Telescope Guide

  • January 29, 2019
  • Earth

The soul without imagination is what an observatory
would be without a telescope.

Henry Ward Beecher

If you ever look at the sky and wonder how beautiful and at the same time fascinating the night sky is , each tiny dot represents a sun like or bigger star that may or may not consist a life and I am sure the second thought in your mind would be how it looks like from telescopes …
And next thought in your mind would be buy one (telescope) but at the same time I know its expensive and hard to choose which one is right one for you ..right ..?
Well , I can solve one of the problem for you , I can guide you through the points and thigs to consider if you are planning to buy a telescope ..

There are total 5 things you need to understand before buying telescope (physics high school concept you probably know it but let’s discusses in context of telescope.

  • Aperture
  • Focal Length
  • Magnification
  • Focal Ratio
  • Resolving Power

Aperture (Bucket of light)

It is diameter of main lens or mirror of your telescope
More (bigger) aperture = Brighter Image (because it capture more light rays) and Dipper into space you can see.
Tip : Good Backyard telescope with mature stargazer have 80 mm to 300 mm(3.21” to 12”) aperture scope

Focal Length (Show me the Image)

Once light falls onto the mirror or pass through a lens, It’s directed by curvature of the optic to come to focus at a plane some distance area, the length over this happens is called Focal Length.
This length directly affects the magnification potential of the telescope when paired with an eyepiece.
In astronomy , where objects are infinitely, the image comes to focus at a plane that pass through the focal length how fascinating it is …

Magnification (Far and Away, Up Close)

To get an image suitable for observation with our eyes, a telescope uses second lens or collection of lens called eye pieces at the end of focal plane.
It’s the number of times in size an object appears, compared to viewing it with the naked eye, for example magnification of 32x means what you are looking at will look thirty-two times larger than when viewed unmagnified.
How to calculate magnification? Divide Focal length of objective lens with focal length of that eye piece. For example a telescope that has a 1500mm focal length, using a 25mm eyepiece will produce a magnification of 60x, and a 10mm eyepiece produces 150x.

Focal Ratio (Faster, Brighter and Smaller)

This term is defined as the ratio between the focal length of the scope and the aperture (the F-Number)
It’s a focal length of Objective mirror divided by Objective mirror’s Diameter for example, A 100mm aperture 1500mm focal length telescope will have a focal ratio of f/15.
So f number gives you idea about overall size and portability of the scope. Lets say you’re considering buying a 12" f/5 or a 12" f/15 Dobsonian (don’t worry I explain it later in blog). Just by looking at the f/ratios, you can tell that the f/5 one will have a much shorter tube length and can probably be handled by one person, whereas the f/15 will be massive.
And A longer Focal length Higher magnification but narrow field of view with given eyepiece so grate for observing moon crater, nebula or double star on the other side Lower focal length gives you wide field of view so you can see galaxies ..

Resolving Power (Sorting one star from other)

Resolution of a telescope is a measure of its ability to distinguish small details of a object of differentiating 2 very close space objects from each other
Resolving power is directly proportional to the aperture of the scope.

I am pretty sure you get the basic idea about what aperture and focal length telescope you want , Now let’s discuss something more practical , types of telescope you will find in market or online site when you search…

There are main 3 types of telescope available for stargazers…

  • Refractor
  • Reflector
  • Compound


It has lens at front of the tube.
These are excellent for observing objects within our solar system—planets and the Moon and, with the right accessories, they can be used for terrestrial viewing.
Pros: The simplicity and reliability of the design makes it easy to use and requires little maintenance.
Cons: Quickly get expensive as the aperture increase (because of lens).


Gather light using mirror at the rear end of the main tube.
Reflectors are ideal for seeing the deep-sky objects that refractors often miss, such as galaxies and nebulae.
Pros: it’s less expensive compared to refractor when aperture increase.
Cons: You need to do an optional alignment every now and then (collimation)


It’s combination of lens and mirror.
It comes in Compact tube and relatively light weight.
It has 2 popular design

  1. Schmidt- cassegrain
  2. Maksutove – cassegrain

These are what telescope tubes looks like now let’s talk about the Telescope mount.
There are 2 major types when you search for mount.
    There are 2 major types when you search for mount.
  1. Altazimuth
    - Simplest type of mount
    - With vertical and horizontal alignment
    - Some newer version comes with slow motion screw as well
    - Also available in computer driven motor version
  2. Equatorial Mount It’s further divided into 2 subtypes.
    • German equatorial mount
    • Fork mounnt
    • German equatorial mount
      - Uses in both Newtonian and refractor uses this kind of mount
      - Also available in computer driven motor version
      - It has larger counter weight opposite to telescope.
    • Fork mounnt
      - Most catadioptric and other shorter optical tubes uses this kind of mount
      - Fully computer driven telescope using internal digital equatorial drive to calculate the altazimuth settings for mount
    1. example

When Galileo first trained his optic telescope on the heavens and opened up modern optical astronomy, that was the first of the electromagnetic windows out of the universe: light.

Kip Thorne