Choosing a Telescope


The first telescope was invented by a man named Hans Lippershey in 1608. He developed the telescope solely for use on his ship, to see other possible invading ships from a distance. Following Lippershey’s invention of the telescope, Gallei Galileo was the first to point the scope at the night sky to view and study astronomical bodies. Today there are two well-known types of telescopes, refracting and reflecting. Both of these telescopes appear physically different but have the same overall purpose, to collect as much light (photons) from outer space as possible to better observe the sky.


Eyepieces, binoculars, eyeglasses and refracting telescopes all use lenses to focus incoming light. As light enters the glass, it is suddenly slowed down and focused, also known as refracting. Refracting telescopes can use two different lenses to refract the light, convex and concave. If you do not recall from elementary school, a concave lens is thicker around the edges than the center while a convex lens is thicker in the center than the edges.   Large convex lenses are called objective lenses and are used in telescopes instead of a primary mirror. Also, a smaller lens is used as the eyepiece lens where the image is magnified for viewing. Calculation of a refracting telescopes magnification power is the same as for a reflecting telescope.

Although refracting telescopes are popular with amateur astronomers because of the affordability and easiness to build, these telescopes suffer from a variety of problems that limit their use as research instruments:

  1. Chromatic aberration is where different colors of light are refracted by different amounts, resulting in different colors having different focal lengths. This causes the image to appear blurry, or 3d like when you don’t put the glasses on.
  2. Air bubbles in the glass will also cause distorted images
  3. The lens can only be supported around the edges to avoid blocking any incoming light. If the lens is large, it may cause it to sag, and in turn, distort the image.
  4. Glass does not allow all ranges of wavelength through it, it is opaque to some wavelengths.


Isaac Newton built the first reflecting telescope in the seventeenth century.  The basic principle of reflection is that the angle between the incoming light and the perpendicular line to the mirror is equal to the angle of the outgoing/refracted light and the perpendicular line to the mirror. Newton discovered, by using this principle, that by using a concave mirror, all of the light rays merge to a focal point within the telescope. The distance between this point and the primary mirror is called the focal length. A secondary mirror must be used to reflect the light rays to one side of the telescope so that the astronomer can view the image through an eyepiece lens.

To determine a telescopes magnifying power, divide the focal length of the primary by the focal length of the eyepiece.  If you try to magnify beyond that limit, the image will become distorted.

There are two main types of reflecting telescopes, a Newtonian and a Cassegrain.  The Newtonian telescope is very popular with amateur astronomers because it is easy to use while standing up. The Cassegrain telescope is mainly used by research observatories. Within the Cassegrain, the equipment is spread out and balanced so that the image is not distorted; also an advantage over using a Newtonian focus.


There are quite a few mounts you can choose from when purchasing a telescope. Alt-Azimuth and the Dobsonian are the most well-known and popular. Below I have listed the pros and cons for each.

Pros: It is compact, easy to set up, store, and carry. Also, the eyepiece is convenient for viewing.
Cons: Must be computerized to track objects in the sky, very expensive.

Pros: Least expensive, easy to set up and use.
Cons: Eyepiece is often in an inconvenient position. You cannot track objects in the sky or take pictures without specialized equipment.


Although binoculars are a quick and easy way to view astronomical objects such as the moon, star clusters, the Great Nebula of Orion, and so on, telescopes allow us to observe SO much deeper into space. Previously I have explained the different types of telescopes so it is up to you to choose which is best for you. If you plan to take photographs, a Cassegrain telescope will be your best option given that it is very balanced and can track objects in the sky. You will need a tracking motor, to follow objects in the sky, and a camera adapter. After purchasing a telescope it is important that you buy a few different eyepieces. This will enable you to look at large areas of the sky at low magnification and to view smaller areas/details of the sky at high magnification.  Unless you purchase a computerized telescope, it is vital that you buy a finder scope, with crosshairs, to attach to your telescope. This will help you zero in on certain objects of interest.

WARNING: If you choose to view the sun and its sun spots, be sure to buy a very good sun filter. Looking at the sun is very exciting but viewing it with the naked eye is very harmful and could cause blindness without you feeling any pain!!


About MissCrys66

I am a 20 year old student at Saginaw Valley State University. I have just completed my third year of college in pursuit of a finance degree. This summer I am attending Delta College as a guest student and have created this blog for use within my professional writing class.

Posted on June 17, 2011, in Astronomy, Sun and tagged , , , , , . Bookmark the permalink. 2 Comments.

  1. Wow! That’s a lot of information! Have you purchased a telescope for yourself? Is it something you would like to do now that you have taken the astronomy class and learned more about studying the night sky? I don’t know many people who own telescopes – it seems that for a while, when space travel was new, more people had them. Cheap ones, but still, it was something. Nowadays, we seem to have lost interest in it – or is it just my perception? I wonder. Good collection of information. You link to sources, but it’s also good if you can indicate where you got your information – online or from your class text – just so readers know the source.

  2. I have not purchased a telescope and probably won’t in the near future (too pricey). I am very intrigued by the night sky but naked eye obnservations will do for now. You are right, it does seem like the people who do own telescopes, are letting them collect dust in the basement.

    All of the information in this blog post came directly from my notes from class. I linked the types of telescopes so that people could get a picture of what they look like. I did not want to overflow my post with a ton of pictures.

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