19 Telescopes and Observations

Naseer Iqbal

Learning Outcomes

After studying this module, you shall be able to

Learn about telescopes, why telescopes are so important and why do not we just use our eye to study the universe
Learn how image is formed in the telescopes, telescope field of view and sky coverage
How do we track or follow the apparent motion of the stars caused due to rotation of the earth
Telescope observation across the electromagnetic spectrum and role of atmosphere in the observation and how to overcome the limits put by atmosphere.

1. Telescopes:

A telescope is an optical instrument that helps in the observation of far objects by gathering the visible light of electromagnetic radiation. The first working telescope was the refracting telescopes constructed in Netherlands in 1608. Thomas Harriot became the first known person, who pointed telescope towards sky to make telescopic observation of sky objects. Ground based astronomical telescopes around the world costs a capital investment of billions of dollars. A single space telescope likes Hubble Space Telescope had a price tag of 3 billion dollars with an annual operating budget large enough than if we build large ground based telescope per year. The question is why do we spent huge money to build telescopes and why not just use our eye to study the universe? There are enough answers to these questions and some of them are:

Telescope collects more light than the unaided eye.
Telescopes has higher angular regulations (to see fine details) than the unaided eye.
Telescopes allow us to observe the portion of electromagnetic spectrum which is not visible to eye.
Telescopes with the help of detector permanently records the details of observation.

Bigger telescopes have better angular resolutions and they collect more light. This helps us to study fainter objects more accurately and allows us to observe finer details of the sources.

1.1 Image Formation

A telescope requires a pair of lenses shown in figure 1. The front lens is known as objective lens, it has a focal length as long as possible and focuses an image. The lens in back is known as eyepiece lens, it magnifies the image. A telescope forms an image in the focal plane as indicated in Figure 3.1. Since astronomical objects are very far, the first lens forms a real, inverted image just beyond its focal point. The magnification of object lens is generally small number. The eyepiece is positioned such that the image formed by the objective lens is essentially at the focal distance of it. Instead of eyepiece we can put some sort of detector, or device to record the image (such as a piece of film or a CCD), in the focal plane, that is also a telescope.

1.2 Field of View and Sky Coverage:

The field of view (FOV) determines sky area covered by telescope observation. It depends on the focal length of the telescope and the area of the imaging detector. It varies inversely with magnification. A telescope having small field of view will have greater magnification and light gathering power. Using single CCD detectors, the angular area covered will be smaller with larger telescopes because larger telescopes usually mean longer focal lengths.

Using the idea of angular area or solid angle, the sky area covered by an image can be measured. Angular area corresponds to angle just as area is to length; area of the sky 1 degree on a side will have angular area of 1 square degree. Solid angle is measured in Steradian, which is the angular area of sky patch that is 1 radian by 1 radian. There are 4π steradians in a complete sphere and In analogy the entire sky should has a solid angle of 4π. However the angular covered by the CCDs and telescopes is often small fraction of the Steradians. The FOV covered by the CCD is less than a square degree and there FOV are usually given in square arc minutes. Therefore we need huge number of CCDs in the same plane to get image of entire sky.

Figure 3.1:

1.3 Angular Resolution:

Angular resolution is the ability of telescope, to distinguish small details of an object. The resolution of the imaging system can be limited either by aberration or by diffraction causing blurring of the image. Aberration is related to geometrical phenomena and can be solved by increasing optical quality at the expanse of the cost of the system, while as diffraction is due to wave nature of light, determined by the finite aperture of optical element.

By definition point source is one having no angular extent. This mean image of a star should be a point. But actually this does not come out and the reason for this is that the atmosphere smears out the light from a point source (seeing). Even though if we put our telescope outside the atmosphere, it will not focus a point source into a point image and instead produces a “bulls eye” pattern (Airy disk). This happens because waves from different parts of the mirror interfere with each other in such a way as to result in the Airy disk pattern with a central peak and then a series of dark and light annuli. Figure shows a cartoon of an Airy disk along with intensity along Airy Point Spread Function. If the aperture of the lens is narrower the PSF will get dominated by diffraction and hence we can calculate angular resolution of system by Raleigh criterion. The angular resolution of the first dark ring is given by

Θ=1.22λ/?

Where λ the wavelength of the radiation, D is the diameter of the primary lens. Larger the diameter of the primary lens (mirror), the smaller will be the angular size of the image of the point source. The smaller the angular size, the easier is to separate two point sources close together in the sky. In case of radio, the λ/D is much larger than in the optical this means that in single dish radio telescopes the sharpness is limited by diffraction. Also the angular resolutions of single dish radio telescopes are much poorer. Therefore to achieve better resolution, radio telescopes are coupled together in interferometers. Many radio telescopes are parts of interferometers.

