What is a telescope? How good are modern telescopes?

What is a telescope? How good are modern telescopes?

Sub: Sci

Sec: Space sector

Context:

• The modern telescope allows us to explore the universe, acting like a powerful tool that reveals the wonders of space. It gives us a better understanding of our place in the cosmos.

Types of Telescopes

• Celestial objects emit light in all directions, but only the light traveling towards Earth reaches us, and by the time it arrives, the rays are nearly parallel. To concentrate these rays and create an image, we can use one of two types of telescopes:

1. Reflecting Telescopes: These use a concave mirror to focus light onto a point. The image produced is real, inverted, and smaller. Most modern telescopes are reflecting telescopes. Larger ones use parabolic mirrors to avoid image blurring from multiple focal points. In these telescopes, the primary mirror reflects light onto a secondary mirror, which then directs the light to an eyepiece for viewing.
2. Refracting Telescopes: These use lenses to bend light and create an image directly. However, refracting telescopes need very large lenses to observe fainter objects, which can distort the image under their own weight. The largest practical refracting telescope, located at Yerkes Observatory in the U.S., has a 02-meter lens.

Main Purpose of Telescopes:

• Contrary to popular belief, telescopes aren’t primarily used to make objects look larger.
• Their main purpose is to enhance the brightness of celestial objects by collecting more light.
• For example, the human eye’s pupil has an aperture of around 153.9 square millimeters, while even a small 0.07-meter reflecting telescope has 118.5 times more light-gathering area. This allows the telescope to detect much fainter objects than the human eye.
  • The opening size that regulates how much light may pass through an optical device is called the aperture.
  • In the below image: Various apertures for a Nikon AF Nikkor lens with focal length 50 mm. Changing the aperture by one stop changes the aperture area by a factor of two, i.e. the area at f/1.4 is twice as big as the area at f/2.0. Each step is specified by the diameter of the aperture as a fraction of the focal length. At f/1.4 the aperture has a diameter of 50 mm/1.4 = 35.7 mm.

Features of Telescopes:

• Brightness and Apparent Magnitude: The brightness of an object is measured by its apparent magnitude on a logarithmic scale. The lower the magnitude, the brighter the object. For example, the Sun has a magnitude of -26.78, while Sirius, the brightest star in the night sky, has a magnitude of -1.46. Telescopes allow us to see fainter objects with much higher magnitudes.
  • The Andromeda Galaxy, which has trillions of stars and an apparent magnitude of +3.44, is the furthest object we can see with our eyes.
• Resolution: A telescope’s resolution determines how much detail it can reveal. The human eye has a resolving power of 60 arcseconds, while even a basic toy telescope can have a resolution of around 1.47 arcseconds, revealing much more detail.

Why Are Telescopes Set Up on Mountains?

• The Earth’s atmosphere can interfere with telescopes by causing stars to twinkle due to air turbulence. To reduce this effect, telescopes are often placed on mountains where the air is more stable.
• Space telescopes, like the Hubble, avoid atmospheric interference entirely and have significantly better resolution.
• New Technologies:
  • In recent years, scientists have developed technologies to correct for atmospheric distortions using lasers to create artificial stars and mirrors that adjust to eliminate image distortions. This approach, called tomography, provides clearer images.
• Limits of Telescopes:
  • Larger mirrors are needed to observe deeper into the universe, but there is a practical limit to their size due to weight. To solve this, modern telescopes use segmented mirrors to maintain stability without sagging.
• Advanced Telescopes:
  • The largest current telescope is the Large Binocular Telescope (LBT) in Arizona, with two 8.4-meter-wide mirrors.
  • Even larger telescopes, like the Extremely Large Telescope (ELT) in Chile, are under construction.
  • The ELT will have a combined aperture of 39.3 meters and will be capable of resolving incredible details from vast distances.
  • Telescopes also capture faint objects by increasing exposure time. For example, the Subaru Telescope in Hawaii recently captured an object 100 million times fainter than what the human eye can detect using 10 hours of exposure.

The five-mirror optical system of the under-construction Extremely Large Telescope

Source: TH