Chromatic Aberration

Chromatic Aberration
Chromatic Aberration. Source: Wikipedia

[/caption]

Some colours just can’t keep up with the others! Well, that’s probably the simplest way to put it. But when scientists talk about the characteristics of light, it would be more accurate to say that different colours of light propagate at different speeds, orhave different wavelengths, and therefore refract differently. A well-known example of this is the prism effect, where a beam of white light is broken into a rainbow of colours. The result of this is that when objects are viewed through a simple lens, light will refract (bends) at different angles, meaning that it will not image all in the same place. A distortion results in which “fringes” of color appear along the boundaries that separate dark and bright parts of the image. This effect, known as Chromatic Aberration, can be a real pain for astronomers, surveyors, photographers, or just about anyone who wants to view an object (or objects) through a lens and needs to do so clearly!

Sir Isaac Newton was the first to demonstrate this effect some two-hundred years ago when he discovered that light was composed of multiple wavelengths. These colours refract unevenly, with blue-appearing light refracting at shorter wavelengths and red-appearing light refracting at longer, with green refracting in the middle. Since that time, scientists, astronomers and opticians have come to identify two basic kinds of aberration. The first is axial (or longitudinal) where different wavelengths are focused at a different distance because the lens in unable to focus different colours in the same focal plane. The second is transverse (or lateral) aberration, where different wavelengths are focused at different positions in the focal plane and the effect is a sideward displacement of the image. In the former case, distortion occurs throughout the image whereas in the latter, distortion is absent from the centre but increases towards the edge.

There are many ways to remedy Chromatic Aberration. During the 17th century, telescopes had to be very long in order to correct for colour distortions. Sir Isaac Newton remedied this problem by creating the comparably compact, reflecting telescope in 1668 that used curved mirrors to get around this problem. The achromatic lens (or achromatic doublet) is another; a double lens that uses two kinds of glass that focuses all white light coming in at the same point on the other side of the lens. Many types of glass, known as low dispersion glasses, have been developed to reduce chromatic aberration, the most notable being glasses that contain fluorite.

The discovery of Chromatic Aberration and the development of corrective lenses were major steps in the development of the optical microscope, the telescope; which in turn was a boon for astronomers and biologist who were able to gain a greater understanding of the universe and the natural world as a result.

We have written many articles about chromatic aberration for Universe Today. Here’s an article about optical aberration, and here’s an article about achromatic lens.

If you’d like more info on Chromatic Aberration, check out Hyperphysics for a great article on chromatic aberration, and here’s a link to Wise Geek’s discussion about chromatic aberration.

We’ve also recorded an entire episode of Astronomy Cast all about Choosing and Using a Telescope. Listen here, Episode 33: Choosing and Using a Telescope.

Sources:
http://en.wikipedia.org/wiki/Chromatic_aberration
http://toothwalker.org/optics/chromatic.html
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/aber2.html
http://www.yorku.ca/eye/chroaber.htm
http://www.yorku.ca/eye/achromat.htm

Absorption Spectroscopy

Absorptivity
Absorption Spectrum by the Hubble Space Telescope

[/caption]

In terms of physics, absorption is defined as the way that energy from photons is taken up by matter, and transformed into other forms of energy, like heat. All of the light in the electromagnetic spectrum is made up of photons at different energy levels. Radio waves are photons with lower amounts of energy, and gamma rays are photons with very high levels of energy. When a photon strikes matter, it can either be reflected or absorbed by the material. And if it is absorbed, the energy of the photon is transformed into heat.

The absorbance of an object is a measure of what percentage of the electromagnetic radiation it’s likely to absorb. Transparent or reflective objects absorb much less than opaque, black objects.

This concept is very important to astronomers, who are able to measure which wavelengths of light are being absorbed by an object or cloud of gas, to get an idea of what it’s made of. When you put the light from a star through a prism, you get a spectrum of the light coming from that star. But in some spectra, there are blank lines, gaps where no photons of a specific wavelength are being emitted. This means that some intervening object is absorbing all of the photons of this wavelength.

