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Currently, refractor telescopes are limited in aperture because gravity will deform the main lens over time. According to Wikipedia the largest practical refracting telescope is 1m.
If we were to overcome this limitation, what would be the theoretical benefit of such a telescope that other types of telescopes currently can't give us?
This is a really interesting question!
The Dragonfly telescope described below takes advantage of the fact that on Earth, the optical surfaces of some lenses can generate less diffuse scattering than the optical surfaces of reflecting telescopes. If you were worrying about scattering from secondary mirrors you could make an off-axis primary, but at a nanometer scale a metalized mirror will have more roughness than a glass lens with antireflection coatings (or so the links say). I suppose there are broadband dielectric reflective coatings for telescope mirrors, but that's probably a topic for a new question.
note that in many cases surface brightness limitations are often the result of skyglow (artificial) and airglow (natural) rather than telescope optics, so applications for large lenses might be found above the atmosphere.
If we ignore the fact that in the solar system space is filled with micrometeorites, then a lens in space might maintain it's lower diffuse scattering for quite a long time! But we can't, so it may not. It may get beaten up fairly quickly in fact. A long tube in front of the lens will decrease the solid angle of primary impacts, but impacts inside the tube may still cause trouble, and if you don't shade the tube from sunlight, diffuse scattering from the close-to-but-not-completely-black coating might also cause trouble.
So far there are no answers to How fast do optical surfaces get dirty or damaged in space? that I can point you to.
Below's discussion of and links to the Dragonfly Telescope is taken from the question What (actually) is the “deprojected half-light radius” of this almost-all-dark-matter Galaxy?
The recent news of the Ultra Diffuse Galaxy (UDG) Dragonfly 44 is an excellent example of what could be termed 'observe different' thinking. The dragonfly telescope is noted not for the size of its collective aperture, but for the absence of the diffracting effects of secondary mirrors and surface roughness that limit the contrast of dim objects in conventional telescopes when brighter sources are nearby. See here and here and here.
above: image of a Dragonfly refractive array telescope from here. Image: P. Van Dokkum; R. Abraham; J. Brodie
above: The Dragonfly 44 ultradiffuse galaxy from here. "Dragonfly 44 is very faint for its mass and consists almost entirely of dark matter. (Pieter van Dokkum, Roberto Abraham, Gemini Observatory/AURA)"
I’ve conducted some extensive research around these two primary types of telescopes. I will first begin with Refractors, before moving swiftly on to the Reflecting Counterpart.
The the first reported use of telescopes appeared in Europe during the early years of the 1600s. They were designed using lenses to enlarge distant objects.
This type of telescope, that utilizes lenses alone, is what we now know as and refer to as a Refracting Telescope.
The first known Refracting telescopes worked using a design known as a Galilean Refractor.
In this design, a convex lens objective and a concave lens eyepiece are used to magnify distant objects and their apparent diameters.
These telescopes are relatively easy to use because they produce an image that is shown the correct side up left to right.
Today, Galilean Refractors are not typically used they are primarily sold as collectors items. Sometimes you will seem them sold under the name “Spy Glasses”.
If you are to purchase a modern Refracting Telescope today, it will most likely be constructed using a slightly adapted design, a Kaplerian Refractor
Kaplerian Refractors still use a convex lens objective but instead have a longer focal length with a convex lens eyepiece set behind the focal plane.
How A Refractor Works: The purpose of the objective lens, which is at the opposite end to the eyepiece, is to collect the light from an object you are observing in the sky, e.g. a star. It then bends that light into a single point of focus. The second lens (which is the eyepiece) has a purpose to then enlarge your focused image for your eyes in many ways it is just like a magnifying glass. In other words, imagine the focused light being collected from the first lens as an insect, and then think of the eyepiece like a Magnifying Glass. This is pretty much how it works.
However, it is important to note that this method of magnification creates an image that is inverted both vertically and horizontally. However, the Kaplerian Design is still preferred over the Galilean because it allows for a larger apparent field of view (see more at any one time).
Therefore, it is important to note that if you purchase a Refractor Telescope, your images will be inverted and appear upside down. This can take some getting used to, but if you wanted to correct this, then you would need to apply a Erect-Image Prism Diagonal to correct and re-orientate the image
Keplerian Refractors are good if you are a beginner in astronomy and are commonly used. These are some of the benefits and as to why they are used:
Advantages of Refractor Telescopes
- Easy Maintenance, Light and Transportable.
