What are telescopes?

Telescopes are scientific instruments that help you see distant objects by gathering and focusing a lot of light. They work by creating a magnified image using glass lenses, mirrors or combination of both. Telescopes are used in a variety of applications such as astronomy, birdwatching, camera lenses, land surveys and so on, however, for our context we limit the discussion to astronomy.

Terminology/ Specifications

  • Objective - The optical part of the telescope that gathers light is called the objective. The objective in a refractor telescope is made of lens, in a reflector telescope is made of a primary mirror.

  • Aperture/ Objective Diameter - the diameter of the objective lens/ mirror is called the aperture/ objective diameter of the telescope. This value is typically specified in milimeters (mm) or in inches (in). The larger the objective lens, the more is the light gathering capacity of the telescope, meaning it will give you brighter, crisper images with more detail. Also, bigger apertures allow you to see fainter objects in the night sky better as compared to smaller apertures

  • Focal length - The distance light has to travel after entering the objective and before reaching your eye is the focal length of the telescope.

  • Magnification -

    • Mathematical definition: the focal length of obj. lens divided by the focal length of eye piece. The higher the focal length of an eyepiece, the lower will be the magnification it will provide, and vice versa.

      • Example 1: using a 25mm eyepiece on a 700mm focal length telescope gives (700/25) = 28x Magnification

      • Example 2: using a 10mm eyepiece on the same 700mm focal length telescope gives (700/10) = 70x Magnification

      • Example 3: using a 10mm eyepiece on a 1200mm focal length telescope gives (1200/10) = 120x Magnification

      • Example 4: using a 10mm eyepiece with a 2x Barlow lens on a 1200mm focal length telescope gives (1200/10)*2 = 240x Magnification

    • Practical significance: The magnifying power of the telescope can be described as how closer it brings the image, or, how much larger it makes the image appear, compared to the original object viewed from the same position. For example, a 10x magnification (telescope+eyepiece combination) magnifies 10 times/ brings the object 10 times closer/ makes the object appear 10 times larger than it appears without the telescope.

    • Although there are different ways of interpreting/ understanding magnification, another easy way to understand magnification is - it is the factor by which the distance of the viewed object reduces. Example: If you are viewing an object at a distance of 100 Km and the magnification of your telescope+eyepiece combination is 10x, then the object will appear as though it is just (100 Km/10) = 10 Km away. Similarly, if the magnification power of the combination is 50x, the same object would now appear (100/50) = 2 Km away, and so on. So on a given telescope, it is possible to achieve different magnifications by using eyepieces of different focal lengths.

    • As it will be clear, magnification isn't always the deciding factor for purchasing a telescope, beyond a certain extent. For more information, see Choosing the right Telescope

  • Maximum Theoretical Magnification - The maximum value of magnification as described above, that you can achieve using the your eyepieces and/or barlow or other attachments. For example, if you have a 700mm foc. len telescope, 3 eyepieces (20mm, 12mm and 6mm) and a 2x barlow lens, the maximum magnification you can achieve would be by using the 6mm eyepiece and the 2x barlow, and will equal (700/6)*2 = 233x

  • Highest Practical Power - Although theoretically the telescope is capable of achieving a certain magnification, practically speaking, the highest magnification you can achieve on a telescope without losing the image quality (brightness and clarity) is twice the aperture. i.e., the highest practical power of an aperture 70mm telescope is 70*2 = 140x, for a 114mm telescope is 114*2 = 228x, and so on.

  • Resolving Power - Consider 2 points drawn very close to each other on a piece of paper. When you view this paper at, say, an arm's length, you can see the 2 points as separate objects on the paper. However, the further you go from the sheet, it becomes more and more difficult in seeing the 2 points clearly and at some point, they seem to merge and appear as a single entity, beyond which it is no longer possible to clearly make out whether it is a single point or 2 separate points. Resolving power of the eye is the minimum angle of separation from the eye towards the objects, at which the objects can still be seen as separate entities by the eye. Similarly, resolving power of a telescope is the smallest distance (in angular measurement) at which 2 objects/ points can be and still be distinctly viewed as separate objects through a telescope (the points in this context would be 2 stars/ 2 celestial bodies in space). Typically the resolving power is measured in arc seconds (angular measurement). The smaller this value for a telescope, the better the telescope can differentiate between objects and provide clearer views.