For example, imagine looking at how the light from a star passes through a planet’s atmosphere which is rich in sodium. This sodium will absorb photons at a specific wavelength, creating gaps in the spectrum from the light of the star. By comparing these gaps to the absorption line pattern of known gasses, astronomers can work out what’s in the planet’s atmosphere. This general method is used in many ways by astronomers to learn what distant objects are made out of.

The opposite of absorption is emission. This is where different elements will release photons when they’re heated. Different elements will release photons at different levels of energy, and their colors on the electromagnetic spectrum help astronomers discover what elements the object is made out of. When iron is heated, it releases photons in a very specific pattern, different from the pattern released by oxygen.

Both the absorption and emission serve as a fingerprint to help astronomers understand what the Universe is made out of.

We have written many articles about Absorption Spectroscopy for Universe Today. Here’s an article about amateur spectroscopy, and here’s an article about the light spectrum.

If you’d like more info on Absorption Spectroscopy, check out the Principles of Spectroscopy, and the Infrared Spectroscopy Page.

We’ve also recorded an episode of Astronomy Cast all about the Hubble Space Telescope. Listen here, Episode 88: The Hubble Space Telescope.

Source:
Wikipedia

Armazones Chile to be the Site for the 42 meter European Extremely Large Telescope?

Artist impression of the Extremely Large Telescope. Credit: ESO

Question: Where are the night skies always dark, cloud-free 360 days a year, bone-dry, and orbiting 3.5 km above sea level?
Answer: Armazones Mountain, Atacama desert, Chile.
Question: Who wants to go live there?
Answer: The European Extremely-Large Telescope (E-ELT)!

“We are talking about the biggest telescope in the world, the biggest for a long time to come. That means we have to choose the best spot. Chile has a superb location. It’s the best in the world, there’s no doubt,” the European Southern Observatory’s astronomer, Massimo Tarenghi, told AFP. He is one of four astronomers – two Chileans, an Italian (Tarenghi) and a German – who were in the desert this week to evaluate its suitability compared to the main other contender: the Spanish isle of La Palma in the Canary Islands off western Africa.

The European Southern Observatory (ESO), an intergovernmental astronomical research agency that already has three facilities operating in the Atacama desert, including the Very Large Telescope array in the town of Paranal which is currently considered Europe’s foremost observatory.

Work on the E-ELT is to begin in December 2011 and cost 90 million euros (120 million dollars) … once a decision is made on the site, which will be as early as March this year.

When complete, the E-ELT will be “the world’s biggest eye on the sky,” according to the ESO, which hopes it will “address many of the most pressing unsolved questions in astronomy.”

The E-ELT is likely to be as revolutionary in the field of astronomy as Galileo’s telescope 400 years ago that determined that the Sun, and not the Earth, was the center of our universe, according to the European agency based in Munich, Germany. The German astronomer in Chile, Wolfgang Gieren, waxed happily about the possibilities of the future telescope. “In no more than 15 years we could have the first good-resolution spectra of planets outside our universe that are the same size of Earth and see if we can detect signs of life,” he said.

One of the Chilean astronomers, Mario Harmuy, said the Armazones provided an ideal location. “Several things come together here. The cold Humboldt Current, which passes by Chile’s coast, means that there is a high pressure center in the Pacific that deflects high clouds and prevents cover over this part of the continent,” he said. “To the east, the high Andes mountains prevent humidity from moving in from the Atlantic with clouds. The higher you are, the less humidity there is, and thus the light from the stars go through less of the atmosphere and is distorted less when it hits the telescope.” To boot, the Chilean location is free of the storms that hit the Canary Islands and the Sahara, he said.

Tarneghi added that the ESO’s existing Paranal observatory nearby also meant that much of the ground infrastructure was already in place.

