The curiosity to learn astronomical insights has been increasing ever since the first telescopes were developed during the 17th century. Till date, we’ve achieved countless fascinating breakthroughs in space which wouldn’t have been possible without the developments of innovative telescopes. Using today’s telescopes, we’re able to see deeper into space, coming across enthralling distant stars and galaxies and detecting events that occur in space.
Several different types of telescopes exist today based on how they work and what they’re used to view. They can be broadly categorized into optical telescopes and non-optical telescopes. These are further classified into various different types.
Following is our take on the different types of optical and other telescopes developed over time:
Optical telescopes are more popular since they are the ones that astronomers have been using with their eye pressed against the eyepiece. They’re used to view distant planets, stars and galaxies.
Refracting telescope is a simple telescope comprising of two lenses. Light entering through the end of the telescope is bent or refracted by the lens to magnify the view for a human eye. The size of the telescope is pretty large and heavy containing a massive aperture that’s meant to allow the maximum amount of light to enter through the primary lens.
Similarly, the lenses of refracting telescopes are also big; the overall body of the telescope is quite long. High-quality lenses result in a higher price.
Refracting telescopes comprise of fixed lenses which won’t get misaligned as would mirror in reflecting telescopes. Reflecting telescopes are highly durable owing to their tough design, which makes them hard-wearing. This means they’re able to handle tough shocks and bumps that could affect the optical properties during transportation.
Moreover, refracting telescopes produce images with far better contrasts than reflectors because they don’t incorporate the secondary mirror that obscures the path of light to a reflector’s eyepiece.
Since a refractor’s tube is sealed from the outside, the external temperature or air currents have no impact on the lens. This results in much sharper images than those produced by reflectors. Moreover, with refractors, you won’t have to stand for extensive time periods to view the sky. This is because unlike reflecting telescopes, their eyepiece is located at the end of the tube that allows you to look through it while sitting.
On the other hand, there are some drawbacks of refracting telescopes as well. Because of being expensive, refractors aren’t the ideal type of telescopes for beginners or part-time hobbyists. They are also subject to chromatic aberration, which refers to the lack of focus by different wavelengths of light, producing a rainbow of colors. This ultimately affects the image quality.
The weight of large lenses, along with other factors, complicates the production of refracting telescopes when trying to incorporate very large apertures. Old refractor designs, as well as some newer versions, are characterized by long objective focal lengths, making them difficult to carry.
Source: Telescope observer
Let’s now examine the different forms of refracting telescopes:
Binoculars are a combination of two parallel telescopes fixed to one another. As opposed to other telescopes, binoculars are more comfortable to use since they allow users to observe objects from both eyes instead of a single eye. In contrast to a monocular telescope, binoculars give a three-dimensional effect and a merged view of two perspectives obtained from both the eyes.
Galileoscope is a special beginner’s telescope that was designed in 2009 as an inexpensive yet efficient option for those planning to take the first steps in astronomy. Its design is based on the refracting telescopes developed by the Italian scientist Galileo Galilei some 400 years ago. However, telescopes of the time were subject to serious chromatic aberration, which indicates optical aberration at the edge of bright extraterrestrial objects in space.
Galileo telescope is a refractor with a long tube consisting of a large lens at the front end and a small lens at the back. It has an achromatic objective lens that eliminates chromatic aberration. When a diverging lens is used an eyepiece, the telescope provides the same perspective of view and the same observing properties as with Galileo Galilei’s original instruments.
Contrarily, when the converging lens is used as an eyepiece, referred to as Kepler mode, you experience much more pleasant observing properties that are comparable to today’s standards. You can view Jupiter’s moons with a field of view of approximately 2 lunar diameters. If someone wishes to gain higher magnifications, switching it to the standard connection of 1.25 inches would make that possible.
Monocular telescopes also referred to as half-pair binoculars, are single-lens refracting telescopes. They make use of lens and prism to magnify distant objects for a better view. While most telescopes show inverted images, monocular portray images with the correct orientation.
The earliest monocular telescopes were created in the 17th century when they were used for jungle safaris, hunting expeditions, and wars. Astronomers then started using them to looking into space but they weren’t as effective in penetrating that far.
Owing to the limitation of being able to view from one eye, most people prefer binoculars over monocular telescopes.
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Reflecting telescope is the catoptrics version of optical telescopes that most novice users prefer to start with. It’s a simple telescope made of mirrors instead of lenses. In this way, it helps overcome the problem of chromatic aberration associated with lenses. Since mirrors reflect all wavelengths of light equally, they have the potential to capture better images.
Unlike refractors, reflecting telescopes are cheaper and possess a simpler design. To keep your passion alive, you should consider easy-to-use telescopes rather than complex ones may cause frustration as the lack of understanding can lead to frustration. Thus, many consider reflectors a better choice.
However, like other instruments, reflecting telescopes are subject to certain issues. One of the issues is coma, which refers to a defect that causes focused images to appear like a comet around the edge of the field of view. Moreover, reflecting telescopes can give you fuzzy images if there’s a significant difference between the telescope mirrors and the external air.
Furthermore, reflecting telescopes consist of a primary and a secondary mirror. This secondary mirror, together with its supporting structures, can block some light, decrease contrast, as well as cause diffraction spikes. Diffraction spikes are the cross-shaped lines that are emitted from stars and other bright objects.
Reflecting telescopes need to be handled with care to keep the primary and secondary mirror aligned. This alignment is known as collimation that every user must be aware of. They may lose the correct alignment with poor handling and transportation. Mirrors need regular cleaning and recoating after every few years.
Following are some common classifications of reflecting telescopes:
Cassegrain telescope is a form of reflector telescope that comprises of a large concave paraboloidal primary mirror and a small hyperboloidal convex mirror. These mirrors help achieve a folded optical path. The primary mirror reflects the image to the smaller convex mirror which in turn reflects the image through the center hole to the eyepiece.
