Astronomers and stargazers use telescopes to observe faint, distant objects in the universe because they have much higher resolution and can gather more light from a small portion of the space than our eyes, obviously. But do you know there are actually more than one type of telescope available?
The earliest known telescope in the human history appeared back in 1608 in the Netherlands and was supposedly invented by a Dutch eyeglass maker Hans Lippershey. However, the name ‘telescope’ was non-existent until 1611, when it was conceived by a Greek chemist and mathematician Giovanni Demisiani.
By 1610, the Italian polymath Galileo Galilei had already developed his own improved version of the telescope with which he discovered four of the Galilean moons. Then around late 1660s, Issac Newton designed the first ever reflecting telescope, which is now known as the Newtonian reflector.
This is basically the early history of telescope’s 400 year old journey. But by the early 20th century, researchers became aware of the need for advanced sets of telescopes which can observe at different electromagnetic wavelengths, which were not possible with traditional visible light telescopes.
The Electromagnetic Spectrum
In this article, we have explained all the major types of telescopes that are used by astronomers to gain a better understanding of the Sun, different planets in the solar system and other distant objects.
Ground Based Telescopes
1. Optical Telescopes
Optical telescopes are the most commonly used telescopes by astronomers and even hobbyists to gaze the night sky. It gathers light from the visible wavelength of the electromagnetic spectrum. With this, we can observe a magnified image of a distant object in space.
Optical telescopes can be categorized into three broad categories; refractor, reflector, and catadioptric optical designs. Each has their own pros and cons and have different uses in astronomy.
Image Courtesy: Szőcs Tamás
Refracting or dioptric telescopes are the oldest type of optical telescope. Every refracting telescope has similar working principle and mechanical ingredients. An objective lens that bends light, converging it at a focal point. Then, with the help of an eyepeice, the telescope captures more light than a human eye.
There are basically four types of refracting telescopes based on design – Galilean telescope, Keplerian telescope, Achromatic and Apochromatic refractors.
While refracting optical telescopes were once popular, they saw a massive decrease due to the adoption of operational reflecting telescopes in the 20th century. Today, only few research grade refracting telescopes are operational in the world.
Reflecting telescope or reflector was invented by Issac Newton in the 17th century as an alternative to ‘flawed’ refracting telescope. While they still cannot produce the ‘ideal‘ image, they are used in almost every other research telescopes due to its physical merits.
Much like refractors, reflecting telescopes can be divided into three broad categories based on design – these are Gregorian, Newtonian and Cassegrain telescopes. Several sub types and specialized extensions are also there.
The third and lesser known type of optical telescope is catadioptric telescopes. In catadioptric telescopes both reflection and refraction are combined to create a special optical system. This optical combination is generally used in vehicle headlights. However, they are also used in certain telescopes and astronomical cameras.
This type of telescopes can have few advantages over other telescopes, such as greater error correction due to a wider field of view. They also have lesser mass and are easier to manufacture. Few examples of catadioptric telescopes are Argunov–Cassegrain telescope, Maksutov telescope and Schmidt camera.
2. Radio telescopes
The Atacama Compact Array Image Courtesy: ESO
Radio telescopes studies radio frequencies coming from various celestial bodies. Unlike telescopes operating at optical wavelengths, radio telescopes can be used during daytime too. They are powerful enough to observe faint radio waves originated from far off sources such as exoplanets, stars and galaxies.
A typical radio telescope has a large antenna, also known as parabolic antenna, which may or may not be connected in an array. Since, radio frequencies can easily penetrate the Earth’s atmosphere, there is no need for space based radio telescopes. However, they can potentially assist ground based ones.
The radio frequencies constitute a large part of the electromagnetic spectrum. In order to listen different radio frequencies, radio telescopes of different shape and size are needed. Some of the frequency bands currently used by radio telescopes are-
- Channel 37
- Hydrogen Line
- The Waterhole
- 23 GHz, 33 GHz, 41 GHz, 61 GHz, 94 GHz, 1406 MHz and 430 MHz
Commercial use of these frequencies are banned in many nations for performing radio astronomy.
In radio interferometry, radio signals captured by multiple antennas over a large area are combined together in order to maximize the overall resolution. This technique was introduced back in 1946.
3. Solar Telescopes
Solar telescopes, formerly known as photoheliograph are specially designed to observe the Sun in frequencies just outside the visible spectrum. While there are many variations in the structure of solar telescopes, they are generally placed on top of a white tower. Solar telescopes usually have long focal lengths.
McMath–Pierce is currently the largest solar and largest unobstructed aperture telescope in the world, located in Arizona, USA.
Space Based Telescopes
Hubble Space Telescope Image Courtesy: NASA
It all started back in 1923, when German physicist Hermann Oberth along with K. Tsiolkovsky and Robert Goddard, the three founding fathers of astronautics pondered upon the idea of a space based telescope that could be sent into Earth’s orbit by a rocket. This was the beginning of the era of a new class of telescopes.
Then in 1946, theoretical astrophysicist Lyman Spitzer of Princeton University, elaborated the advantages of such an instrument and how a space based telescope can completely eliminate Earth’s atmospheric turbulence from telescopic observations.