Chile’s government was equally enthusiastic about hosting the E-ELT. Gabriel Rodriguez, in charge of the foreign ministry’s science and technology division, said Chile was ready to cede the 600 hectares (1,500 acres) needed for the project. The government is to submit its offer to the ESO next Monday, with a decision expected early March.

The Italian astronomer cautioned that despite Chile’s obvious advantages, the tender had to be weighed carefully for all its aspects. “Neither any of us nor the ESO know what the final decision will be. We need to receive the Chilean and Spanish proposals and evaluate factors of operation, work and production costs,” Tarenghi said.

The other Chilean astronomer, Maria Teresa Ruiz, remained fired up at the potential of the new instrument. The “surface area of this telescope is bigger than all the others in Chile combined, which will allow us to explore things in the universe that we can’t even imagine today,” she said.

Source: AFP

Online Telescopes

Have you ever wondered what it would be like to look through a telescope, but don’t have one? Are you curious if there is such a thing as an online telescope? The answer is yes. If you have a computer, you can use it to virtually look through the eyepiece of a telescope… and even aim it at the objects of your choice!

One of the most exciting concepts to come about in a long time is the SLOOH Space Camera. Here’s an opportunity to look through a variety of online telescopes located around the world and take a look at space from the comfort of your home. It’s not difficult and you don’t need complicated instructions to use it. SLOOH’s patented instant-imaging technology and user-friendly interface let’s astronomers of all ages and skill levels remotely control a real telescope!All you need is a Mac or PC computer and Internet browser to explore the deepest reaches of space. To use the Slooh online telescope you must become a member of the Club, which includes mission cards, activity books, and online gift certificates. Once enrolled, you can articipate in group missions or control the online telescopes yourself. Says PC Magazine: “Would-be astronomers can gaze at live images of the night sky, but in the comfort of their homes. Kids – even big ones will marvel when they see the Andromeda Galaxy and other distant objects slowly materialize on their computer screens.”

iTel-Logo-UTIf you’re a bit more advanced and would like to try your hand at astrophotograpy with an online telescope, then check out iTelescope.Net. iTelescope.Net also has a variety of telescopes positioned in observatories around the world, and you can view live images as they are being created by remote astrophographers. Because taking images of the sky can involve very costly equipment and years of practice, how cool would it be just to tap into an on-line telescope and begin imaging? Now it’s as easy as taking lessons and renting the equipment – and you don’t even need clear skies or a special place to go. It’s as close as your PC!

Another type of opportunity to enjoy an online telescope in a different format is the WorldWide Telescope. While this online telescope doesn’t offer “live” views, the WorldWide Telescope (WWT) will allow your computer to act as a virtual telescope by displaying images from the foremost ground and space-based telescopes. You can even take a tour of all the most incredible places in space narrated by a real astronomer! This online telescope can provide views in multiple wavelength. Imagine seeing an x-ray view of a supernova and fading into visible light! Now you can take a look with H-alpha to view star-forming regions and examine high energy radiation coming from nearby stars in the Milky Way. Are you skies clouded out? No more. With the WorldWide Telescope you can view the Moon and planets anytime, from any location on Earth and any time in the past or future!

Would you like to use an online telescope to look at our nearest star? Then take a look at Eyes On The Skies. This simple and easy to use website offers “live” views of our Sun with an online telescope. This is the home of the internet-accessible robotic solar telescope, built by Tri-Valley Stargazers member Mike Rushford. Of course, you can only control the online solar telescope if the skies are sunny in Livermore California, USA!

Why Are There So Many Celestron Reviews?

Collimation
Collimation

I’ve had a couple of readers write me, wondering what was going on with all the Celestron telescope reviews. Are we sponsored by Celestron, or something? Nope. Let me just make this clear. We don’t get any money from any of the telescope manufacturers, or any kind of sponsorship at all. If and when we do, I’ll let you know.

So far, Celestron, Vixen and Sky Watcher are the only telescope manufacturers willing to send out a telescope for us to review, and then willing to pay for the return shipping to take it back off our hands again. If I have to pay to receive a telescope, or ship it back, we can’t afford to review it on Universe Today.

