Indian Space Station

The Magic of Glass - Part - 2

The Magic of Glass
Part 2

In this part of the series, we take a look at the different types of telescopes readily and cheaply available (mind you, cheap is only a relative term) in the market today.

  • Note- Focal length is nothing but the distance that from the lens to the convergence point.
  • The F-ratio of the telescopes is calculated as the focal length of the lens divided by aperture of the lens (the size of the objective lens or mirror in your telescope).
  • F.O.V(Field of View) is quite simply how much area you are seeing at any point

The Focal length and the F/ratio play a vital role in the use of a telescope, it is very important you know what they mean.

The F-ratio varies in classification from “fast” to “slow”, a fast f/ratio means the image will be brighter and will give a wide field of view, the Fast f/ratio’s up to f/6 provide great wide field views. Faint nebulas and Star Clusters which are spread across a large area are much better rendered, but there is one shortcoming, the magnification is not the greatest, if you have other viewing interests, the telescopes with f-ratios of f/10 and above are best for planetary viewing and looking at binary stars. The Telescopes between f/6 and f/10 are a middle route that most tend to take if you are not really interested in the details of it all. Many hobbyists keep multiple telescopes for this very reason.

The Eyepieces

The eyepiece can be called the heart of the telescope, for without the eyepiece, the Telescope is quite useless. Having mentioned that, you need to know what type to buy and which one, there are plenty of options. The first thing to remember about the Eyepiece is, the lower the focal length of the Eyepiece, the higher the magnification you are going to achieve. The Magnification can be very simply calculated, simply divide the focal length of the telescope by the focal length of the eyepiece. For example, if the Focal Length of the Telescope is 1500mm and the focal length of the eyepiece is 25mm, the magnification will be 1500 / 25 = 60x, but I have to disappoint you at this point by saying the earlier statement does not mean that if you use a 1 mm eyepiece with a telescope of focal length 1500mm, you will get 1500x of magnification. There is a limit on every telescope called the magnification threshold; it is roughly 60 x per inch of aperture. For example, I use a 6 inch Schmidt-Cassegrain Telescope, the Magnification threshold of my telescope is 6 x 60 = 360x. This means for all practical purposes, I cannot “bump up” the Magnification beyond 360 x and get a good, clean image.

focal lenght

If I choose to buy a 4mm eyepiece will that be useful to me? Lets check, 1500 / 4= 375x, My magnification threshold is 360 x, so in effect the eyepiece will not be useful to me, so before you buy any eyepiece, always make a quick calculation.


For easy reference and to avid any confusion, I have categorized them in 3 basic types of telescopes, and Binoculars

1- Refractors

2- Reflectors

3- Schmidt-Cassegrain or Maksutov Cassegrain.

Let’s have a look on what these can do and which one should you buy, if you intend to buy one,

Add comment

Security code