A refractor telescope or refracting telescope, also known as dioptric telescope, is a type of optical telescope that uses a lens to form an image. This type of telescope has at least one primary lens and one lens at the eyepiece where the viewer sees the image. The objective lense refract the light penetrating through the device and converges straight from the objective lens to a focal point, giving a magnified view.
Refractor telescopes are long, thin telescopes known for sharp, crisp, detailed, contrasty images and are best for viewing the moon and planets. A quality achromatic refractor of 60mm to 80mm aperture will make a fine starter scope. They are portable, maintenance free, and less expensive. A refractor is the scope of choice if you will be doing most of your observing from the city or suburbs, where the night skies are moderately light-polluted. A refractor telescope with bigger aperture will yield washed out and poor images as it will amplify the sky glow.
The refractor boasts of an optical system that is highly resistant to misalignment. Greatly reliable and very convenient to travel with, this telescope entails lesser cleaning as the lenses are sealed inside the tube.However, refractor telescopes are more expensive than reflector telescopes. The best for the eye is apochromatic refractor telescopes, but they are more expensive.
In the second half of the 19th century large refracting telescopes were very popular, the reflecting telescopes are preferred by the researchers. As refractor telescopes use lenses to collect and focus light, it is difficult to make large refractor telescopes as finely ground glass lenses are expensive and heavy. The Yerkes Observatory in Wisconsin is the largest refractor telescope ever made and has lenses that are 40 inches in diameter.
Refracting telescope’s advantages:
The refractor telescopes are low in maintenance
The refractor telescope produces high-contrast, sharp images.
Due to air tight seal of a refractor telescope, the air currents does not affect the view.
The refractor telescope is ideal for long distance terrestrial viewing as well as viewing the planets, stars and other deep sky objects.
There are several disadvantages of this type of optical telescope:
This type of telescope costs more than the reflector and catadioptric telescope when comparing models of the same aperture size.
Due to the design of the telescope, the refractor telescope isn’t the best for viewing faint astronomical objects.
Although the images are clear, there may be unexpected color fringes around the objects in view.
Inexpensive models do not result in decent astronomical viewing as it lack the quality needed.
Despite the disadvantages of these types of telescopes, they are the most user-friendly versions and would be best suited for amateur star gazers.
Catadioptric Telescopes gives best of both Refractor and Reflector telescopes. They use both lens and mirror as light gathering and focusing devices. The Maksutov-Cassegrain and the Schmidt-Cassegrain are two main types of Catadioptric telescopes which are very popular. They both has some advantages and disadvantages. They have a primary thick, curved correcting lens and a secondary mirror. The Maksutov telescopes have secondary mirror smaller than the Schmidt’s which gives it a slightly better resolution for planetary observation.
The Schmidt are lighter than the Maksutov as Maksutove have thick correcting lens. At night, it can take longer time to get thermal stability telescopes with larger apertures. The Maksutov optical design is usually easier to manufacture, but requires more material for the corrector lens than the Schmidt-Cassegrain.
The telescopes that uses lenses instead of mirrors to gather light are called as Refractor Telescopes. They produce high-contrast images with sharp and crisp views of the astronomical objects like moon, planets, stars, galaxies and nebula. Refractors are known for their rugged simplicity. They are very easy to aim at, need less of a maintenance and once the lenses are aligned they rarely need it again. They are equally popular amongst beginners of astronomy as a hobby and armature star gazers.
Reflectors are telescopes that use mirrors. They most commonly use a main concave mirror at the bottom of the tube, which focuses light back up to the top of the tube. There, a small flat mirror placed at right angel, reflects the light to an eyepiece placed on the side of the telescope body. Reflectors telescopes offer larger aperture for the money you spend than refractors and providing enough light to see moon, planets and hundreds of deep-sky astronomical objects. As the mirrors in reflector telescopes not covered due to the design, they are expose to dust and grime which may need occasional cleaning and adjustments to keep them properly aligned.
In its simplest terms, an optical telescope is one that is designed to collect light in the visible spectrum of electromagnetic radiations. In other words, light that can be seen with the eyes rather than through using specialized equipment. This is done through the use of lenses and mirrors which collect the light and create an enhanced image for the viewer.
Mainly there are three types of optical telescopes:
1. Refracting telescopes, also called dioptric, which uses lens to gather light.
2. Reflecting telescopes which use mirrors to form the image, also known as catoptrics which is another term for using mirrors and reflections.
3. And finally there are catadioptric telescopes which use both lenses and mirrors together.
A telescope’s ability to gather light and resolve small detail is directly related to the primary lens or mirror that collects and focuses the light. The larger the primary lens is, which is also called the objective, the more light the telescope can collect and the clearer the image will be.
Very hot or more energetic objects in space can emit x-rays and the hottest or most energetic objects in space give off gamma-rays, the most energetic waves we know. X-rays and gamma rays from space, however, cannot penetrate the Earth’s atmosphere, because the short wavelengths get disrupted and weaken in the atmosphere. Scientists like to put these types of telescopes in space where atmosphere is not a problem and observe the object from the space. Several countries have launched x-ray and gamma-ray telescopes on satellites that orbit the Earth.