A space based telescope is basically a telescopic instrument located outside the Earth’s atmosphere to observe distant astronomical objects including planets, galaxies and stellar bodies in different electromagnetic wavelengths.
Unlike ground based telescopes, space telescopes offer more accurate observations since they are free of any atmospheric turbulence and radiation distortion. Below are the different types of space telescopes.
4. Infrared Telescopes
Artist’s conception of Spitzer space telescope Image Courtesy: NASA
Infrared astronomy is an important branch of the modern astrophysics. Since most of the infrared radiation is blocked by the Earth’s atmosphere (relatively smaller wavelengths can make it through), many infrared telescopes are space based.
The infrared telescopes can detect far off astronomical objects in the dusty regions of the space. They also play a crucial role in studying the early state of the universe. However, unlike most of the other wavelengths, observation in infrared frequency is a bit tricky since every hot body emits infrared radiation.
In order to cope with that, infrared telescopes are fitted with special cameras which are constantly kept at cryogenic temperatures (below −150 °C) and are coupled with solid-state detectors.
NASA’s legendary Spitzer Space Telescope is one of the most important infrared space based telescope to date.
5. Ultraviolet Telescopes
Our atmosphere blocks most of the harmful radiation from entering the Earth. This includes ultraviolet rays. Due to this reason, ultraviolet light can only be observed from above the Earth.
Ultraviolet radiation plays an important role in astronomical observations as it revels important characteristics such as chemical composition, temperature and densities of different celestial bodies. Observation in UV spectrum also provides invaluable information about the evolution of galaxies.
Notable Ultraviolet Space Telescopes
The first ever space telescope capable of observing the UV spectrum of the sky was the Far Ultraviolet Camera/ Spectrograph, which was deployed on the Moon’s surface by the Apollo 16 mission in 1972.
Currently, NASA’s Far Ultraviolet Spectroscopic Explorer or FUSE and the Hubble Space Telescope are operational, and most prominent examples of UV space based telescopes.
Image of Crab nebula at multiple wavelengths Image Courtesy: NASA
6. X-Ray Telescopes
X-ray telescopes are designed to study extremely distant objects in X-ray frequency. Since X-ray frequencies are almost blocked by Earth’s atmosphere, only space based telescopes can capture these wavelengths.
The core component of any X-ray telescope are mirrors (focusing or collimating) that collect radiation and project them on the detectors. X-ray telescopes with focusing mirrors needs long focus, i.e., the mirrors must be at few meters from the detectors.
Notable Space Based X-ray Telescopes
Since 1970, more than thirty different X-ray telescopes are launched into space. The first was Uhuru satellite, which extensively studies the Cygnus X-1 and other distant X-ray sources. NASA’s Chandra X-ray Observatory launched in 1999 was a breakthrough in the field of X-ray astronomy.
Chandra is 100X more sensitive to weak X-rays than any other telescope before its launch. This was possible only due to the higher angular resolution of its mirrors. Other noteworthy X-ray observatories are NuSTAR (Nuclear Spectroscopic Telescope Array) and Japanese Hitomi satellite.
7. Microwave Telescopes
All Sky map of CMB. Based on Nine years of data from WMAP Image Courtesy: NASA
Like X-rays and ultraviolet radiation, Earth’s atmosphere blocks most of the radiation from microwave wavelength, so astronomers have to rely on space based microwave observatories and telescopes to observe the cosmic microwaves.
Cosmic microwaves or cosmic background radiation are the oldest electromagnetic radiation in the universe; the remnants of the Big Bang. While microwave space telescopes are generally used to study the cosmology of the early universe, they can also observe Synchrotron radiation and other phenomenon.
Notable Space Based Microwave Telescopes
Telescopes mounted on NASA’s WMAP (Wilkinson Microwave Anisotropy Probe) and ESA’s Planck satellite are perhaps the only two space-based microwave telescopes currently operational. The only other prominent space based microwave telescope was on the Cosmic background Explorer or COBE, which went down in 1993.
8. Gamma-Ray Telescopes
Gamma-rays are the most dynamic form of the electromagnetic radiation. While low energy gamma-rays (in MeV range) are produced by solar flares, high energy gamma-rays (GeV) on the other hand, are only generated by extreme events outside our solar system such as superluminous stellar explosion, etc., therefore gamma-rays are important for various extra-galactic studies.
However, they are much more difficult to observe than the X-rays. In fact, there is no focused gamma-rays telescope to date. Instead, astronomers use secondary means to detect gamma-ray flux in the sky, i.e. Cherenkov light.
Though the earth’s atmosphere act as a barrier for gamma-rays, in many cases they can be observed from multiple ground based observatories including HESS, HAWC and VERITAS.
Notable Gamma-ray Telescopes
Currently there are only five operational space-based telescope observing in gamma-ray frequency. NASA’s Swift explorer, which was launched in 2004, detects mysterious gamma-ray burst from all over the universe. Another NASA observatory Fermi is specially designed to observe high energy bursts from pulsars and black holes.
Apart from telescopes observing in X-ray, gamma-ray, UV and microwave frequencies, there are multiple space based telescopes that focuses on visual, infrared and radio frequencies. Hubble Space Telescope is a perfect example of multi-wavelength telescope observing in near infrared, visible light and ultraviolet frequencies.