Telescopes, do you have one ?

Types of  telescopes
Evolution of  telescopes
Future telescopes projects
Achievements due to optical observation with telescopes

Refractors
Reflectors
Compound Telescopes

Commercial Refractor

    Refractor telescopes are those that most people perceive as the appearance of an optical telescope. In fact, most professional telescopes are not these kind.

    The telescope works as shown below:

    No mirrors are involved. The image is inverted but if you have a spotting scope with an upright image, then it is simply this arrangement plus another lens in between these two.

    Advantages:
        - Simple maintenance
        - Good for beginners

    Disadvantages
        - Expensive, in terms of aperture size
        - Suffers from chromatic aberration, resulting in blue fringes around bright objects
        - Sagging of objective lens, so large apertures are unrealistic
        - For large apertures, the optical tube gets very long and bulky

    Recommendation:
        For beginners, buy a spotting scope instead, then move on to compound telescopes as you        mature.

    Reflectors, or Newtonian telescopes, are the least popular of the three in the amateur sense. In the professional sense, this configuration is highly preferred due to its simplicity in structure. It is not popular with amateur because it gathers dust easily which is not easily removable.
 
    If you worry about the secondary mirror blocking light to the primary mirror, relax, the blockage is negligible. The internal workings are shown below:

    Advantages
        - Least strain on the observer, as the light comes out from the side rather than the bottom
 
 
    Disadvantages
        - Mirror may tarnish
        - Difficult to maintain as dust easily accumulates on the mirror
        - Unstable setup for astrophotography

Commercial compound telescopes
 
    This type of telescope can be made in large apertures and still maintain a short tube because of the multiple folding of light. For professional astronomy, this type of telescope offers apertures from 8-inch to 16-inch. This is the type of telescope that is suitable for all kinds of astrophotography that requires a telescope. The telescope also gives a wide angle view due to the spherical shape of the the primary mirror rather than the parabolic shape in Newtonians.

    Advantages:
        - cheaper than refractors of the same aperture
        - enough back focus for astrophotography
        - wide angle view, 8-inch with 30mm eyepiece has 0.4⁰
        - stable as tube is short
        - computerised models are easily available
        - very easy to maintain

    Disadvantages
        - very heavy, 8-inch is about 16 kg, with tripod
        - many screws, easily lost and no replacements are easily available
        - very wide tube results in bulkiness during transportation

    For this part, two important figures must be mentioned, Galileo Galilei and Newton.

    1609, a Dutch lens maker, noticed that when he put together two lenses, distant objects appeared closer and Galileo heard about it and start making refractors. He is given credit for using it for it for astronomical discoveries. He faced much opposition from Greek philosophers who were firm believers of Earth-centred universe.

    Towards the end of the 17th century, Newton thought of a way to overcome the defect of chromatic aberration of lenses by using mirrors. He bulit a reflector with a spherical mirror (wrong shape).

    Refractors and reflectors are built to greater sizes in the 19th century. Refractors reached a limit when the Yerkes 1 metre refractor is built. Larger lens will sag under its own weight and require an extremely long tube to house it.

    Hence, reflectors are being looked forward to greater apertures. In 1917, the 100-inch Wilson telescope became the largest telescope in the world. George Hale was in favour of even larger telescopes, so he proposed and funded for the 200-inch Hale telescope. This took 14 years to build and the construction itself was legendary. The polishing of the mirror took many years and involve using thumbs as the polishing tool.

    A new way was discovered to make bigger mirrors, that is to use 36 segment mirrors, each of 2m in diameter to make the world largest, Keck 1 & 2 telescopes. They each have 10m aperture. Both are located on Mauna Kea in Hawaii.

    The best telescope is Hubble Space Telescope. Although a 2.4m aperture, it has the greatest view because it is above the atmosphere. During its first light, scientists realised that it has a fatal error, the mirror is of the wrong curvature. Despite that, it still provides better views that ground telescopes. It is in fact the most expensive telescope in the world, so repairs must be done to make the telescope work better. In 1993, the repair went on flawlessly and the scope is working better than the scientists had expected.

    The next method of improving the views of the ground telescopes is to use aperture synthesis method, where the light of 2 or more telescopes superimpose to bring out faint details.

    European Southern Observatory project is to put 4, 8m aperture telescopes side by side in Chile and employ aperture synthesis.

    In fact, the 2 Keck telescopes intend to use aperture synthesis to achieve better views than the single Keck telescope.
 

    Galileo used his telescope to discover craters on the Moon, phases of Venus, Moons of Jupiter  and sunspots. (Note: in his notebook with drawings of Jupiter's moons, an eighth magnitude star is seen, that is Neptune.) His discoveries eventually overthrew the concept of an Earth centred universe.

    The 100-inch Wilson telescope led Edwin Hubble to discover that Andromeda Cloud is not in the Milky Way, but far from it. This led to the famous Shapley-Curtis Debate.

    The Hubble space telescope led to more amazing discoveries. The most luminous stars, white dwarfs in 47 Tucanae, black holes in the bulge of spiral galaxies and galaxies evolution in the Deep Space View, whose exposure lasts for 10 days. It also found that quasar are actually ultra bright centres of galaxies.

  Quasars are actually galaxies !
  The pistol star.