Figure 3.2. Cartoon of an Airy disk along with intensity along Airy Point Spread Function

2. Telescope Mount:

Telescope mount is a mechanical structure which is designed for supporting the mass of the telescope and allowing the accurate pointing of the instrument at any spot on the sky and to track or follow the apparent motion of the stars caused by the rotation of the earth. To track motion of the stars with the rotating Earth, different types of mounts have been developed over the years. Among these Alt-azimuth mount and Equatorial mount are mainly used. Both types of mountings have their own advantages and disadvantages.

2.1 Altazimuth Mount:

The advantage of alt-azimuth mount is simplicity in its mechanical design. This uses a two perpendicularly axis (horizontal and vertical axis) for support and rotation of telescope. So that rotation of horizontal axis varies angle of elevation (up and down) of the pointing direction of the instrument and rotation about the vertical axis varies the azimuth (side to side) of the pointing direction. This mount will not be able to follow the motion of the astronomical sky objects as Earth spins on its axis. The other limitation is that as telescope tracks, telescopes field of view rotates at varying speed while as telescope body does not. This requires counter rotation (microprocessor based) of field of view during astrophotography or astronomical imaging. This mount has also blind spot near the zenith that makes the tracking rate of azimuth too fast to accurately follow equatorial motion, if the altitude is exactly 90 degree, the speed becomes infinite. Therefore alt-azimuth mount cannot track smoothly with in a zenith blind spot though it can point in any direction.

Despite additional costs for incorporating complex tracking and image orienting mechanism, the simple design and structure of alt-azimuth mount allows significant cost reduction. Further simplified mounts enables telescope structure to be more compact, which further reduces the cost reduction of dome structure. These reason makes alt-azimuth mounts more advantages to use large telescopes.

2.2 Equatorial Mount:

An equatorial mount have one rotational axis tilted and parallel to the Earth’s axis of rotation. This compensates the rotation of earth and allows the instrument to stay connected on any objet in the sky having diurnal motion by driving one axis at constant speed. However tilting the polar axis adds complexity to the mount, therefore mechanical systems should be engineered to support one or both of the axis. Equatorial mounts therefore differ from mechanically simpler alt-azimuth mounts as they require variable speed motion around both axes to track a fixed object in the sky. Also in this case the image does not rotate in the focal plane; therefore equatorial mounts are important, particularly for astrophotography. These mounts come in different shapes like German Equatorial Mount (GEM), equatorial fork mounts, Poncet Platform. These designs need large counter weights to counterbalance the mass of the telescope, larger domes needed to cover the bigger mechanical size and range of movement of equatorial mounts. These makes equatorial mounts less viable in large telescopes and are largely replaced by the alt-azimuth mounts.

3. The Atmosphere from Observation Point:

The Earth’s atmosphere provides essential to life. It insulates the humans of Earth from the extreme temperatures of space and protects us from harmful radiation like x-rays and ultraviolet radiation and cosmic rays. It contains the oxygen that we breathe. Without atmosphere there would have been no life on Earth. But the same atmosphere hinders ground based astronomical observation to greater extent. The offhand problems are clouds and air pollution. These two are not that important as we can locate our telescopes at places where clouds are least frequent and far from civilization areas. Besides these two, the other main deleterious effects of the atmosphere are

On ground, we observe small window of electromagnetic spectrum as Earth’s atmosphere allows small portion of electromagnetic wave to pass and attenuates it major portion. The two windows that Earth’s atmosphere passes are optical window and radio window. Infect, the atmosphere is not completely transparent even in these windows.
The second limitation is seeing. As the light passes from celestial objects passes through the atmosphere, smearing images of these objects are produced. This not only causes us to lose details of the object, it also makes it much hard to measure the brightness of faint objects.
Even far from city lights, the atmosphere glows due to atomic processes in the air. This light emitted called sky glow and again puts severe problem such as degrading the accuracy of measurement and adds further constrain on observing faint objects. Near cities, the situation becomes much worse, as the atmosphere, besides glowing, also scatters light from artificial sources, making the sky appear even brighter.
Atmospheric refraction is one more irritant introduced by the atmosphere. In this case atmosphere spreads out light in small spectrum along the line pointing to the zenith. Refraction smears out Images as we observe with a filter covering wide range of wavelength.
Atmospheric extinction is another problem which atmosphere introduces, causing the object to look dimmer than it would be without the atmosphere. Because atmosphere absorbs and scatters fraction of light at optical wavelengths. However we can measure this and correct for this effect on observation. These are therefore least deleterious effect compared to others.

