What is a Telescope?

A telescope is an optical instrument that allows us to observe distant objects by collecting and focusing light. 

• Contrary to the common belief that telescopes are primarily designed to make objects appear larger, their main function is to increase the brightness of celestial objects. This is achieved through their light-gathering power, which determines how much light they can collect from faint or distant sources.
• The key factor that influences a telescope’s light-gathering ability is its aperture (the size of the opening through which light enters). The larger the aperture, the more light the telescope can capture, resulting in brighter and clearer images of celestial bodies.
  • Aperture refers to the diameter of the telescope's opening (objective lens) that controls how much light is allowed to pass through. A larger aperture allows more light to be gathered, making faint objects, such as distant stars and galaxies, visible.
  • For instance, when the human eye’s pupil is fully dilated, its aperture area is about 153.9 square millimetres. In contrast, a small 0.07-meter reflecting telescope (commonly available as a toy) has an aperture area of 18,241.4 square millimetres. This means the telescope has 118.5 times more light-collecting area than the human eye, allowing it to capture much more light and make dim objects easier to see. 

Two types of telescopes:

Celestial objects emit light in all directions. But only light rays travelling in the direction of the earth will reach us. And when these rays reach us after a lengthy journey, they are virtually parallel.

There are two ways to concentrate these rays and create an image. 

Reflecting Telescopes:

• In a reflecting telescope, rays reflected by the primary mirror (concave mirror) are diverted to a secondary mirror, which reflects them into an eyepiece with a small lens (convex lens) to enhance the image. 
• The image produced by this reflecting telescope is real, inverted, and smaller. Most contemporary telescopes are such reflecting telescopes. E.g., Hubble Space Telescope, James Webb Space Telescope, Very Large Telescope (Chile) etc. 
• Primarily used for Deep-Sky observation: Reflecting telescopes have larger mirrors (and thus larger apertures). Larger apertures allow reflectors to gather more light, which is crucial for viewing faint objects like distant galaxies, nebulae, and star clusters. 

• Advantages:
  • More cost-effective to produce larger mirrors (for reflecting telescopes) than larger lenses (used in refracting telescopes).
  • No chromatic aberration since mirrors reflect all wavelengths equally.
• Disadvantages:
  • Requires regular maintenance (e.g., mirror alignment).

Refracting telescope:

• A refracting telescope is an optical instrument that uses lenses to gather and focus light in order to magnify distant objects. It typically consists of two main lenses:
  • Objective Lens: The primary lens that collects light and brings it to a focus, forming an image.
  • Eyepiece Lens: The secondary lens that magnifies the image formed by the objective lens for viewing.
• Primarily used for high-magnification observations: Refractors excel at viewing bright objects (e.g., planets, Moon, stars) because their lenses can focus light sharply without the interference of additional mirrors. They are better suited for high-magnification observations where sharp, clear images are priority. E.g., Yerkes Observatory Refracting Telescope, the US. 

• Advantages:
  • Produces sharp, high-contrast images, especially for planetary observations.
  • Generally easier to maintain than reflecting telescopes.
• Disadvantages:
  • Can become expensive as lens size increases. (Can be more expensive than reflecting telescopes of the same aperture)
  • Limit on lens size: To observe fainter cosmic objects, much bigger lenses are required, which will slump under their own weight and distort the image. The maximum practicable lens size in a refracting telescope is around 1 m. The world’s largest refracting telescope is at Yerkes Observatory in the U.S., with a 1.02-m lens. 
  • Chromatic aberration (colour fringing) can occur, especially in larger refractors.

Chromatic aberration:

• Chromatic aberration is a type of optical distortion that occurs when a lens fails to focus all colours of light at the same point. This results in a blurred image with colour fringes around objects, especially noticeable in high-contrast scenes.
• Cause: Light consists of various wavelengths (colours). Lenses bend (refract) light differently based on its wavelength. Shorter wavelengths (blue light) are bent more than longer wavelengths (red light).
• Since, mirrors reflect light (not refract), so, chromatic aberration is present in lenses, not in mirrors. Reflection does not depend on the wavelength of the light, all colours of light are reflected uniformly. 

Limits to reflecting telescopes: 

• A telescope with a higher limiting magnitude (Limiting magnitude is the brightness of the faintest object visible to an optical instrument) is required to look deep into the universe, which demands a larger primary mirror. However, there is a limit to the size of the primary mirror. A mirror wider than around 8.5 m will sink under its own weight, distorting its surface.
• Hence, instead of a single primary mirror, today’s large reflecting telescopes have many small mirrors. Each piece is small enough to remain firm without slumping. And when they are combined, the overall light-collecting area (aperture) is still large. E.g., James Webb Space Telescope’s primary mirror is composed of 18 hexagonal segments. These segments work together to form a single, large mirror with a diameter of 6.5 metres. 

Advanced telescopes around the world: 

• Large Binocular Telescope: The largest telescope to date is the Large Binocular Telescope (LBT), which has two 8.4-m-wide mirrors and an effective combined aperture of 11.9 m. It is located at the Mount Graham International Observatory in Arizona, USA.
• Extremely Large Telescope: The Extremely Large Telescope (ELT) is under construction atop the Cerro Armazones in the Atacama Desert in Chile, as part of the European Southern Observatory. It has five mirrors and a combined aperture of 39.3 m. It is expected to be completed by 2028. The ELT’s light-gathering power will exceed that of any telescope to date. Our eyes can discern two lights burning 30 cm apart and kept 1 km away. In perfect conditions, the ELT can distinguish two lights kept 30 cm apart from 12,000 km away.
• Subaru Telescope is an 8.2-m-wide Japanese telescope located at the Mauna Kea Observatory in Hawaii. It recently used 10 hours of exposure time to capture a faint celestial object with a visual magnitude of 27.7, which is 100-million-times fainter than what any human eye can detect. 
• James Webb Space Telescope (JWST): Launched in 2021 by NASA, JWST is orbiting the Sun at the L2 Lagrange point (1.5 million km from Earth). The infrared telescope has a 6.5 metre primary mirror. It detects near-infrared and mid-infrared wavelengths to observe faint and distant objects.

Why are telescopes setup on mountains?

The earth’s tumultuous atmosphere interferes with the telescope’s functioning. Telescopes are often set up on mountains for several key reasons:

• Reduced Atmospheric Interference: At higher altitudes the atmosphere is thinner. The thinner atmosphere absorbs and scatters less light, improving the visibility of faint celestial objects.
• Less Air Turbulence: Higher altitudes often experience less air turbulence compared to lower elevations, where weather systems can cause more turbulent air movement. This reduces blurring or distortion of images caused by atmospheric turbulence.
  • Space telescopes are more than 400 km above sea level, allowing them to entirely escape atmospheric disturbances. That is why the Hubble Space Telescope has a resolving power of around 0.04 arcsec, 10-times greater than the best ground-based telescopes.
• Clearer Skies: Higher elevations generally have lower humidity levels, which means there is less water vapour in the atmosphere. This helps reduce cloud cover and atmospheric absorption, allowing for more frequent and prolonged observations.
• Minimised Light Pollution: Mountain locations are often remote and away from large cities, which helps reduce light pollution and makes high-altitude locations ideal for clear, unobstructed observations.