Some Useful Web Pages

<Hubble Telescope>

<Colliding Galaxies>



5-1: RADIATION: Information from space
Light as a Wave and a Particle: electromagnetic radiation, wavelength (Greek letter, l) and photon
Electromagnetic Spectrum: nanometer (nm), angstorm (Å), AM, FM, ozone (O3), atmospheric windows
Two types: Refracting and Reflecting; mirror, objective lens, eyepiece, focal length, chromatic aberration, achromatic lenses
Subtypes: Prime focus, Newtonian, Cassegrain, Schmidt-Cassegrain
Telescope mountings: sidereal drive, equatorial mountings, polar axis, altazimuth mountings,
The powers of a telescope: light gathering power, resolving power, diffraction fringe, seeing, magnifying power, light pollution, formulas for three kinds of power
Buying a telescope: magnification, diameter of objective mirror, mounting, cost and usefulness
New-Generation of telescopes: revolving oven, segment mirrors, computer controlled thrusters, floppy mirror, active optics, binocular telescope,
Imaging systems: charge coupled devices (CCDs), false color images
The Spectrograph: grating, comparison spectrum


Operation of a radio telescope: dish reflector, antenna, amplifier, recorder, contour maps, false color
Advantages of a radio telescope: imaging cool hydrogen clouds (90% hydrogen in the universe), 21 cm wavelength, birth place of stars, power of penetrating distance, radio luminous objects
Limitations of a radio telescope: poor resolution, raio interferometer, very large array (VLA), very long baseline array (VLBA)
Infrared astronomy:
Ultraviolet astronomy
X-ray astronomy
The Hubble Space Telescope
KEY CONCEPTS: Telescopes are the tools of the astronomers to study the unverse. The instruments such as spectrographs, CCD detectors and Photometers make telescopes powerful analyzing tool. Electromagnetic waves cover a wide range of radiations, starting from radio waves to gamma rays. The variety of radiations have the same physical properties as wave and differ only in wavelength or frequency. Understanding the basic properties of light as in wave and particle behaviors. Telescopes are designed for different types of radiations. They perform certain tasks most efficiently. Each design has certain advantages and limitations. Special instruments are required for analyzing the radiations received by a telescope. Spectrograph has revolutionized astronomy. Over 80% of all allocated telescope time is for spectroscopic studies of one or another kind.




Radiation: Ordinary light reaching the earth from the sun, stars, reflection from the surfaces of the moon and the planets, brings quite a bit of information about the heavenly bodies.

NATURE OF LIGHT: If you have observed and possibly admired the beauty of the rainbow in the sky, you have seen light as a wave, much like water waves on a pond or sea. Another example of a wave is sound, the subject area called Acoustics. Sound requires a medium to travel unlike light waves which can penetrate through the vast empty space. Light is made up of electric fields and magnetic fields, hence called Electromagnetic waves. Light is only one type of electromagnetic waves. Radio waves, microwaves, infra-red, visible light, ultra-violet, x-rays and gamma rays are all part of the electromagnetic waves. Speed of light is tremendous.

c = 186,000 miles per second = 300,000 km per second

A wave has a Wave Length = l . It completes periodic cycles of vibrations per second called frequency = f.

Speed = distance/time = Wave length/Time period.

Some experiments in physics cannot be explained unless light is considered as a particle (named photon). Energy of electromagnetic radiation is,

E = hf = hc/l

Here, h is called Planck's constant. Energy relation shows that as wavelength becomes shorter, the energy becomes larger. 


 Radio waves: AM, FM, VHF and UHF ; Range of wavelength from 10 cm to 1000m.

Microwaves: 0.1mm to 1cm. We are familiar with microwave ovens used to cook our food.

Infrared radiation: 800 nm to 1 mm. Our eyes do not see it as a color. Our skin can sense it as heat. 1 nanometer = one in a ten thousand parts of width of average human hair.

Visible light: wavelength from 400 to 700 nanometers (nm).

Wavelength is a measure of color just as pitch the wavelength of sound.

Ultraviolet: 10 nm to 300 nm. These are harmful rays that may cause skin cancer.

X-rays: 0.01 nm to 10 nm. Used in the hospital to take pictures of the bones for possible fractures.

Gamma rays: wavelength less than 0.01 nm, highly energetic and penetrating waves.

Radiation reaching earth from heavens: Earth's atmosphere absorbs most electromagnetic radiations except in the visible and some high frequency radio waves. Astronomers make extensive use of radiations in these two ranges to study the heavens.