These irritant severely effects our observation on ground and can occur regardless of weather and location. In addition to these effects there are other nuisance like Wind shakes degrades image quality, high humidity coupled with pollution can degrade the optical surface, clouds block lights caused by the Earth’s atmosphere.

4. Ground and Space based Observations:

Astronomy is the branch of science limited to observation and study of cosmic sources. The main source of cosmic information is the electromagnetic radiation that we receive on or near earth. Electromagnetic radiations virtually carries all the information that constitutes our knowledge of the Universe and based on this physics is built. Though electromagnetic waves acts as main sources, there are others also such as gravitational waves, neutrinos, elementary particles like protons or meteorites. The aim of observation is to develop high sensitive technique/telescopes to collect this information. However most of radiations are absorbed or distorted before they reach the ground based telescope. Only portion of radiations from electromagnetic spectrum reaches the earth and this limited radiation has provided astronomers enough information to know the general shape and size of the universe, but there is much left to learn. It is therefore essential to study the entire spectrum. The only way to study remaining portion of electromagnetic spectrum and also to get rid of deleterious effects of atmosphere is to put telescope into the space. However, putting telescopes in space opens up its own set of problems such as extreme cost, need to control remotely, and inability to service easily etc. We can build bigger telescopes on the ground than space.

4.1 GROUND BASED OSERVATION:

Ground-based observatories are located on the surface of Earth and far from major population to avoid contamination of noise. The best locations for having good observations are regions that have dark skies, a large percentage of clear sky nights per year, dry air, and are at high elevations. To study objects in space, astronomers use a number of telescopes sensitive to different parts of the electromagnetic spectrum and based on the observation in the radio, optical, we have mainly two types of ground based telescopes viz radio telescope, optical telescope. However, with surprise astronomers also use ground based astronomy to detect very high energy (VHE) gamma-rays. These telescopes don’t detect the gamma-rays directly. Instead they convert the limitation of atmosphere of not allowing high energy radiation to pass through into an advantage by using atmosphere itself as the detector. We will briefly introduce these observations across the electro-magnetic in the coming section.

Figure: 3.3

The Earth’s atmosphere attenuates electromagnetic radiation most at infrared, ultraviolet, X- ray, and gamma-ray frequencies; therefore there are only two atmospheric windows namely the radio and visible wavebands that are suitable for ground-based astronomy. The visible window is relatively narrow in terms of logarithmic frequency or wavelength. Early view of universe was limited only to visible objects such as stars, clusters and galaxies of stars, hot gas ionized by stars because only visible light can be seen without the aid of instruments. The radio window is much wider than the visible window. It spans roughly five decades in frequency (10 MHz to 1 THz) and therefore includes wider range of astronomical sources and emission mechanisms (thermal and non thermal). Radio astronomers usually measure frequencies instead of wavelengths.

4.1.1 Radio observation:

Radio telescopes usually do not have domes. Radio waves can pass through the Earth’s atmosphere without significant obstacles and can observe even on cloudy days. A special technique called interferometry is used in radio telescopes. Using this technique we can combine data from telescopes separated by large distance and create images as if taken by single big telescope with size equal to the distance between the two telescopes. Therefore these telescope arrays can scan incredibly small details of the object. This window was used by astronomers before space based telescope observations. Therefore early radio astronomy was the science of serendipity and discovery. It had revealed a parallel universe with unexpected sources never seen in optical window. This parallel universe is often violent with high energy and explosive phenomena going in contrast to steady light output from visible stars in optical universe. These radio sources are mainly powered by gravity instead of nuclear fusion (the main energy source in visible stars).

4.1.2 Infrared (IR) observation:

Ground-based IR telescope needs high altitudes and dry climates to overcome absorption of IR from water vapor present in the atmosphere. We know anything that has heat emits IR. This means atmosphere, the telescope, and even the infrared detectors themselves all emit infrared light. To account for atmosphere, the infrared radiation from the atmosphere is measured simultaneously with the cosmic object being observed. Then, for correct measurement of the cosmic object, the emission from the atmosphere is subtracted. Also telescopes are designed to control IR radiation from reaching the detector and detectors are cooled to limit their IR emission.