- Better Performance in Weaker Conditions – Images Appear Steadier
- Near permanent optical Alignment.
Easy Maintenance: Due to the design of a Refractor Telescope, the glass surface is protected in the tube and is therefore closed from the atmosphere. This means that it barely needs cleaning.
Better Performance in Weaker Conditions: A secondary benefit to the design, is that the lenses are less affected by changing temperatures. This results in steadier and sharper images than can be typically achieved from a Reflector telescope (of the same size).
Optical Alignment: Refractor telescopes only really require a one-off alignment. This is because their optical system is more resistant to misalignment than the reflector telescopes.
However, refracting telescopes do have their limitations and its good to be cognizant of these:
Disadvantages of Refractor Telescopes
Spherical Aberration happens due to the use of spherical lenses. These are unable to bring parallel light rays into a perfect focus as each part of the lens refracts light somewhat differently. The result is that this causes a slight blurring of images. Thankfully, this can be corrected by using an objective lens that is parabolic rather than spherical.
Chromatic Aberration occurs because simple glass lenses are only able to refract shorter wavelengths of light over longer wavelengths. This problem can be mostly corrected by using compound lenses. These are lenses made with two different types of glass each with different refractive properties.
Lens Sag happens only in the largest Refracting telescopes. However, it is the principal limiting factor of Refracting telescopes as they get larger. Glass is not perfectly equal so as lenses are made larger and larger they become too heavy to retain the correct shape lens. The result is that the lens becomes distorted due to its own weight.
Unfortunately, there’s no real way to fix Lens Sag, apart from refraining from using a lens to capture and focus the light. Enter the Reflecting Telescope.
Learn Astronomy HQ
Amateur astronomers have a simple decision to make when they are just starting out. What kind of telescope should I buy? Most people will tell you there is a simple choice between a scope called a refractor, or a scope called a reflector. They have unique characteristics, strengths and limitations.
However, there are two unique challenges you need to understand related to the refractor/reflector decision: Chromatic distortion and Collimation. There are also some strengths and limitations related to viewing angle, but let's get through the two "C's" first.
Chromatic distortion affects refractors. It's the classic telescope design used by Galileo and allow lenses in a certain alignment to magnify objects. The occasional problem at high magnification is the way that light behaves. Light passes through lenses in a refractor and is bent and modified to form an image. Given the fact that light disperserses through any lens, you may not see the true color of an object. Chromatic distortion is a failure of a lens to focus all colors to the same convergence point.
Can you fix it? Not really although a longer focal length (the length of the tube on your scope) can compensate. NASA has done significant research on the distortion, but even they admit it's difficult to manage. The bottom-line is simple, if you choose a refractor you will simply have to accept that you will have some degree of chromatic distortion at some point.
On a positive note, refractors are excellent for observation of near-earth objects such as the moon and the planets. If you enjoy spending your time studying our solar-system, a refractor is the right choice for you.
Reflectors don't have this chromatic issue given that the construction of a reflector is essentially designed around mirrors that magnify and reflect light. However, reflectors present a unique alignment challenge called "Collimation." The fundamental challenge with any reflector is to make sure the light captured at the base of the tube in the polished mirror is accurately directed towards a small mirror at the top of the scope directing the image to the eye-piece. It sounds easy enough and is fairly basic, but if the base mirror or reflecting mirror to the eye-piece is out of alignment you need to "Collimate" your scope.
An example of good collimation: Image by Jeremy Stanley
A simple way to do this is with a " collimator ." It's fairly inexpensive and uses a laser beam to help you align your adjustments. Most collimators come with full instructions including the usual warnings about looking directly into a laser beam. If you purchase a reflector you might want to purchase a collimator as well.
A reflector scope being collimated
Reflectors are good for near-earth objects, but are also the scope of choice for observing distant galaxies or moons of Jupiter or Saturn. They're also more economical relative to a refractor for the viewing power.
The general consensus is that refractors are typically used for viewing of the moon and planets, while the light gathering ability of a reflector are best for viewing of galaxies and deep space objects. However, a reflector also works quite well for viewing the moon and the planets so the decision is up to you.
