How to Choose a Telescope

How to Choose a Telescope

If you love the sky and the mysterious realms above it then you probably dream of having your own telescope. But, if you don’t possess a scientific background or much knowledge about the tool then your first purchase could end up a nightmare. As such, here is some basic knowledge you need to know about these sky-peeking tools.


The most important specification of a telescope is the aperture. The aperture refers to the diameter of its main optical component which may be a lens or a mirror. The aperture defines your telescope’s light gathering ability which defines how bright your image appears and its resolving power that defines how sharp the image appears.

How to choose an aperture? The bigger, the better. The larger the aperture is, the clearer you can view the object in the sky. Depending on the specifications of your aperture, you can clearly view the moon, even to the faintest details in its craters.


It may come as a surprise but the aperture does not determine the telescope’s magnification at all. If you are new to the field then perhaps you’d ask “How much does this telescope magnify” and the answer to that question would be “Any amount that you want”. Any telescope can provide an almost infinite range of magnification. That is, depending on the attached eyepiece.

There are two main factors that limit the power that shows a decent view with a given instrument: aperture and the atmospheric pressure. It is important to note that to view the details of the image created by the scope’s main mirror or lens, you must find the optimum magnification.

This needs to be done without spreading out the target’s precious light too much. That’s why astronomers have certain telescopes set to observe certain bodies in the galaxies. Low powered telescopes observe faint things like nebulae.

How do you calculate for magnification? Every telescope has a focal length or the distance from the primary lens or mirror to the image it forms and so does every eyepiece.

Given these two focal lengths, solving for the magnification is simple – just divide the focal length of the scope with that of the eyepiece.

For example, given a 1,000 mm focal length scope used with a 10 mm eyepiece then the telescope delivers 1000/10 = 100 (10x) power.

Types of Telescope to choose from


The stereotypical image of a telescope is that of a refractor – a long, gleaming tube with a large lens in front and an eyepiece at the rear. The front lens focuses light to form an image at the rear. The eyepiece is a little magnifying glass where you look at the image.

High quality refractors are often sought by lunar and planetary observers who value a high contrast image that can take high magnification.


This type of telescope uses mirrors instead of lenses in order to gather and focus light. The most common form of this type of telescope is Isaac Newton’s Newtonian reflector with its specially curved concave primary mirror in the bottom end of the telescope.

Near the top of the scope, a small secondary mirror directs the light from the primary mirror to the side of the tube where an eyepiece is conveniently attached.

Reflectors are known for their high apertures. When well-made and well-maintained, the reflector can provide a sharp, contrasting image for all forms of celestial objects for a fraction of the cost of an equal aperture refractor.

Another benefit to this reflector is that it is the only type of telescope that shows you a correct reading image rather than a mirror image.

Best of Both Worlds

Possessing both the characteristics of the refractor and reflector is the catadioptric or compound telescope.

This type of telescope employs both lenses and mirrors to marry the best characteristics of a refractor and a reflector. The greatest benefit from this type of telescope is that, in its most commonly encountered form, they are compact.

Their tubes are just two times or more as long as wide which is made possible by optical folding of the light. The smaller tube is lighter and so is more manageable in mounting. In other words, you can obtain a large-aperture, long-focus telescope that is also very transportable.

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