With these telescopes scientists see regions in deep space where high energy radiation is being created. Some of these regions include stellar explosions, galactic centers, and regions near pulsars and black holes. The is one telescope that looks at these objects and events.
X-ray telescopes help astronomers study the sun, supernova, and other stars, while the gamma ray telescopes study supernovas, pulsars and black holes.
Objects that are cooler than stars don’t give off visible light, but they often give off infra-red waves (sometimes called heat waves). One of the biggest problems with trying to detect faint infra-red waves from space is that everything on Earth (including everything around the astronomer, and the astronomer himself for that matter) is giving off infra-red waves. So infra-red telescopes need to be isolated and cooled to very low temperatures so their own infra-red waves don’t interfere with the waves coming from objects in space.
Since water vapor in our atmosphere gobbles up infra-red waves (not letting it through from space), infra-red astronomy is best done from above the clouds. Thus, modern infra-red telescopes are found in airplanes that can fly at high altitudes or on satellites in space.
There are objects and processes in the universe that put out natural radio waves. Radio telescopes look at very long wavelength light, the radio frequency portion of the electromagnetic spectrum.
To gather radio waves from space, astronomers use giant metal antennas otherwise known as radio dishes (as most resemble a soup bowl of some sort). Radio waves are collected by these antennas and are brought to a focus, where a radio detector can then capture the waves.
Radio waves are so large that they don’t notice small imperfections in the telescope surface, so unlike optical telescope mirrors, radio telescopes often have dishes with lots of holes to reduce the weight. You can see many diverse things with radio telescopes; for example, you can investigate how hydrogen gas is distributed in our galaxy and other galaxies, and you can time the rotation period of pulsars. Such cosmic radio waves can, for example, tell us about the material that lies between the stars, about sites of cosmic explosiveness, and about the remains of some types of dead or dying stars.
Individual radio telescopes generally can’t show as much detail in cosmic objects as visible light telescopes do. To overcome this difficulty, astronomers frequently tie together a number of radio dishes into an array. The more widely spaced the telescopes in such an array, the more detail they can make out in the objects they observe. In the largest arrays, radio dishes are spread out over many miles, so that we can see the details of distant objects much more clearly than with any single radio telescope. Scientists build them far away from populous areas to avoid interference from radio and TV. Radio telescopes look at very long wavelength light. You can see many diverse things with radio telescopes. For example, you can investigate how hydrogen gas is distributed in our galaxy and other galaxies, and you can time the rotation period of pulsars. (See for example, http://www.nrao.edu/imagegallery/php/level1.php).
A telescope is an instrument for collecting radiations or light from the distant objects in space.
Telescopes have been one of the most awe-inspiring instruments to not only the ardent scientists and astronomers, but to almost every human being. To be able to view fascinating objects billions and billions of miles away from us is truly a phenomenal experience that this instrument provides.
Visible light, the light humans see is only a tiny part of the huge range of electromagnetic waves the universe sends us. But stars like the Sun are especially good at giving off visible light, so visible light telescopes, also known as optical telescopes are extremely useful for studying the stars. Visible light is the middle range wavelength of electromagnetic spectrum. Short wavelengths include X-rays and gamma rays. Longer wavelengths include microwaves and radio waves.
Various telescopes differ from each other as they cater to observe differing types of electromagnetic radiation (EM). Although optical telescopes are the type most commonly used for stargazing and amateur astronomy by observing visible portion of light (waves) in electromagnetic spectrum, lot more astronomical research is done on telescopes that look at wavelengths other than that of visible light. Each of these wavelength ranges shows us something unique about the universe and allows us to view events and objects which are beyond the visibility of human eyes.
People have been looking to the sky to understand astronomical objects for much of human history. In fact, the earliest telescope can be traced all the way back to 1608. The first telescopes used a convex objective lens at one end of the telescope and a concave eyepiece as a way gather more light than can be done by just looking at something with the naked eye. Sir Isaac Newton is given credit for the first usable reflector telescope in 1668 by incorporating a small mirror to reflect the light to an eyepiece on the side of the telescope. The basic technology of optical telescopes, using lenses and mirrors to collect light, hasn’t altered much over the centuries, but the way they are used in combination has.
Telescopes can be categorized in several ways. While some are classified according to their specification, ranging from professional to amateur, others are classified by the determining which portion of the electromagnetic spectrum it deals with.
There are large varieties of telescopes available, although the safest bet would be the optical telescopes or visible light telescopes. In other words, these telescopes deal with middle range of electromagnetic spectrum, the light that can be seen with the eyes. Optical telescopes are basically focus on the visible light of the electromagnetic spectrum. Lens and mirrors are imperative to achieve the motive of this astronomical invention. The telescopes which focus on the longer wavelength of EM spectrum are Radio Telescopes and Infra-Red Telescopes and those focus on shorter wavelength are X-ray and Gamma ray Telescopes.
We’ll delve deeper into the different types of telescopes in different areas of this site.