a big difference in the design between an optical telescope and a radio telescope.

Optical astronomy is the study of the visible part of the electromagnetic spectrum, that is wavelengths of approximately 400 nm (purple) to 700 nm (red).

Radio wavelengths are much longer; the radio spectrum ranges from approximately one millimeter to hundreds of meters. This means that optical photons have much higher energies than radio photons. This property of photons affects the way one would detect them.

Optical detectors such as charge coupled devices (CCDs) detect the individual photons that strike the detector surface. The photon strike creates a response in the detector - this response is proportional to the number of photons striking the surface. In some cases, one can count the photons. The result is a measure of the intensity of the source that is generating the photons. Such a detection mechanism (called "coherent detection") will not work on radio photons. The energy carried by these photons is much too low to cause a reaction in a detector.

So in radio astronomy "incoherent" detection techniques are used. In simple terms, the receiver will now detect the wave nature of the radio wave rather than the photon nature. This means that one cannot count radio photons but rather get information about the phase and amplitude of the wave.

Electromagnetic Spectrum Diagram

Electromagnetic Spectrum Diagram

Optical Telescopes

In optical astronomy there are three basic types of telescopes; therefractor, the catadioptric and the reflector.

Refractor TelescopeThe refractor telescope receives light through the objective lens (the large lens closest to the subject being viewed) and sends it to the eyepiece for magnification. The image below shows a schematic diagram of a refractor telescope.

The second type of optical telescope is the catadioptric. This type of telescope is similar to the Cassegrain reflector. It uses both mirrors and lenses, as the light entering the tube changes its direction twice. The tube length is shorter than its focal length. Focal length is the distance between the optical center of the objective lens and the optical center of the eyepiece. The longer this distance, the greater the magnification.

Reflector TelescopeThe basic reflector telescope uses a concave mirror as its objective lens to collect light from distant objects and then reflects that light up the tube to an overhead diagonal mirror. The diagonal mirror redirects the light into the eyepiece for magnification. The image below is a diagram of a reflector telescope.

There are four basic types of reflecting telescopes; they are the prime focus (as shown here), Newtonian, cassegrain, and the Schmidt. In the prime focus, the detector lies in front of the mirror. The Newtonian has a small mirror that reflects the light off the side of the telescope tube. The cassegrain utilizes a subreflector to reflect the light back through a hole in the primary mirror, and the detector can be placed behind the mirror. The Schmidt reflector, uses both a mirror and a correcting lens to produce a perfect image over a wide field.

Radio Telescopes

Radio telescopes are mainly either prime focus or Cassegrain reflectors. However, the radio telescope looks very different from the optical telescope; radio telescopes are much larger than optical telescopes. The reason for this is that the angular resolution (or the angular area of the sky from which the telescope can collect emission) of a telescope is proportional to the wavelength divided by its diameter. So in order for a radio telescope to be able to detect the same angular resolution as an optical telescope the radio telescope has to be much larger. In addition, the sensitivity of the telescope or the ability to detect weak emission is also related to the area of the reflecting surface.

There are four basic elements to a radio telescope, the reflector, the subreflector, the feed and transmission line and the receiver.

The reflector collects power from astronomical sources. The subreflector is a surface that directs the radiation to the feed at the center of the reflector. Behind the feed is the receiver system (at the cassegrain focus). The receiver amplifies the radio signal, selects the appropriate frequency range that detects the signal.

Radio telescopes use a large metal dish, usually parabolic, to reflect radio waves to the subreflector situated close to the prime focus. The signal from the antenna is sent to an amplifier, which magnifies the faint radio signals. The amplified radio signal is then processed by a computer. The receiver is configured in such a way that throughout the amplification process, the signal remains directly proportional to the strength of the incoming radiation. So the resulting image or spectrum is a true representation of the emission from the astronomical source. The following image is a schematic diagram of a radio telescope.

Schematic diagram of a radio telescope

The resulting images from the radio telescope are very different from the optical telescope. These difference are mainly due to the different mechanisms that cause the emission. For example, here is an image of the Milky Way using an optical telescope (photo courtesy of APOD). At visible wavelengths, the sky is dominated by thermal emission from the visible surface of the stars. Optical astronomers measure the brightness of objects by measuring the apparent magnitude, or the flux density of the object. The flux density is a measure of the power received from the object per unit frequency, per unit area.

Image of Milky Way through optical telescope

Here is the Milky Way using a radio telescope tuned to 408MHz.
(image courtesy of APOD 12/14/97)

Image of Milky Way through a radio telescope

None of the bright stars in the night sky are prominent radio emitters. The emissions that are measured in radio astronomy come not from the stars, but from the gasses, etc. It can be seen that these views are very different from one another. The radio sky is not dominated by the light from stars and, depending on the wavelength, may not be dominated by thermal radiation. At short radio wavelengths thermal emission sources dominate the sky, and at long radio wavelengths the sky is dominated by non-thermal emission sources. In the radio sky there are also sources of both continuous emission and line emission. An example of radio line emission is the 21-cm line of neutral atomic Hydrogen. At long wavelengths the emission occurs primarily from synchrotron emitting sources such as pulsars, supernovae remnants, radio galaxies, and quasars. At short wavelengths the emission is dominated by thermal sources. Small, hot sources such as stars can be detected but are not an important part of emission. Large, cold sources such as the gas and dust clouds of interstellar medium and hot, large sources such as HII regions are important sources of emission.

[Previous] [Next] [Contents] [Home]