Light is the visible form of electromagnetic radiation, an electric and magnetic disturbance that transports energy at the speed of light. The electromagnetic spectrum includes, gamma rays, X rays, ultraviolet radiation, visible light, infrared radiation, and radio waves.
You can think of a particle of light, a photon, as a bundle of waves that acts sometimes as a particle and sometimes as a wave.
The energy a photon carries depends on its wavelength. The wavelength of visible light, usually measured in nanometers (10-9 m) or Angstroms (10-10 m), ranges from 400 nm to 700 nm. Radio and infrared radiation have longer wavelengths and carry less energy. X-ray, gamma ray, and ultraviolet radiation have shorter wavelengths and more energy.
Wavelength is related to frequency, the number of waves that pass a point in one second.
Earth’s atmosphere is transparent in only two atmospheric windows—visible light and radio.
Astronomical telescopes use a primary lens or mirror (also called an objective lens or mirror) to gather light and bring it to a prime focus where it can be magnified by an eyepiece. Short-focal-length lenses and mirrors must be more strongly curved and are more expensive to grind to shape.
A refracting telescope uses a lens to bend the light and focus it into an image. Because of chromatic aberration, refracting telescopes cannot bring all colors to the same focus, resulting in color fringes around the images. An achromatic lens partially corrects for this, but such lenses are expensive and cannot be made much larger than about 1 m in diameter.
Reflecting telescopes use a mirror to focus the light and are less expensive than refracting telescopes of the same diameter. Also, reflecting telescopes do not suffer from chromatic aberration. Most recently built large telescopes are reflectors.
Light-gathering power refers to the ability of a telescope to produce bright images. Resolving power refers to the ability of a telescope to resolve fine detail. Diffraction fringes in the image limit the detail visible. Magnifying power is less important because it can be changed by changing the eyepiece.
Astronomers build observatories on remote, high mountains for two reasons. Turbulence in Earth’s atmosphere blurs the image of an astronomical telescope, a phenomenon that astronomers refer to as seeing. Atop a mountain, the air is steady, and the seeing is better. Observatories are remote from cities to avoid light pollution.
Light first comes to a focus at the prime focus, but secondary mirrors can direct light to other focus locations such as a Cassegrain focus or a Newtonian focus. The Schmidt-Cassegrain focus is popular for small telescopes.
Because Earth rotates, telescopes must have a sidereal drive to follow the stars. An equatorial mounting with a polar axis makes this possible, but alt-azimuth mountings are becoming more popular.
Very large telescopes can be built with active optics maintaining the shape of floppy mirrors that are thin or in segments. High-speed adaptive optics controls the shape of telescope mirrors and partially cancels out seeing turbulence.
Interferometry refers to connecting two or more separate telescopes together to act as a single large telescope which has a resolution equivalent to that of a telescope as large in diameter as the separation between the telescopes.
6-3 Special Instruments
How do astronomers record and analyze light?
For many decades astronomers used photographic plates to record images at the telescope, but modern electronic systems such as charge-coupled devices (CCDs) have replaced photographic plates in most applications.
Astronomical images are often computer enhanced and reproduced as false-color images to bring out subtle details.
Spectrographs using prisms or a grating spread starlight out according to wavelength to form a spectrum revealing hundreds of spectral lines produced by atoms in the object being studied. A comparison spectrum allows astronomers to measure the wavelengths of spectral lines.
6-4 Radio Telescopes
Why do astronomers use radio telescopes?
Astronomers use radio telescopes for three reasons: They can detect cool hydrogen in space; they can see through dust clouds that block visible light; and they can detect certain objects invisible at other wavelengths.
Most radio telescopes contain a dish reflector, an antenna, an amplifier, and a data recorder. Such a telescope can record the intensity of the radio energy coming from a spot on the sky. Scans of small regions are used to produce radio maps.
Because of the long wavelength, radio telescopes have very poor resolution, and astronomers often link separate radio telescopes together to form a radio interferometer capable of resolving much finer detail.
6-5 Astronomy from Space
Why must some telescopes go into space?
Earth’s atmosphere absorbs gamma rays, X rays, ultraviolet, and far-infrared. To observe at these wavelengths, telescopes must be located in space.
Earth’s atmosphere distorts and blurs images. Telescopes in orbit are above this seeing distortion and are limited only by diffraction in their optics.
Cosmic rays are not electromagnetic radiation; they are subatomic particles such as electrons and protons traveling at nearly the speed of light. They can best be studied from above Earth’s atmosphere.