Telescopes

There are many different kind of telescopes such as Optical telescopes, radio telescopes, gamma ray telescopes, infrared telescopes, X-ray telescopes etc. But Optical telescopes are the first of the kind and hence has maximum derivatives available. Also these telescopes are used in amateur astronomy and hence the telescopes are very much developed and evolved in true sense. So we are now going to learn the classification of telescopes for better understanding of what telescopes are used and why.

Optical Design: The telescopes can be classified in terms of the optical design in the following categories:

• • • Refractors: Refractor telescopes are telescopes which uses a convex lens as the primary optical surface. The rays of light as pass through the lens is converged and meet at a point called focus. The eyepiece used is also adjusted for its focus to meet at the same point and hence a share image is obtained. Refractors work on the principle of refraction. Refractors have a size constraint and the largest refractor telescope has an aperture of 1m which makes the lens very heavy and hence operations and holding the lens is a challenge.
    • Reflectors: Reflector telescopes are telescopes which uses a concave mirror as the primary optical surface. The rays of light when falls on the surface of the mirror are bounced back converging and are intercepted by a highly polished secondary mirror which further reflects the light towards the eyepieces. The reflector telescopes have further different design pattern modified over the period of time to use it to its maximum potential. One advantage of mirrors over lens is that the mirrors does not come with size limitations. Since mirrors can be supported at the back so we can infinitely big mirror and still the size won’t be a problem where as the lens will collapse due to its own weight beyond a certain limitation. Lets now focus on different types of reflector telescopes:
      • Newtonian Reflectors: Newtonian reflectors are the oldest and the first ever reflector telescopes designed in the late 1600s when Issac Newton decided to improve the Galilean telescope for the imperfections it had. The newtonian telescope is named after Issac Newton. A newtonian telescope places the secondary mirror at an angle of 45 degrees and let the light bounce off the secondary mirror moving perpendicular to the direction of incidence and the ray or light comes out on the side. The eyepiece is placed on the side of the tube and can be seen. This is the most common design and hence is used in almost all contemporary reflector telescopes.
      • Schmidt Cassegrain: Schmidt Cassegrain Telescopes (SCT) are highly modified newtonian telescope, modified so much that now it has its own class of telescopes. SCTs are often compact and light weight with wider FOV as compared to their newtonian counterparts and hence preferred for its versatility offering wider FOVs, brighter object and higher magnifications. There are a number of changes made, first being adding a glass element called as corrector plate which ensures the light entering the telescope is parallel and there is no disturbance in the waves. This makes sure the image will be free of any error. Second modification has been to put a secondary mirror exactly parallel to the primary mirror however the reflecting surfaces facing each other. This alteration bounces the light back to the primary mirror towards the centre. This lead to cut a small hole at the centre of primary mirror so that the light can come out and hence we can put an eyepiece and observe the celestial objects. The mirror used here is convex lens.
      • Maksutov Cassegrain: The Maksutov (also called a “Mak”) is a catadioptric telescope design that combines a spherical mirror with a weakly negative meniscus lens in a design that takes advantage of all the surfaces being nearly “spherically symmetrical”. The negative lens is usually full diameter and placed at the entrance pupil of the telescope (commonly called a “corrector plate” or “meniscus corrector shell”). The design corrects the problems of off-axis aberrations such as coma found in reflecting telescopes while also correcting chromatic aberration. It was patented in 1941 by Russian optician Dmitri Dmitrievich Maksutov. Maksutov based his design on the idea behind the Schmidt camera of using the spherical errors of a negative lens to correct the opposite errors in a spherical primary mirror. The design is most commonly seen in a Cassegrain variation, with an integrated secondary, that can use all-spherical elements, thereby simplifying fabrication. Maksutov telescopes have been sold on the amateur market since the 1950s. There are many Maksutov designs that use a Cassegrain configuration, mounting a convex secondary mirror near the focus of the primary mirror. Most types use full-aperture correctors and are therefore not very large, since the corrector plate rapidly becomes prohibitively large, heavy and expensive as the aperture increases, with very long cool-down times to reach optimal optical performance. Most commercial manufacturers usually stop at 180 mm (7 in).
      • Ritchey – Chretien: A Ritchey Chretien telescope or RCT is a specialised variant of the cassegrain telescopes that has a hyperbolic mirror and a hyperbolic secondary mirror designed to eliminate off-axis optical errors (coma). The RCT has a wider FOV free of optical errors compared to a more traditional reflecting telescope configuration. Since he mid 20th century, a majority of large professional research telescopes have been RCT, some well know examples are the Hubble and Keck and ESO-VLT.

Mount Design: The telescope tubes are so big and are so heavy, they require some kind of mounting system which will help the observer use the telescope. Here are a few mount classification:

• • Equatorial Mount: The most professional type of mounting is “Equatorial mounting”. The mounting allows the user to lock one axis of the telescope movement with respect to Earth’s axis of rotation by pointing it towards the pole star. The second axis then can be moved only to follow the stars in the sky. This kind of movement is preferred in the filed of astrophotography and also will help people do celestial observations easily.
  • Altitude-Azimuth Mount: This kind of mounting is used in most small and basic telescope which are used only for visual observation. They are easier to set than Equatorial mounts up but less easy to use when compared. The mount needs to move in both the axis which is along the horizon and rising or setting in the altitude motion. It is difficult to track the object manually in the telescope.
  • Dobsonian Mount: Built by John Dobson in 1960s, the mount is a modified Altitude Azimuth mount only the difference is the set up takes about a minute and the apertures can be increased drastically without the worry of mounting capabilities. Today Dobsonian mounts are the most popular mounting used along with newtonian optical design among the amateur astronomers around the world.

Telescope Drive: The driving power of the telescope is one of the most important aspects of choosing the telescope. It cost fortune and you do not want to invest into one which does not work the best for you. A telescope can be driven using one of the following methods:

• • Manual Drive: Manual drive mounts are mounts which are controlled manually by the user. Manual mounts are used by the application of force by the user otherwise the telescope will not move on its own.
  • Electronic Drive: Electronic drive mounts are mounts which are controlled by a motor drive. They lack the ability to follow commands but they will continue to move in the direction with predefined speeds, following an object which was in the view.
  • GoTo System: The GoTo system of telescopes is a special capability provided to the telescope mounts to follow the command using an inbuilt computer which contains all the necessary information about the sky. All the used need to input is the name of the object and the telescope will identify the position of the object in the sky and go to that location and hence will be easy for newcomers to start with this hobby.
  • Remote system: Remote systems are special network of electronic circuitry and internet connection to use the telescope from a remote location. Ideal for observatories and research facilities which will enable them to use the telescope from a far away location.