Lenses and mirrors are used for building telescopes. Light gathering power, Resolving power, Diffraction fringes, Magnifying power, Optical quality of the instrument, Atmospheric conditions (thin steady air in mountain tops can avoid the air turbulence resulting in the twinkling of stars),

1. REFRACTING TELESCOPE: Light is transmitted through transparent medium such as glass. Light bends if it passes from one medium to another, and is called refraction of light.

Ordinary Telescopes and Microscopes are built with two lenses, Objective lens and the eye-piece lens.

In a refracting telescope, objective lens (a converging lens) gathers all the light from the object and forms an image at a distance called the focal length of the lens. A second converging lens (called the eyepiece) with a short focal length, is placed such that it receives the image formed by the first lens just inside its own focal length. This enables the image to be much magnified.

2. REFLECTION TYPE OF TELESCOPES: A reflection Telescope uses a large converging objective mirror (instead of lens) to form the image at its focus. A short plane mirror or converging secondary mirror is used to reflect the light back down the telescope tube through a hole at the center of the objective mirror. This arrangement is called Cassegrain telescope. Other popular telescopes are Newtonian focus, Schmidt camera and Schmidt-Cassegrain types.

Cassegrain Reflection Telescope

Light Gathering Power (LGP) ° Diameter of aperture squared

The resolving power is the ability of the telescope to separate out two closely spaced objects in an image.

Resolving Power = a = 11.6/Diameter (in cm)

Magnification = (Focal length of the objective)/(Focal length of eyepiece)

M = Fo/Fe

7x50 Binoculars have magnification of 7 and aperture of 50 mm.

Light Gathering Power is proportional to the area of the telescope objective (primary lens or mirror)


New generation Telescopes use multiple sectional mirrors forming the total area of the objective. They use CCDs (Charge Coupled Devices) as detectors of light instead of film.

Modern detectors of light: CHARGE-COUPLED DEVICE (CCD) is a digital light detecting system that has a very good resolution of images and it is made up of millions of tiny light sensitive cells.

Spectrograph: Uses diffraction grating or prism to seperate the light into characteristic wavelengths

Photometer: Measures the light output

HUBBLE SPACE TELESCOPE (HST): A telescope in the orbit like a satellite around the earth is a dream come true for many astronomers. Hubble telescope has just done that in April 24, 1990. Hubble discovered the expansion of the universe. HST has a large converging mirror 96 inches in diameter. It has a number of light detectors and many fancy equipment for studying the universe. Aberration problem from HST was mostly removed in December, 1993.

Most recently, HST was able to take pictures of the stars being born in the Pegasus galaxy.

Electromagnetic Radiation: Radiations considered waves emitted by a heated object ,span from zero to infinitely long wavelength. These waves carry electric anf magnetic fields while traveling from one place to another and hence the name.

Wavelength: The distance between the adjacent peaks in a wave is called Wavelength.

Photon: For most experiments with light, it seemed to behave as a wave. However, there are a class of experiments in which light behaves as particles. Light when treated as particles, is named Photon.

Chromatic Aberration and Achromatic lens: Ordinary lenses suffer from defects in that the light passing through these lenses are not focussed at one point for all colors. This defect is eliminated by designing the lens with additional elements, such lens is called Achromatic lens.


Sidereal Drive: Literally star drive. Ability to rotate the telescope to compensate for the westward motion of the stars so that a star is always in focus.  

Equatorial Mounting: One axis parallel to earth's Polar axis. Eastward motion of the earth is compensated by the smooth westward motion (sidereal Drive) of the telescope.

Altazimuth Mounting: The mounting that moves the telescope in altitude (perpendicular to observer's horizon) and also moves in azimuth (parallel to the horizon). These two seemingly complicated movements of the telscope are efficiently made by Computers, guiding the telescope.

Thin mirrors used to reduce the weight of telescope. But they easily sag and are called floppy mirrors. Computer controlled supporting legs (thrusters) to hold the mirror in place and to preserve its fine curvature. Astronomers call it Active Optics. 

FALSE COLOR IMAGE: To enhance the finer features or details of the image obtained. Color used are not real colors of the observed object.

DIFFRACTION GRATING: A series of narrow slits or groovings on a film or plate

COMPARISON SPECTRUM: Most astronomers add a calibrated known spectrum on top and bottom of the spectrum of the observed object for comparison and subsequent analysis.

RADIO INTERFEROMETER: Resolving power of radio telescope is not very good even for a dish 30 meters in diameter. An array of radio telescope is used to increase its power. A link up of radio telescopes as in a Y-shape, is called radio interferometer.




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Revised 01/13/2014