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ASTRONOMY: 3. Methods of astronomical observation - types of telescope - how optical refracting and reflecting telescopes work and radio telescopes - radio astronomy

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3. Methods of astronomical observation - types of telescope

Be able to compare methods of observing the Universe using visible light, including the naked eye, photography and telescopes.

In observing the night sky, the naked eye, apart from aesthetic appreciation, has been largely replaced by photography, usually coupled to a telescope.

However, historically, stars, planes, comets, our Moon have all been successfully discovered, observed, mapped and plotted via naked eye observations and astronomical tables of data assembled.

Distant stars can be seen because they are so hot and powerful emitters of electromagnetic radiation eg visible light.

Optical telescopes have much better light gathering power than the naked eye and the lens and lens-reflecting mirror systems can produced greatly magnified images and can peer into deep space totally inaccessible to the naked eye.

Optical telescopes using refracting convex lens were the earliest types used to examine the 'universe'.

Optical telescopes using reflecting mirrors were developed later, but do employ an eyepiece lens.

Optical telescopes detect visible light and convex refracting lenses or concave reflecting mirrors are used in their construction - some telescopes use both lenses and mirrors.

To improve the quality of the image you can increase the diameter of the objective lens - the primary light gathering lens at the front end of the telescope.

The bigger the diameter of the refracting objective lens (the larger the aperture) the more light is collected to improve resolution - image quality.

The same argument applies to a larger diameter mirror in a reflecting telescope.

You also get better resolution the greater the optical quality of the lenses too - the chemical composition of the glass refracting convex lens is another important factor in image quality.

Angular resolution describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object, high resolution = a good image, low resolution = a poor image.

how a refracting telescope works gcse physics igcs objective lens eyepiece

The basic design of a refracting telescope

The objective lens collects and focuses the light onto the eyepiece lens.

The eyepiece lens position can be adjusted to produce a clear focussed image on the eye, photographic plate or photocell plus computer.

For mirror based reflecting telescopes, you can increase the diameter of the concave mirror to gather more light and improve the quality and resolution of the image.

Optical telescopes are limited to visible light observations, so you need other types of telescopes.

For more on detecting different EM radiations see the first section of my Life Cycle of Stars page.

Photographing the same patch of sky and comparing images from one night to another can show up whether an object is moving eg asteroid or comet or some new star appearing or an old star exploding in a massive supernovae explosion,.

So, anything that changing that reflects or emits visible light can be detected and by using long-time exposures you can detect very faint very distant objects.

The result of all these historical and continuing contemporary observations with telescopes of all kinds is to give us a pretty good picture of the observable universe, even if we don't fully understand how it all works!

Problems with observations and ways to improve matters - image quality

The most obvious problems with the use of optical telescopes is absorption of light by the Earth's atmosphere and light pollution.

The Earth's atmosphere both absorbs, refracts and scatters light from an astronomical source which reduces the quality of the image - reduces resolution.

Reminder: Angular resolution describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object, so high resolution = a good image and low resolution = a poor image.

Light pollution comes from any light source on the Earth's surface that emits light into the sky at night eg from road traffic, office blocks, street lights etc., all of which makes it more difficult to observe dim-faint objects in the sky.

The 'twinkling' of stars, giving an unstable image, is caused by incoming light refracting several times in the Earth's atmosphere.

Air pollution eg particulates/dust can also absorb or scatter light diminishing the quality of ANY image.

All of these problems can be considerably reducing by getting a telescope to operate high in the atmosphere OR above the Earth's atmosphere completely.

Observatories using optical telescopes can be sited high up on mountains in dark places where the atmosphere is less dense (thinner), especially in remote places where there is little pollution of any nature - dust/particulates or artificial light sources.

The Hubble Space Telescope has been put into orbit around the Earth acting just like satellite with an adjustable optical telescope. Since it is above the atmosphere, many of the problems of image quality described above are greatly reduced.

For any type of telescope, the larger the size of the electromagnetic radiation collector, the greater the resolution AND the farther you can look across the universe to the most distant objects, and back in time too!

The lenses, mirrors or radio dishes etc. of the telescopes are linked to powerful light detection systems (eg. photocells rather than the naked eye), which in turn, are linked to powerful computers to generate extraordinary detailed images that can be analysed to help develop and test out astrophysics theories.

How a reflecting telescope works

A reflecting telescope uses a concave mirror

A relatively large concave mirror collects as much light as possible from distant astronomical object e.g. a star.

The collect light is reflected by a small plane mirror at ~45o into an eyepiece or camera to record the image.

By means of a magnifying lens in the eyepiece tube you can produce a clear focussed and greatly magnified image of the star.

Advantages over convex collecting lens.

Atomic emission line spectroscopy - used by astronomers to identify elements in stars

If the atoms of an element are heated to a very high temperature in a flame they emit light of a specific set of frequencies (or wavelengths) called the line spectrum. These are all due to electronic changes in the atoms, the electrons are excited and then lose energy by emitting energy as photons of light. These emitted frequencies can be recorded on a photographic plate, or these days a digital camera.