I know that really sets the bar pretty low. Universe Today received almost 2 million visitors last month, with 50,000 people subscribed to the RSS feed and daily email newsletter. Many of them are very interested in owning a telescope and would love to read about all the telescopes on the market. But I’m honestly exhausted trying to justify this to the manufacturers.

But so we’re clear, we’re not paid to give Celestron good reviews. If Tammy comes across as kind of enthusiastic in her reviews, well… that’s Tammy; she’s an enthusiastic force of nature. The manufacturers pay to ship the telescopes to and from our reviewers (well, Tammy), and then I pay Tammy for her reviews. If the telescope companies advertise on Universe Today, through Google, or through direct advertising, it doesn’t influence what Tammy has to say about them.

And if you’re a telescope manufacturer who wants to join this elite club of companies getting reviews on Universe Today, you just need to pay for the shipping. And if you want to advertise on Universe Today, just drop me an email.

P.S. I picked up a Celestron First Scope for the, uh, kids, and I really like it. Thanks to Tammy for the review, and thanks to the IYA for helping get it built.

The Extremely Large Telescope

The European Southern Observatory (ESO) is planning on building a massive – and I do mean massive – telescope in the next decade. The European Extremely Large Telescope (E-ELT) is a 42-meter telescope in its final planning stages. Weighing in at 5,000 tonnes, and made up of 984 individual mirrors, it will be able to image the discs of extrasolar planets and resolve individual stars in galaxies beyond the Local Group! By 2018 ESO hope to be using this gargantuan scope to stare so deep into space that they can actually see the Universe expanding!

The E-ELT is currently scheduled for completion around 2018 and when built it will be four times larger than anything currently looking at the sky in optical wavelengths and 100 times more powerful than the Hubble Space Telescope – despite being a ground-based observatory.

With advanced adaptive optics systems, the E-ELT will use up to 6 laser guide stars to analyse the twinkling caused by the motion of the atmosphere. Computer systems move the 984 individual mirrored panels up to a thousand times a second to cancel out this blurring effect in real time. The result is an image almost as crisp as if the telescope were in space.

This combination of incredible technological power and gigantic size mean that that the E-ELT will be able to not only detect the presence of planets around other stars but also begin to make images of them. It could potentially make a direct image of a Super Earth (a rocky planet just a few times larger than Earth). It would be capable of observing planets around stars within 15-30 light years of the Earth – there are almost 400 stars within that distance!

The E-ELT will be able to resolve stars within distant galaxies and as such begin to understand the history of such galaxies. This method of using the chemical composition, age and mass of stars to unravel the history of the galaxy is sometimes called galactic archaeology and instruments like the E-ELT would lead the way in such research.

Incredibly, by measuring the redshift of distant galaxies over many years with a telescope as sensitive as the E-ELT it should be possible to detect the gradual change in their doppler shift. As such the E-ELT could allow humans to watch the Universe itself expand!

ESO has already spent millions on developing the E-ELT concept. If it is completed as planned then it will eventually cost about €1 billion. The technology required to make the E-ELT happen is being developed right now all over the world – in fact it is creating new technologies, jobs and industry as it goes along. The telescope’s enclosure alone presents a huge engineering conundrum – how do you build something the size of modern sports stadium at high altitude and without any existing roads? They will need to keep 5,000 tonnes of metal and glass slewing around smoothly and easily once it’s operating – as well as figuring out how to mass-produce more than 1200 1.4m hexagonal mirrors.

The E-ELT has the capacity to transform our view not only of the Universe but of telescopes and the technology to build them as well. It will be a huge leap forward in telescope engineering and for European astronomy it will be a massive 42m jewel in the crown.

Convex Mirror

Convex Lens

A convex mirror is a spherical reflecting surface (or any reflecting surface fashioned into a portion of a sphere) in which its bulging side faces the source of light. Automobile enthusiasts often call it a fish eye mirror while other physics texts refer to it as a diverging mirror.