One other consideration is related to viewing position. A refractor and a reflector are wonderful when we're watching Venus rise on the horizon in the western sky or the full moon above the horizon in the east. It all becomes a bit problematic when we're viewing objects directly above.
A refractor with a short focal-length (a short tube) is not a problem especially if you use a 90° elbow. However, you may need a high tripod and most of us sit in a chair when using a refractor in any position.
An example image of a high tripod used for this large refractor due to the viewing position at the back of the telescope.
A Reflector is typically a standing proposition unless you're viewing objects on the horizon. This is both comfortable and easy for adults, but can be a problem for children. That's something to think about with regards to scope choice. Refractors tend to favor a lower viewing position, while reflectors tend to favor a taller viewing position.
What is a Refracting Telescope – Prices
Prices on these scopes vary, but considering the difficulties they present for makers of large scopes, the versions available to the public tend to be within a reasonable price range. For those looking to set up a scope in their backyard for clear nights, roughly $200-400 will get you a great quality starter scope. If you have more to spend, you can look into computerized telescopes, ones with tracking devices, larger lenses, or higher magnification rates. The prices depend on the maker and the technology.
The beginner lenses generally measure 60mm, 80mm, 90mm, and offer very clear pictures regardless of their small size. Telescopes come with a tripod to set them on, usually with a rotating ball at the center for easy navigation. You can buy a finderscope, which is essentially a miniature telescope that sits on top of the main scope and helps you focus in on the object you’re viewing. It makes observing objects at a distance much easier, and much more accurate.
Overall, if you’re looking to view the moon’s surface or planets when they are within range, a refractor telescope is a perfect purchase for you as a beginner.
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What Are Telescopes and Its Benefit?
A telescope "telescope" is an instrument designed for the observation of remote objects. The term usually refers to optical telescopes, but there are telescopes for most of the spectrum of electromagnetic radiation and for other signal types. An optical telescope is an optical tool that gathers and focuses electromagnetic radiation. Telescopes increase the apparent angular size of distant objects, as well as their apparent brightness. Telescopes work by employing one or more curved optical elements - lenses or mirrors - to gather light or other radiation and bring that light or radiation to a focus, where the image can be observed, photographed or studied. Optical telescopes are used for astronomy and in many non-astronomical instruments.
The main purpose of a telescope is to gather light, i.e. to collect and focus photons. We can think of a telescope then as a "light bucket" - the bigger the bucket, the more photons a telescope can collect.
Types of telescopes:
There are three basic types of telescopes -- Refractors, Newtonian reflectors, and Catadioptrics.
Newtonian Reflector Telescopes:
Newtonians usually use a concave parabolic primary mirror to collect and focus incoming light onto a flat secondary mirror that in turn reflects the image out of an opening at the side of the main tube and into the eyepiece.
Refracting telescopes have an objective lens at the front of the tube. The light exits out through the back of the tube to the eyepiece. Since many observations are made high in the sky, a right-angle diagonal is used to avoid neck strain. This also provides an upright image making them suitable for terrestrial observations. A refractor has several advantages over other designs. The tubes are enclosed so that dust and moisture do not enter the tube, they have fixed optics that do not normally require collimation, and they do not have a central obstruction which reduces the light entering the tube. A refractor typically will give higher quality images of planets than other telescopes of similar aperture.
Telescopes using a combination of both mirrors and lenses are called catadioptrics. There are many different designs. Examples of these are the Schmidt-Cass grain and the Maksutov-Cassegrain. Usually a full aperture lens is used to correct aberrations in a compound reflecting telescope. The corrector lens also increases the performance of the instrument as air currents are eliminated. The main advantage of the design is that, because the light path is folded back on itself, it provides a very portable, short physical length telescope with a long focal length.
1. The aperture of a telescope is several times larger than the aperture of human eye so that the objects that can not be normally seen by unaided eye can be seen. Light- gathering power of a telescope is proportional to the area of its aperture and hence depends on the square of the radius of the mirror. Therefore a 20 cm diameter telescope collects four times more photons than a 10 cm diameter telescope.
