CDS (I) 2024 General Knowledge · Question 53 of 120

Correct — B, multiple parabolic antennas. The Giant Metrewave Radio Telescope is not one dish but an array of thirty fully steerable parabolic dishes, each 45 metres across, spread over a region about 25 kilometres wide near Khodad, some 80 km north of Pune. Fourteen of the dishes stand close together in a compact central array and the remaining sixteen run out along three arms in a roughly Y-shaped layout, so that the signals can be combined interferometrically and the array behaves like a single instrument with an aperture as wide as its longest baseline. Built and operated by the National Centre for Radio Astrophysics of TIFR, it works at metre wavelengths and is among the largest such arrays in the world.

• (a)a single parabolic antenna — The dish shape is right but the count is wrong. Single-dish radio telescopes do exist — the 305 m Arecibo dish, now collapsed, and China's 500 m FAST — but the GMRT's whole design principle is to synthesise a large aperture from many separated dishes.
• (c)multiple circular antennas — Says nothing about the reflecting surface. A radio dish must be a paraboloid so that incoming parallel waves are brought to a single focus; a merely circular flat or spherical element would not do that. The dishes are circular in outline, but 'parabolic' is the property that matters.
• (d)a combination of circular and parabolic antennas — The array is uniform — thirty identical 45 m parabolic dishes. There is no mixed-element design at the GMRT.

A radio telescope collects radio waves with a reflecting surface and brings them to a focus at a receiver, exactly as an optical reflecting telescope does with light, but at wavelengths thousands of times longer. Because resolving power depends on aperture divided by wavelength, a single radio dish of practical size resolves very poorly. The way round it is interferometry — spread many dishes over a wide area, record their signals with precise timing and combine them, so that the array resolves as well as a single dish the size of the longest separation between elements. That is the design of the GMRT, of the Very Large Array in New Mexico and of the Square Kilometre Array now being built.

The word to key on in the stem is 'consists of', which is asking about construction rather than function. Two of the four options differ only in how many antennas there are and one differs only in their shape, so the item is really testing two independent facts — that the GMRT is an array, and that its elements are paraboloids. 'Giant' in the name refers to the array, not to any one dish; 'metrewave' tells you the observing band, which is why the dishes are made of stretched wire mesh rather than solid metal, since mesh reflects metre waves perfectly well and weighs far less. The array has since been upgraded to the wider-band uGMRT, but the thirty-dish configuration is unchanged.

• The GMRT is an array of thirty fully steerable parabolic dishes, each 45 metres in diameter.
• It is located at Khodad near Narayangaon, about 80 km north of Pune, and is run by NCRA-TIFR.
• The dishes are spread over baselines of up to about 25 kilometres and are combined interferometrically.
• It observes at metre wavelengths, and the upgraded instrument is known as the uGMRT.

• Reading 'Giant' as a claim about one enormous dish; it describes the array's overall extent.
• Treating 'circular' and 'parabolic' as interchangeable — only a paraboloid focuses parallel rays to a point.
• Confusing the GMRT with the Ooty Radio Telescope, which is a single cylindrical paraboloid antenna.

Asked as a one-line construction item about a named Indian facility — CDS repeatedly tests observatories and telescopes this way.

• practice — not a real PYQ

  The Giant Metrewave Radio Telescope is located near which city?

  • (a)Bengaluru
  • (b)Pune
  • (c)Hyderabad
  • (d)Nainital

  Answer(b) Pune — at Khodad near Narayangaon, about 80 km north of the city.

• practice — not a real PYQ

  Why do radio astronomers combine many separated dishes rather than build one very large dish?

  • (a)To observe more colours at once
  • (b)To obtain the resolving power of an aperture as wide as the separation between dishes
  • (c)To reduce the cost of the receivers
  • (d)To avoid atmospheric refraction

  Answer(b) To obtain the resolving power of an aperture as wide as the separation between dishes — this is the principle of aperture synthesis.