The term “diverging mirror” is based on this mirror’s behavior of making rays diverge upon reflection. So when you direct a beam of light on a convex mirror, the mirror will allow the initially parallel rays that make up the beam to diverge after striking the reflective surface.

Since convex mirrors have wider fields of view than other reflective surfaces, such as plane mirrors or concave mirrors, they are commonly used in automobile side mirrors. Having a fish eye on your automobile will allow you to see more of your rear.

A convex mirror is also a good security device. Store owners, for instance, install a number of them inside their stores and orient them in such a way that a single security personnel can see large portions of the store even while monitoring from a single location. They are the large disk-like reflective surfaces that you see near the ceilings of grocery or convenience shops.

The same kind of security devices are installed on automated teller machines to give the person withdrawing a good view of what is happening behind him. Some cell phones are also equipped with these mirrors to aid users when performing a self portrait shot.

Unlike images formed by concave mirrors, an image formed by a convex mirror cannot be projected on a screen. Such an image is called a virtual image. If one is to visualize the location of such a virtual image, then the image is found behind the surface of the mirror.

The complete description of an image formed by a convex mirror is: virtual, diminished in size, and upright. When we say upright, we mean that if you position an arrow in front of this kind of reflecting surface, then the arrowhead of the reflection will point to the same direction as that of the object (the real arrow) itself.

Want to see an object that is both a convex and a concave mirror? Take out a metallic spoon – the inner side is a concave mirror while the outer side is a convex mirror. Notice how your reflection is diminished in size. You may compare that with your reflection on a typical wall-mounted mirror.

Want to read more about mirrors? Here are some articles from Universe Today featuring them:
Parabolic Mirror
Nano-Engineered Liquid Mirror Telescopes

There’s more from NASA

NASA’s Largest Space Telescope Mirror Will See Deeper Into Space
Mirror Production Begins on Webb Telescope

Here are episodes from Astronomy Cast you might be interested in. Lend us your ears!
Shooting Lasers at the Moon and Losing Contact with Rovers
The Moon Part I

Source: The Physics Classroom

Parabolic Mirror

Herschel in 3-D. Credit: Nathanial Burton-Bradford.

[/caption]
Sometimes, in astronomy, the name of a thing describes it well; a parabolic mirror is, indeed, a mirror which has the shape of a parabola (an example of a name that does not describe itself well? How about Mare Nectaris, “Sea of Nectar”!). Actually, it’s a circular paraboloid, the 3D shape you get by rotating a parabola (which is 2D) around its axis.

The main part of the standard astronomical reflecting telescope – the primary mirror – is a parabolic mirror. So too is the dish of most radio telescopes, from the Lovell telescope at Jodrell Bank, to the telescopes in the Very Large Array; note that the dish in the Arecibo Observatory is not a parabolic mirror (it’s a spherical one). Focusing x-ray telescopes, such as Chandra and XMM-Newton, also use nested parabolic mirrors … followed by nested hyperbolic mirrors.

Why a parabolic shape? Because mirrors of this shape reflect the light (UV, IR, microwaves, radio) from distant objects onto a point, the focus of the parabola. This was known in ancient Greece, but the first telescope to incorporate a parabolic mirror wasn’t made until 1673 (by Robert Hooke, based on a design by James Gregory; the reflecting telescope Newton built used a spherical mirror). Parabolic mirrors do not suffer from spherical aberration (spherical mirrors cannot focus all incoming, on-axis, light onto a point), nor chromatic aberration (single lens refracting telescopes focus light of different colors at different points), so are the best kind of primary mirror for a simple telescope (however, off-axis sources will suffer from coma).

The Metropolitan State College of Denver has a cool animation of how a parabolic mirror focuses a plane wave train onto a point (the focus).

Universe Today has many articles on the use of parabolic mirrors in telescopes; for example Kid’s Telescope, Cassegrain Telescope, Where Did the Modern Telescope Come From?, Nano-Engineered Liquid Mirror Telescopes, A Pristine View of the Universe … from the Moon, Largest Mirror in Space Under Development, and 8.4 Metre Mirror Installed on Huge Binoculars.