The design of Cassegrain telescope was introduced by the seventeenth-century French priest Laurent Cassegrain. However, it was not before the eighteenth century that his work began to be appreciated. It was found that this design serves to prevent the images from getting blurred.
Cassegrain telescopes are versatile telescopes that help produce images for almost all types of sky observations. They have been used in radio transmitters and receivers.
The Newtonian telescope is a classic reflector telescope invented by Isaac Newton in 1668. With Newtoniantelescopes, you won’t face the problem of color fringing around bright objects. However, the quality and reflectance of mirrors also impact the image quality. Hence, you should rely on well-known brands.
As compared to refractors, Newtoniantelescopes are cheaper. Yet, they’re known for their promising performance. But the obstruction caused by the secondary mirror remains there as the human eye receives a lesser amount of light than originally entered the tube.
Newtonian telescopes come with an equatorial mount that enables the telescope to follow the rotation of the sky with axis parallel to the earth’s axis of rotation. This mount comes in the form of a tripod with controls at the top.
Catadioptric, also known as compound telescopes are considered the hybrids of refractors and reflectors, combining the best features of both types. They are shorter in length. Not only are they highly portable, but also over the problem of chromatic aberrations that separate telescopes face.
However, catadioptric telescopes are much more expensive than either reflective or refractive telescopes. While they’ve helped overcome many problems separate telescopes faced, some issues remain unsettled such as the need to collimate, which means you still have to ensure the proper alignment of primary and secondary mirrors as in reflecting telescopes.
Similarly, the light remains obstructed owing to the presence of the secondary mirrors.
As a form of catadioptric telescope, Maksutov-Cassegrain telescope uses both lenses and mirrors. It serves more like a reflecting telescope but uses the corrector lenses to eliminate the aberrations.
The light enters from the front of the telescope into the Maksutov corrector lens and is reflected from the concave mirror situated at the back of the telescope towards the front. The smaller, convex secondary mirror in the front then reflects it back towards the primary mirror where it passes through the hole to finally reach the eyepiece. This mechanism allows Maksutov telescope to be much smaller than an equivalent refractor or Newtonian telescope.
However, what makes Maksutov Cassegrain telescopes unsuitable for deep sky observations are their longer focal ratios. This is because longer focal points demand longer exposure times which create problems when trying to observe dim objects. Additionally, most Maksutov Cassegrain telescopes create the secondary mirror by aluminizing a small spot on the inside of its corrector lens. While this is inexpensive and convenient, it is not considered ideal. Instead, a separate secondary mirror with a slightly different, yet more optimized curvature is much better.
The best performing Maksutov Cassegrain telescopes comprise of optimized, aspherical secondary mirrors rather than aluminized spots. These offer tremendous results when it comes to planetary observations. However, these are significantly more expensive.
Other non-optical Telescopes
On the other hand, the telescopes working beyond the optical spectrum are used to obtain other insights such as creating images of huge celestial bodies or detecting any significant event in space.
X-Ray and Gamma-Ray Telescope
Gamma rays are the most extreme type of radiation that emerges from the most extreme sources of the universe. Scientists are trying to find out about the high-energy environments in space that emit Gama rays. Examples of objects releasing Gamma rays include exploding stars, black holes, and supernovas. These rays have an incredible impact on the surrounding space and on the evolution of the galaxies. Hence, they help understand how the laws of physics apply to the dynamics of outer space.
X-ray and gamma-ray telescopes detect rays from sources outside the earth’s atmosphere and use them to create images of different celestial bodies. These could be rays given out by the sun, stars, supernovas and other objects.
Gamma-ray telescope is typically used to notice events that occur deep in space such as supernovas, black holes, star formation, and destruction, etc. These are usually events that can’t be detected with visual confirmation.
One common gamma-ray telescope in use by NASA is the Fermi Gama Ray Space telescope. It is placed in space to detect the most powerful sources of radiation in the universe.
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Every object that exists in space emits radio waves. A radio telescope is the type of telescope used to detect radio-frequency radiation emitted by extraterrestrial bodies. Based on these observations, it creates images of far-off celestial bodies.
Radio telescopes appear as huge dishes comprising of three spokes in the middle. They consist of a radio receiver and a large antenna system. Radio Telescopes need to be extremely large to attain the optical resolution required to detect the radio wavelengths that are much longer than wavelengths of visible light.
Three critical aspects impact the sensitivity of a radio telescope, that is, the ability to measure the weak sources of radio emission: The duration of the observation, the sensitivity of the radio receiver, and finally, the area and efficiency of the radio antenna.
Source: University of Oregon
Space telescopes are huge telescopes positioned at high elevations in outer space to observe distant planets, stars, galaxies and other celestial objects. The major space telescope is popularly known as Hubble. It’s named after Edwin Hubble, one of the most prominent astronomers of the early twentieth century. The invention and launch of the Hubble telescope are regarded as the most significant astronomical invention after Galileo’s telescopes.
The Hubble space telescope, placed way above the earth’s atmosphere and light pollution, helps obtain an unobstructed view of the universe. It orbits the earth at about 17,000 mph while capturing images of heavenly bodies in outer space. The discovery of the most distant stars, planets, and galaxies of our solar system wouldn’t have been possible without this telescope. Since its launch in 1990, more than 1.3 million observations have been made.
The 13.3 meters long Hubble telescope resembles a large bus. It has the capacity to observe extraterrestrial objects with an angular size of 0.05 arcseconds. At this capacity, you can spot a pair of fireflies in Tokyo that are at least 10 feet apart from Washington, DC. It uses energy from the sun to fulfill its power needs and has the storage capacity to store 22 average car batteries.