4.1.3 Optical Observation:

Optical astronomy is as old as humanity because visible light can pass through our atmosphere. Ancient people were doing there astronomy by looking with their naked eye up at the night sky. Today we have large number of high sensitive optical telescopes for doing visible astronomy. Optical telescope collects light mainly from the visible part of the spectrum and focuses them. These telescopes increase the apparent brightness and angular size of distant objects. The optical telescopes may be are refracting, reflecting or catadioptric (combination of mirrors and lenses). Optical telescopes are constructed in a dome like structure, to shield the fragile instruments from the outer elements. These domes have a slit opening in the roof and the entire portion of the telescope dome can be rotated. This slit opening of these telescopes makes them possible to operate only during observation period only and helps them to remain closed when the telescope is not in use. Rotation of the dome allows the instrument to observe different sections of the night sky.

4.2 Space Based Observation:

It is impossible for all portion of the electromagnetic spectrum to pass through the Earth atmosphere. Even there will be limits for doing the optical astronomy using the ground based observations. The light passing through the atmosphere gets distorted by the turbulence within the air, this limits the quality of the images for faint sources. We can avoid these limitations using the space based observations. The advantage of space based telescope is that we don’t have to look through the Earth’s atmosphere and very detailed information of the source can be obtained. Space based telescopes orbiting the Earth outside atmosphere avoids twinkling and light pollution from artificial light sources on Earth. This helps us in obtaining the angular resolutions in these telescopes much smaller than the ground based telescopes with similar aperture. Hubble Space Telescope is the most famous optical telescope in orbit. Also with the interferometry technique, if we put radio telescope in orbit around the earth, we can make images as if there is telescope with size equal to that of entire planet.

Space based astronomy is particularly important in frequency outside the optical and radio window of electromagnetic spectrum. These are the only two wavelengths that are not severely attenuated by the atmosphere. X-ray astronomy is currently important only because of orbiting X-ray telescopes such as ASTROSAT, Chandra observatory etc. The X-ray astronomy is nearly impossible to be carried from Earth; infrared and ultraviolet are also largely attenuated. However, space based telescopes are more expensive than ground based telescopes and they are difficult to maintain and also it is much difficult to carry out services of telescopes in space.

4.2.1 Ultraviolet observation:

The Earth’s atmosphere is opaque to ultraviolet light, so this part of astronomy must be done using telescopes in space. An ultraviolet telescope is much like a regular visible light telescope other than selected filters. The Hubble Space Telescope Swift UVOT telescope both can perform a great deal of observing at ultraviolet wavelengths.

4.2.2 X-ray observation:

X-rays are more energetic than UV photons and therefore are also blocked by the Earth’s atmosphere. Because these radiations are so much energetic and small in wavelength, they pose additional challenge of not getting bounce back from the reflecting surface and passes right through the material. Therefore, traditional orientation of lenses and mirrors in telescopes just didn’t work in these telescopes. These rays can be made reflecting only if they have mirrors that are made of material that will reflect an X-ray photon and X-ray photons just glances the surface of the mirror, called “grazing incidence”. To achieve grazing incidence the mirror orientation should be such that the mirror surfaces are nearly parallel to X-rays and therefore hit the mirror at a very shallow angle. For example Wolter telescopes composed of glancing mirrors made of heavy metals and sections of rotated parabola and hyperbola or ellipse, reflects the rays in few degrees. Chandra X-ray Observatory, ROSAT and Einstein Observatory uses this type of telescopes.

Figure: 3.4 Illustration of grazing incidence.

The scale in this image is exaggerated the angle of incidence (the angle between the mirror surface and the X-ray) is actually shallower.

4.2.3 Gamma-ray observation:

Gamma-ray is very energetic having millions of times more energy than visible photons. Therefore they have the difficulty of observing from the ground as high energy and short wavelength photons are blocked by the Earth’s atmosphere. So we have to use space based observations in this case. Secondly, they are more difficult to focus than X-rays. Till date there is no focusing gamma ray telescope. However, there is alternative way to determine the gamma ray sources in the sky. These methods are based on the way they interact with matter such as Compton scattering, photoelectric ionization and pair production process. All these processes cause the motion of electron and the current produced in this way can be amplified. This current will contain the information of energy and direction of the original photon. There are two classes of Gamma-ray detectors viz spectrometers and imagers. Spectrometers are like “light buckets” directed towards the region of the sky containing the object of interest and collects as many photons as possible. These use solid state detectors to transform gamma-ray signal into the electric signal. Second class i.e imager, perform the task of gamma-ray imaging and They calculate the direction of incoming photon either by interaction of photons with matter such as pair production process or by using the coded mask to allow an image to be reconstructed.

For More Details (on this topic and other topics discussed in Text Module) See

1. Photoelectric Photometry D.S. Hall & R.M. Genet, Willmann-Bell 1988

2. Astronomical Spectroscopy C.R. Kitchin, IOP 1995