Telescopes, the Next Level is an excellent Astronomy Cast episode, containing material on parabolic mirrors.

Who Invented the Telescope

Galileo Galilei's telescope with his handwritten note specifying the magnifying power of the lens, at an exhibition at The Franklin Institute in Philadelphia. Credit: AP Photo/Matt Rourke

The history of the telescope dates back to the early 1600s. Galileo Galilei is commonly credited for inventing the telescope, but this is not accurate. Galileo was the first to use a telescope for the purpose of astronomy in 1609 (400 years ago in 2009, which is currently being celebrated as the International Year of Astronomy). Hans Lipperhey, a German spectacle maker, is generally credited as the inventor of the telescope, as his patent application is dated the earliest, on the 25th of September 1608.

Lipperhey combined curved lenses to magnify objects by up to 3 times, and eventually crafted sets of binocular telescopes for the Government of the Netherlands.

There exists some confusion as to who actually came up with the idea first. Lipperhey’s patent application is the earliest on record, so this is usually used to settle the debate, although another spectacle-maker, Jacob Metius of Alkmaar, a city in the northern part of the Netherlands, filed for a patent for the same device a few weeks after Lipperhey. Another spectacle-maker, Sacharias Janssen, also claimed to have invented the telescope decades after the initial claims by Lipperhey and Metius.

Regardless of the inventor, most of the earliest versions of the telescope used a curved lens made of polished glass at the end of a tube to magnify objects to a factor of 3x. To learn more about how a telescope lens works, read our article on the telescope lens in the Guide to Space.

Galileo heard news of the telescope, and constructed his own version of it without ever seeing one. Instead of the initial 3 power magnification, he crafted a series of lenses that in combination allowed him to magnify things by 8, 20 and eventually 30 times. You can obtain a version of Galileo’s original telescope today, at the Galileoscope web site.

The lens telescope is still in use today in smaller telescopes, but many larger and more powerful telescopes use a reflective mirror and eyepiece combination that was initially invented by Isaac Newton. Called a “Newtonian” telescope after its inventor, these types of telescopes have a polished mirror at the end of a tube, which reflects the image into an eyepiece at the top of the tube. More information about Newtonian telescopes can be found in our Guide to Space article here.

Here’s a few more links on the history of the telescope:

What are Telescopes?

This artist’s rendering shows the Extremely Large Telescope in operation on Cerro Armazones in northern Chile. The telescope is shown using lasers to create artificial stars high in the atmosphere. Image: ESO/E-ELT
This artist’s rendering shows the Extremely Large Telescope in operation on Cerro Armazones in northern Chile. The telescope is shown using lasers to create artificial stars high in the atmosphere. Image: ESO/E-ELT

Early theories of the Universe were limited by the lack of telescopes. Many of modern astronomy’s findings would never have been made if it weren’t for Galileo Galilei’s discovery. Pirates and sea captains carried some of the first telescopes: they were simple spyglasses that only magnified your vision about four times and had a very narrow field of view. Today’s telescopes are huge arrays that can view entire quadrants of space. Galileo could never have imagined what he had set into motion.

Here are a few facts about telescopes and below that is a set of links to a plethora of information about them here on Universe Today.

Galileo’s first telescopes were simple arrangements of glass lenses that only magnified to a power of eight, but in less than two years he had improved his invention to 30 power telescope that allowed him to view Jupiter. His discovery is the basis for the modern refractor telescope.

There are two basic types of optical telescopes; reflector and refractor. Both magnify distant light, but in different ways. There is a link below that describes exactly how they differ.

Modern astronomer’s have a wide array of telescopes to make use of. There are optical observation decks all around the world. In addition to those there are radio telescopes, space telescopes, and on and on. Each has a specific purpose within astronomy. Everything you need to know about telescopes is contained in the links below, including how to build your own simple telescope.