Reflecting telescope

SOFIA 2.5M Primary Mirror

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Franklin reflector 24

Newton telescope replica 1668

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Great Telescope, Birr, Offaly 1

Gran Telescopio Canarias

Sirius A and B Hubble photo

Reflecting telescope

A reflecting telescope (also known as a reflector) is an optical telescope that uses a single or a combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century as an alternative to the refracting telescope, which uses lenses to collect and focus light.

History

The first reflecting telescope was built by Isaac Newton in 1668. It used a concave primary mirror to gather light and a flat diagonal secondary mirror to reflect the light to an eyepiece. This design, now called the Newtonian telescope, solved the problem of chromatic aberration seen in refractors by eliminating the use of lenses to collect light.

Design and Types

Reflecting telescopes come in various designs, each with its unique arrangement of mirrors and optical elements. The most common types include:

• Newtonian telescope: Invented by Isaac Newton, this design uses a concave primary mirror and a flat diagonal secondary mirror.
• Cassegrain telescope: This type uses a concave primary mirror and a convex secondary mirror that reflects light back through a hole in the primary mirror, allowing for a more compact design.
• Gregorian telescope: Similar to the Cassegrain, but with a concave secondary mirror that reflects light back through a hole in the primary mirror.
• Ritchey-Chrétien telescope: A variant of the Cassegrain with hyperbolic mirrors to reduce optical errors, widely used in professional observatories.

Advantages and Disadvantages

Reflecting telescopes offer several advantages over refractors:

• No chromatic aberration, as mirrors reflect all colors of light equally.
• Larger diameters are possible, allowing for more light collection and better resolution.
• Generally less expensive to manufacture for a given size.

However, they also have some disadvantages:

• Mirrors require a protective coating that can degrade over time and need recoating.
• The optical designs can be more complex, requiring precise alignment of mirrors (collimation).
• Some designs, like the Newtonian, have an obstructed light path due to the secondary mirror, which can reduce contrast.

Applications

Reflecting telescopes are used in a wide range of applications, from amateur astronomy to cutting-edge research in professional observatories. They are the telescope of choice for most professional research because they can be scaled up more easily than refractors, making them ideal for deep-sky observations, astrophotography, and spectroscopy.

Notable Examples

Some of the world's most significant telescopes are reflectors, including:

• The Hubble Space Telescope, a Ritchey-Chrétien reflector orbiting Earth.
• The Very Large Telescope (VLT) in Chile, consisting of four individual telescopes that can work together.
• The upcoming Extremely Large Telescope (ELT), which will be the world's largest optical/near-infrared telescope.

Reflecting telescopes have revolutionized our understanding of the universe, from the discovery of distant galaxies to the detailed study of planets within our solar system. Their development and refinement continue to be at the forefront of astronomical research.