Modern Telescopes: A Gateway to the Universe and Their Amazing Capabilities

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What is a Telescope?: A telescope is a scientific instrument designed to collect and enhance light from distant celestial objects, allowing astronomers to observe and study the cosmos. Contrary to popular belief, telescopes are not primarily meant to make objects appear larger; their main function is to increase the brightness of celestial objects by gathering more light than the human eye can. Modern telescopes, whether situated on mountaintops or in space, serve as windows to the universe, enabling us to explore distant galaxies, stars, and other cosmic phenomena.

Two Types of Telescopes: Reflecting and Refracting

When light from celestial objects reaches Earth, it does so as virtually parallel rays due to the vast distances it has traveled. Telescopes use two main methods to concentrate these rays and create an image:

1. Reflecting Telescopes

Use concave mirrors to focus incoming light at a point. Most modern telescopes fall into this category, using parabolic mirrors to prevent image blurring.
In a basic reflecting telescope, light rays hit a primary mirror and are diverted to a secondary mirror, which then reflects them into an eyepiece with a lens to enhance the image. Alternatively, some designs have a hole in the primary mirror for the light to pass through to the
eyepiece.
Reflecting telescopes are favored in contemporary astronomy because they can be made larger without the image distortion issues that affect lenses in refracting telescopes.

2. Refracting Telescopes

Use lenses to bend and focus light directly, creating an image. The maximum practical lens size for a refracting telescope is around 1 meter due to the
distortion caused by the lens weight. The Yerkes Observatory in the U.S. houses the world’s largest refracting telescope with a 1.02-meter lens.
While most large telescopes today are reflectors, refractors are still valuable in some applications. For instance, the Vera C. Rubin Observatory employs three large lenses, with one measuring 1.55 meters in diameter, to sharpen images.

The Primary Function of Telescopes

A telescope main role is to collect light from distant objects, thereby making them appear brighter. The aperture, or the size of the telescope opening, determines its light-gathering power. The larger the aperture, the more light it can collect, similar to how a larger container collects more rainwater.

For comparison, the human eye fully dilated pupil has an aperture area of around 153.9 sq. mm. A small reflecting telescope, even as a toy, has an aperture area of about 18,241.4 sq. mm, gathering 118.5 times more light than the human eye. This increased light-gathering power is what enables telescopes to reveal faint cosmic objects.

Features of Telescopes: Brightness and Resolution

Brightness: The brightness of celestial objects is measured in apparent magnitude, where lower numbers indicate brighter objects. For instance, the Sun apparent magnitude is -26.78, while Sirius, the brightest star visible in the night sky, has a magnitude of -1.46. The human eye can see objects with an apparent magnitude of up to +6.5. A small telescope, with a higher limiting magnitude of +11.2, can reveal objects 100 times fainter than what the naked eye can perceive.
Resolution: This refers to a telescope ability to distinguish fine details. The human eye has a resolution of around 60 arcseconds, while even a small toy telescope can have a resolution of 1.47 arcseconds, enabling it to see details 40 times finer.

Why Are Telescopes Placed on Mountains?

The Earth atmosphere creates turbulence, causing stars to twinkle and blurring images. The higher a telescope is placed, the less atmospheric interference it faces. Space telescopes, positioned over 400 km above sea level, avoid atmospheric disturbances altogether, giving them far superior resolving power. For example, the Hubble Space Telescope has a resolution of 0.04 arcseconds, nearly 10 times better than ground-based telescopes.

Advanced Technology in Modern Telescopes

Modern telescopes employ techniques like adaptive optics to correct for atmospheric turbulence. By using lasers to create an artificial star in the upper atmosphere, astronomers can measure how this star fluctuates and adjust the telescope mirrors to counteract the distortions. A more advanced version of this technology, tomography, examines different layers of the atmosphere to further enhance image clarity.

Limits to Telescope Size

The light-gathering capacity of a telescope depends on the size of its primary mirror. However, a single mirror larger than 8.5 meters would sag under its weight, distorting the image. To overcome this, modern telescopes use segmented mirrors that fit together to form a large, stable surface without slumping.

Advanced Telescopes Around the World

The Large Binocular Telescope (LBT) in Arizona has two 8.4-meter-wide mirrors, creating a combined aperture of 11.9 meters.
The Extremely Large Telescope (ELT), currently under construction in the Atacama Desert, Chile, will have five mirrors forming a 39.3-meter aperture. Expected to be completed by 2028, the ELT will distinguish objects separated by just 30 cm from 12,000 km away under optimal conditions.

Here is a list of major Indian telescopes along with their locations and primary functions:

Giant Metrewave Radio Telescope (GMRT)

Location: Pune, Maharashtra
Type: Radio telescope
Function: GMRT is used for observing the universe at radio wavelengths. It is one of the world largest arrays of radio telescopes and is used for studying a wide range of astrophysical phenomena, including pulsars, galaxies, quasars, and the evolution of the universe.

Indian Astronomical Observatory (IAO)

Location: Hanle, Ladakh
Type: Optical, infrared, and gamma-ray telescope
Function: The IAO in Hanle houses the Himalayan Chandra Telescope (HCT) and the High-Altitude Gamma-Ray Telescope (HAGAR). The HCT is used for optical and near-infrared astronomical observations of stars, galaxies, and other celestial objects. The observatory is one of the highest in the world, providing ideal conditions for clear astronomical observations.

Astrosat

Location: Space (Satellite-based telescope launched by ISRO)
Type: Multi-wavelength space telescope
Function: Astrosat is India first dedicated multi-wavelength space observatory. It observes the universe in X-ray, ultraviolet, and optical wavelengths. It helps in studying various celestial objects like stars, galaxies, and supermassive black holes.

Devasthal Optical Telescope (DOT)

Location: Nainital, Uttarakhand
Type: Optical telescope
Function: The 3.6-meter Devasthal Optical Telescope is one of Asia largest optical telescopes. It is used for a wide range of astronomical observations, including the study of stars, planets, galaxies, and other celestial phenomena.

Solar Ultraviolet Imaging Telescope (SUIT) on Aditya-L1

Location: Space (Satellite-based, yet to be launched as part of the Aditya-L1 mission by ISRO)
Type: Solar telescope (Ultraviolet)
Function: SUIT will study the Sun outermost layers (chromosphere and corona) in the ultraviolet spectrum. The Aditya-L1 mission aims to provide insights into solar activities like solar winds and space weather effects on Earth.

Neutrino Observatory (INO)

Location: Theni district, Tamil Nadu (Under construction)
Type: Neutrino detector
Function: The INO will be an underground laboratory to study atmospheric neutrinos. Its primary goal is to understand the properties of neutrinos, which are fundamental particles in the universe.

Pushing the Limits: Exposure Time

Astronomers extend exposure times to capture faint celestial objects. By leaving the telescope camera shutter open for hours, they can gather light from extremely dim sources, similar to collecting more rain in a container over time. The Subaru Telescope in Hawaii recently used a 10-hour exposure to detect a celestial object with a visual magnitude of 27.7, which is 100 million times fainter than what the human eye can see.

A New Age of Cosmic Exploration

Modern telescopes are far more than simple magnifiers. They serve as powerful instruments that enhance our understanding of the universe, revealing wonders that were once beyond human reach. With continuous technological advancements, telescopes like the ELT and space observatories promise even greater discoveries, pushing the boundaries of our cosmic knowledge.