  • Stellar Magnitude - Brightness of celestial bodies as appear to us is measured in magnitude. The smaller the magnitude, the brighter is the object, and the higher the number, the more faint it is. Examples: Venus is the brightest object in the night sky after Moon, with a brightness of magnitude -4.2, followed by Jupiter with magnitude of -2.9 to -1.7. the brightness of Saturn is approx -0.5 when its brightest, that of our Moon is -12, that of our Sun is -27! Sirius (in Scorpio) is -1.45, Rigel (in Orion) is 0.15, Polaris is at 1.98.

  • Limiting Stellar Magnitude of a Telescope - The faintest magnitude level up to which you can see through a telescope. For instance, stellar magnitude of our unaided eye is between 5 and 6, meaning, we can only stars of that much magnitude or lower (brighter). A telescope having a limiting magnitude of 8, for example, will allow you to see faint stars that have a brightness of 8 or below (8 or brighter) but not beyond. Similarly, a telescope with a stellar magnitude of 14 would allow you to see up to the faintest of stars of magnitude 14.

  • Focal Ratio - Mathematically, the Focal Length of the telescope, divided by the Aperture of the objective gives the Focal Ratio. It is also referred to as F ratio, and is often denoted as "F/n" where n is the ratio.

    • Examples:

      • A refractor telescope with aperture 70mm and focal length 700mm has a focal ratio of (700/70) = 10, and is often called a "F/10 Telescope"

      • A reflector telescope with aperture 150mm and focal length 750mm has a focal ratio of (750/150) = 5, and is often called a "F/5 Telescope"

      • A maksutov telescope with aperture 90mm and focal length 1250mm has a focal length of (1250/90) = 14, and is called a "F/14 Telescope"

      • Note that an F ratio only indicates the relative sizes of the aperture and focal length, and does not necessarily imply a specific optical design or specific aperture or focal length. For example, a 70/700 Telescope and a 100/1000 Telescope both are called F/10 Telescopes

    • Significance: Telescopes with lower focal ratio are usually said to be "fast scopes" and those with higher focal ratios are said to be "slow scopes", indicating the time it takes for light to travel through through the scope. Accordingly, Fast scopes are typically better for DSO (deep sky) observations as they have a relatively large aperture, and slow scopes are typically better suited for lunar and planetary viewing as they have relatively longer focal lengths.

  • Optical Design - In terms of optical design, telescopes can be either refractors, reflectors or catadioptric/ hybrid. These design references are explained in detail in the below dedicated section.

Types of Telescopes Based on Optical Design


These are telescopes that use one or more lenses as the objective. The objective is usually at the end which points towards the target and the other end has eyepiece where user can view. These are simplest and earliest form of telescope, though at apertures higher than basic ones (70+) they are expensive as costs involved to manufacture are high. They may have a single lens (singlet), 2 lenses (doublet) or 3 lenses (triplet) to correct for chromatic aberration.


These are telescopes that use a large mirror as the primary objective, and another smaller mirror that reflects the light rays coming from the primary, and focuses into the eyepiece. The primary is placed at the end farthest from the end that points at the target, and eyepiece is placed on the side of the OTA. Compared to refractors, reflectors are more economical to make and you can usually get a bigger aperture reflector than refractor in the same price range. Large reflectors are commonly called as light buckets because of the massive light gathering capacity they have due to big apertures


These are telescopes that use combination of lenses and mirrors for gathering and focusing light. Light usually bounces around a couple of times within the scope before being focused at the eyepiece. Cassegrains are most expensive to manufacture but are the most compact scopes enabling them to achieve far greater focal lengths at a fraction of the OTA length of refractors and reflectors.