2. A telescope can be equipped to record light over a long period of time, by using photographic film or electronic detectors such as photometers or CCD detectors while the eye has no capability to store light. A long-exposure photograph taken through a telescope reveals objects too faint to be seen with the eye, even by looking through the same telescope.
3. A third major advantage of large telescopes is that they have superior resolution, the ability to discern fine detail. Small resolution is good. The resolution is directly proportional to the wavelength being observed and inversely proportional to the diameter of the telescope.
Advantages of Refracting Telescope
- Superior resolving power per inch of aperture
- No reflections or disruption of the path of light
- Near permanent optical alignment – requires minimum maintenance
- Long focal ratios mean using longer focal length and simpler eyepieces
- Glass surface inside the tube is sealed from the atmosphere and seldom requires cleaning.
- Since the tube is closed off from the outside, effects of air currents and changing temperature are absent, thereby, giving sharper and steadier images.
What are the benefits of a large refracting telescope? - Astronomy
There are two basic types of telescopes, refractors and reflectors. The part of the telescope that gathers the light, called the objective, determines the type of telescope. A refractor telescope uses a glass lens as its objective. The glass lens is at the front of the telescope and light is bent (refracted) as it passes through the lens. A reflector telescope uses a mirror as its objective. The mirror is close to the rear of the telescope and light is bounced off (reflected) as it strikes the mirror.
- Refractor telescopes are rugged. After the initial alignment, their optical system is more resistant to misalignment than the reflector telescopes.
- The glass surface inside the tube is sealed from the atmosphere so it rarely needs cleaning.
- Since the tube is closed off from the outside, air currents and effects due to changing temperatures are eliminated. This means that the images are steadier and sharper than those from a reflector telescope of the same size.
Though excellent refractors are still made, the disadvantages of the refractor telescope have blocked the construction of very large refractors for use in astronomical research.
The Final Say
Refracting telescopes come in different types and configurations. Each of them has its own key features that are powerful enough to provide sharp, bright images from huge distances. Some of them are too advanced and are made of high-quality lenses and components, but these kinds of telescopes are usually expensive.
Entry-level telescopes, on the other hand, while more affordable, have specifications that are almost similar to more expensive models. These telescopes are ideal for budding astronomers and family outdoor activity.
Now that you know how does a refracting telescope work, we are confident that you are well on your way to using this remarkable invention to good use. Happy viewing!
Tips When Buying a Telescope
Now that you have an idea on the two types of telescopes and the difference when you use a bigger instrument, you might think that you’re all set on purchasing your telescope. Before going on your next shopping trip, here are some tips to remember for your first telescope:
1. It’s not all about power.
Although higher magnification power means that the larger you can see the object, it also means that the image is spread out through your eyepiece, making it fainter with your every zoom. Also, there are high-powered scopes in the market today that requires specific eyepieces, so make sure what type of eyepiece would compliment the kind of scope you’re looking for.
2. Having a good telescope mount is as important as the eyepieces.
There are two most common types of mounts in the market today: equatorial and altazimuth. Equatorial mounts are designed to automatically follow the rotation of the earth while altazimuth mounts allow you to move your telescope back and forth and up and down.
Make sure that you test out the mount of the telescope before purchasing it. It should not easily wobble when you touch and should never wobble because of the wind.
3. Consider the weight and portability of the telescope.
If you’re an amateur who would like to enhance their stargazing skills, then it’s understandable if you purchase a telescope and place it on your balcony. However, if you’re an avid stargazer, you would most probably be looking for different camping areas for you to have darker skies in front of you. With this in mind, you should consider the weight of your telescope and how portable and compact it is when you’re always traveling.
4. Understand that there are different types of eyepieces and they have their own functions.
When purchasing a telescope, you should also consider buying a set of eyepieces that will coincide with your telescope. A higher powered eyepiece can provide you with better detail of the image, while a lower power eyepiece can help you see small clusters and even make it easier for you to find different objects in the sky.
Both the refractors and the reflectors are very good and are used for specific tasks. It is very important that the purpose, required features and budget are considered when making a decision.If you are still debating on which one to go for, then this guide will help you in making an informed decision. At this point, you should have the information that you require, and now the final choice is yours to make.
If you have brought a telescope, tells us which one and share with us how you were able to decide in the comment box below.