Choosing your first Astronomical Telescope
The article below is a quick guide for choosing a first telescope, and is exclusively based on my experience as an amateur astronomer. As the user delves deeper into the subject, he/she will tend towards one or the other optical design, or in other words "favoring" one type or the other. Also, there is no telescope that is of the category "One Size fits all". The telescope is always good for one purpose or good for another.
The Optical design of a telescope may be defined as the configuration in which the optical components of the telescope are arranged. Optical components are generally made of glass, some inexpensive telescopes also use poly carbonate plastic.
The Refractor telescope is perhaps the first optical telescope design to be invented. I shall conveniently skip the history. The design, explained in a simple manner consists of an Objective lens which focuses the light rays to a point called Focal Length, and an Eyepiece which is placed close to the Focal Length of the objective to obtain a magnified image. How the setup works requires a rather extensive class on Geometrical Optics. The figure on the left shows this type of telescope.One might wonder as to why I have shown the lenses in two different colours. Modern telescopes have objective and eyepiece lenses made up of atleast two different types of glass so that all the colors of the rainbow (which makes up white light) focus to a point. In reality there are three types of Refractor telescopes.
a) Achromatic Doublet Refractor: Contains a double lens made of crown and flint glass, typically separated by an air gap. These are good for general observations and visual use. If astronomical photography is attempted with this telescope, one may obtain the photos, but they will show the optical errors.
b) Apochromatic Doublet Refractor: Contains a double lens made of high quality, low dispersion glass and is optically better than the previous design. This telescope will focus all colours in the visual spectrum (white light) to a point. It can be used for astro photography. It also permits the use of higher magnification on moon and planets due to less optical errors.
c) Apochromatic Triplet Refractor: Contains a Triplet lens or Three lens objective made of very high quality, low dispersion glass and by far is the best for any purpose. However, since the manufacture of this telescope requires the grinding and polishing of three lenses of high precision, the price is very expensive as compared to type b or type a described above.
I am deliberately limiting the content of the page and giving a "taster" of the subject. This is owing to the fact that many experienced and advanced amateur astronomers have written entire books on the subject.
The Optical design of a telescope may be defined as the configuration in which the optical components of the telescope are arranged. Optical components are generally made of glass, some inexpensive telescopes also use poly carbonate plastic.
The Refractor telescope is perhaps the first optical telescope design to be invented. I shall conveniently skip the history. The design, explained in a simple manner consists of an Objective lens which focuses the light rays to a point called Focal Length, and an Eyepiece which is placed close to the Focal Length of the objective to obtain a magnified image. How the setup works requires a rather extensive class on Geometrical Optics. The figure on the left shows this type of telescope.One might wonder as to why I have shown the lenses in two different colours. Modern telescopes have objective and eyepiece lenses made up of atleast two different types of glass so that all the colors of the rainbow (which makes up white light) focus to a point. In reality there are three types of Refractor telescopes.
a) Achromatic Doublet Refractor: Contains a double lens made of crown and flint glass, typically separated by an air gap. These are good for general observations and visual use. If astronomical photography is attempted with this telescope, one may obtain the photos, but they will show the optical errors.
b) Apochromatic Doublet Refractor: Contains a double lens made of high quality, low dispersion glass and is optically better than the previous design. This telescope will focus all colours in the visual spectrum (white light) to a point. It can be used for astro photography. It also permits the use of higher magnification on moon and planets due to less optical errors.
c) Apochromatic Triplet Refractor: Contains a Triplet lens or Three lens objective made of very high quality, low dispersion glass and by far is the best for any purpose. However, since the manufacture of this telescope requires the grinding and polishing of three lenses of high precision, the price is very expensive as compared to type b or type a described above.
I am deliberately limiting the content of the page and giving a "taster" of the subject. This is owing to the fact that many experienced and advanced amateur astronomers have written entire books on the subject.
We now come to the type of telescope that utilizes a combination of mirrors to reflect and bend light so that it reaches the eyepiece lens. Some sources have some speculation as to who invented this design, so we shall stick to the fact that Sir Issac Newton made practical use of it. In the refrator design, the lens converges light to a point. Now if we have a mirror, which has the same curvature as the lens, the that mirror will reflect light the same distance, converging it to a point. These mirrors are front coated so that it is the curved portion that reflects the light. The figure on the left explains the optical design.
The optical design therefore consists of a Primary mirror or Objective, which is concave shaped (bowl shaped) that reflects light to a point in front of it. An eyepiece cannot be kept at this converging/focal point because if done so the head of the user will block all the light falling on the objective. Therefore, one needs to divert this light to the side of the telescope to use an eyepiece. The secondary mirror shown in the figure does this work. One may argue that the secondary mirror might block some light. Yes it does block light, but not as much as a human head would, and given the fact that mirrors are easier to manufacture than lenses, a bigger mirror can easily compensate for the small loss of light. In typical mass manufactured reflecting telescopes, the percentage of blocking may vary from 10% to 35% based on how long the focal length of the primary mirror is and what purpose is the telescope intended for. Please refer to the end of this article for a good book on telescopes.
For some comfort of the reader and beginner, some of the largest optical telescopes in the world are reflectors, including the Hubble Space Telescope which is currently the largest optical telescope in space.
The optical design therefore consists of a Primary mirror or Objective, which is concave shaped (bowl shaped) that reflects light to a point in front of it. An eyepiece cannot be kept at this converging/focal point because if done so the head of the user will block all the light falling on the objective. Therefore, one needs to divert this light to the side of the telescope to use an eyepiece. The secondary mirror shown in the figure does this work. One may argue that the secondary mirror might block some light. Yes it does block light, but not as much as a human head would, and given the fact that mirrors are easier to manufacture than lenses, a bigger mirror can easily compensate for the small loss of light. In typical mass manufactured reflecting telescopes, the percentage of blocking may vary from 10% to 35% based on how long the focal length of the primary mirror is and what purpose is the telescope intended for. Please refer to the end of this article for a good book on telescopes.
For some comfort of the reader and beginner, some of the largest optical telescopes in the world are reflectors, including the Hubble Space Telescope which is currently the largest optical telescope in space.
Depending on the purpose, telescopes can be designed in many ways. One other common design is the compound telescope. This telescope is part reflector, part refractor. That is, it has both lens (speciallly designed to bend light in calculated ways) as well as mirors to reflect the light. There are three prominent compound telescopes. the Schmidt Caasegrain, The Maksutov Cassegrain and the Ritchey Chretien(RC). Most professional observatories use RC telescopes. The figure on the left shows the general optical design. The RC telescope does not have a corrector lens. The secondary mirror itself is specially designed to negate some optical defects. All the telescope have advantages and disadvantages and the user will have to weigh his/her options before taking the plunge. It does not matter which telescope you buy, you will enjoy numerous hours under the stars because these instruments will show you the Universe like you have never seen before.