Every element atom/ion has its own unique and particular set of electron energies so each emission line spectra is unique for each element (atom/ion) because of a unique set of electron level changes. This produces a different pattern of lines i.e. a 'spectral fingerprint' by which to identify any element in the periodic table .e.g. the diagram on the left shows some of the visible emission line spectra for the elements hydrogen, helium, neon, sodium and mercury.

The surface of stars is so hot that all the atoms can potentially emit their characteristic frequencies and from the frequency pattern an element can be identified.

This is an example of an instrumental chemical analysis called spectroscopy and is performed using an instrument called an optical spectrometer and is used to identify elements in stars when attached to a telescope.

Other electromagnetic radiation telescopes

Radio telescopes are used to detect and study naturally occurring emissions from objects such as stars, galaxies, quasars and black holes.

Fortunately, not all the extraterrestrial radio waves are absorbed by the Earth's atmosphere, but some are absorbed and others are reflected back into space by the upper atmosphere - much depends on the wavelength of the radio waves.

The radio telescope can consist of a large parabolic dish or large arrays of smaller signal receivers.

The electromagnetic (mainly radio) emissions received, provide information that can be analysed to understand the structure and functioning of these various astronomical objects.

Radio telescopes are used to detect the cosmic microwave background radiation which has helped understand and test out theories of the origin of the universe.

For radio astronomy, the shortest wavelength of radio waves that can pass through the Earth's atmosphere is 100 m.

Calculate the maximum frequency of the shortest radio waves that can be detected.

(speed of 'light' = 3 x 108 m/s):   v = f x λ    *    f = v / λ  = 3 x 108 / 100 = 3 x 106 Hz

X-ray telescopes are very good for detecting very explosive high temperature events e.g. exploding stars - supernova.

These extremely violent events emit the highest energy electromagnetic radiations.

 

The advantages of computerised telescopes

Most modern telescopes are linked to computers and can do many tasks automatically and be programmed to make a specific series of observations e.g. keep pointing at the same distant object in the sky.

Computers can help to produce clearer and sharper images and store them for later more detailed analysis.

They can collect and store huge amounts of images 24/7 and easily process the data with an amazing speed of analysis.

INDEX of my physics notes on ASTRONOMY


Learning objectives for astronomy Part 3

  • Be able to show an understanding of how scientists use waves to find out information about our Universe, including:
    • a) the Solar System - the Sun and orbiting planets and asteroids
      • The movement of the planets and asteroids has been observed from visible light (reflected sunlight) for thousands of years, initially with the naked eye and from the early 16th century onwards, with telescopes.
      • With modern techniques, the Sun can be observed by detecting emissions in various regions of the electromagnetic spectrum eg infrared, visible light, ultraviolet, X-rays and even gamma ray emissions.
    • b) the Milky Way - the view looking through our own galaxy
      • Until relatively recently, the Milky Way galaxy, has been observed with the naked eye and then telescopes on Earth, but now it can be viewed through powerful telescopes on satellites eg the Hubble Space Telescope. Our galaxy, and for that matter distant galaxies, can be continually observed using everything from giant radio telescopes, huge optical\visible light telescopes to gamma ray burst detectors.
  • Be able to compare methods of observing the Universe using visible light, including the naked eye, photography and telescopes.
    • In observing the night sky, the naked eye, apart from aesthetic appreciation, has been largely replaced by photography, usually coupled to a telescope.
    • However, historically, stars, planes, comets, our Moon have all been successfully discovered, observed, mapped and plotted via naked eye observations and astronomical tables of data assembled.
    • Distant stars can be seen because they are so hot and powerful emitters of electromagnetic radiation eg visible light.
    • Telescopes off much better light gathering power than the naked eye and the lens and lens-reflecting mirror systems can produced greatly magnified images and can peer into deep space totally inaccessible to the naked eye.
    • Photographing the same patch of sky and comparing images from one night to another can show up whether an object is moving eg asteroid or comet or some new star appearing or an old star exploding in a massive supernovae explosion,.
    • So, anything that changing that reflects or emits visible light can be detected and by using long-time exposures you can detect very faint very distant objects.
    • The result of all these historical and continuing contemporary observations with telescopes of all kinds is to give us a pretty good picture of the observable universe, even if we don't fully understand how it all works!
  • Be able to explain how the eyepiece of a simple telescope magnifies the image of a distant object produced by the objective lens (ray diagrams are not necessary).
  • Be able to describe how a reflecting telescope works.

Keywords, phrases and learning objectives for astronomy

Be able to describe and explain the different methods of astronomical observation.

Know there are different types of telescopes - optical refracting or reflecting telescopes (optical astronomy) and radio telescopes (radio astronomy).

Be able to describe and explain the usefulness of telescopes and photography e.g. observing the Solar system and the cosmos in general with visible light.


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