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Science & Tech.
Mahesh

18/09/24 11:07 AM IST

How good are modern telescopes?

In News
  • The modern telescope is a window into the universe, a sophisticated paintbrush in the hands of skilled astronomers.
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. We can use a concave mirror to focus incoming photons at the focus point.
  • The image produced by this reflecting telescope is real, inverted, and smaller.
  • Most contemporary telescopes are such reflecting telescopes. Giant telescopes use parabolic mirrors because light rays reflected from the concave produce several focal points, causing the image to blur.
  • In a reflecting telescope, rays reflected by the primary mirror are diverted to a secondary mirror, which reflects them into an eyepiece with a small lens to enhance the image.
  • Alternatively, a hole is drilled in the primary mirror’s centre, and the rays the primary reflects pass through this hole to the secondary, which finally reflects them upward into the eyepiece.
  • The world’s largest refracting telescope is at Yerkes Observatory in the U.S., with a 1.02-m lens.
Function of telescope
  • It’s a common misconception that telescopes are designed to make astronomical objects appear larger.
  • Instead their primary function is to enhance the brightness of celestial objects, measured by their light-gathering power.
  • Say it’s drizzling and you wish to collect rainwater. Place a cup with a small opening and a tub with a larger opening outside.
  • Due to the larger opening, the tub will collect more water than the cup in a given time.
Features of telescopes
  • The brightness of celestial objects is quantified by their apparent magnitude.
  • Its values are logarithmic, meaning each step represents 2.512-times more brightness than the earlier. For example, a star of magnitude 4.0 is 2.512-times brighter than a star of magnitude 5.0.
  • The lower the apparent magnitude, the brighter the object; the larger the magnitude, the dimmer it is.
  • The sun’s apparent magnitude on this scale is –26.78, Venus’s is –4.92, and Sirius, the brightest star in the night sky, is –1.46.
  • 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. It appears as a fuzzy patch and we can’t discern individual stars.
  • The star V762 Cassiopeiae is 1,000,000-times brighter than the Sun.
  • But because it is 16,000 lightyears away, it has an apparent magnitude of only +5.82. It’s the faintest star visible to the naked eye.
  • A telescope’s resolution limit specifies the size of the smallest detail it can spot between two objects that are really close together.
  • The greater the resolving capacity, the more details will be visible.
  • The human eye with 20/20 vision has a resolving power of 60 arcsec. One arcsec is 1/3600th of a degree.
  • The toy telescope’s optimal resolving power is around 1.47 arcsec, over 40-times greater.
Advanced telescopes around the world
  • 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.
  • 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, with a fantastic resolving power.
  • 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.
  • The 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.
Source- The Hindu

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