Parabolic Mirror

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.

Infrared Spectroscopy

Infrared spectroscopy is spectroscopy in the infrared (IR) region of the electromagnetic spectrum. It is a vital part of infrared astronomy, just as it is in visual, or optical, astronomy (and has been since lines were discovered in the spectrum of the Sun, in 1802, though it was a couple of decades before Fraunhofer began to study them systematically).

For the most part, the techniques used in IR spectroscopy, in astronomy, are the same or very similar to those used in the visual waveband; confusingly, then, IR spectroscopy is part of both infrared astronomy and optical astronomy! These techniques involve use of mirrors, lenses, dispersive media such as prisms or gratings, and ‘quantum’ detectors (silicon-based CCDs in the visual waveband, HgCdTe – or InSb or PbSe – arrays in IR); at the long-wavelength end – where the IR overlaps with the submillimeter or terahertz region – there are somewhat different techniques.

As infrared astronomy has a much longer ground-based history than a space-based one, the terms used relate to the windows in the Earth’s atmosphere where lower absorption spectroscopy makes astronomy feasible … so there is the near-IR (NIR), from the end of the visual (~0.7 &#181m) to ~3 &#181m, the mid (to ~30 &#181m), and the far-IR (FIR, to 0.2 mm).

As with spectroscopy in the visual and UV wavebands, IR spectroscopy in astronomy involves detection of both absorption (mostly) and emission (rather less common) lines due to atomic transitions (the hydrogen Paschen, Brackett, Pfund, and Humphreys series are all in the IR, mostly NIR). However, lines and bands due to molecules are found in the spectra of nearly all objects, across the entire IR … and the reason why space-based observatories are needed to study water and carbon dioxide (to take just two examples) in astronomical objects. One of the most important class of molecules (of interest to astronomers) is PAHs – polycyclic aromatic hydrocarbons – whose transitions are most prominent in the mid-IR (see the Spitzer webpage Understanding Polycyclic Aromatic Hydrocarbons for more details).

Looking for more info on how astronomers do IR spectroscopy? Caltech has a brief introduction to IR spectroscopy. The ESO’s Very Large Telescope (VLT) has several dedicated instruments, including VISIR (which is both an imager and spectrometer, working in the mid-IR); CIRPASS, a NIR integrated field unit spectrograph on Gemini; Spitzer’s IRS (a mid-IR spectrograph); and LWS on the ESA’s Infrared Space Observatory (a FIR spectrometer).

Universe Today stories related to IR spectroscopy include Infrared Sensor Could Be Useful on Earth Too, Search for Origins Programs Shortlisted, and Jovian Moon Was Probably Captured.

Infrared spectroscopy is covered in the Astronomy Cast episode Infrared Astronomy.


Who Invented the Telescope

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:


Altazimuth is a contraction of altitude-azimuth; in astronomy it most often refers to a type of telescope mount (and is sometimes called alt-az), but it can also mean a coordinate system.

Altitude means the angular distance above the horizon; straight up (overhead) is 90o (and is called the zenith). Azimuth is also an angular distance, measured clockwise from north (so east is 90o). Any point, or direction, in the sky has one – and only one – altitude and azimuth; in other words, the altitude and azimuth are the coordinates of the point (on the celestial sphere).

An altazimuth telescope mount is one that can move separately in altitude (up and down, vertically) and azimuth (side to side, horizontally). Small telescopes used by amateur astronomers tend to have altazimuth mounts; larger ones tend to have equatorial mounts … unless they are Dobsonian. Why? Because while alt-az mounts are generally cheaper, tracking astronomical objects (like stars) is much easier with equatorial mounts.

Historically, the telescopes used by professional astronomers did not have alt-az mounts, because automatic tracking was impossible. As computers became powerful and cheap enough, they could be used to control the motors on each axis of an altazimuth mount; today, almost all ground-based astronomical telescopes have altazimuth mounts, whether optical, radio, or even high energy gamma ray! The first really large optical telescope to use an altazimuth mount is the 6-meter Bol’shoi Teleskop Azimultal’nyi, in Russia.

Universe Today’s Telescope Mount, Telescope Parts, Telescope Tripod, and How To Use a Telescope are great resources for learning more.

The Astronomy Cast episode Choosing and Using a Telescope covers the benefits of alt-az mounts (vs equatorial), and Telescopes, the Next Level gives insight into tomorrow’s professional ones.

What is Cherenkov Radiation?

Cherenkov radiation is named after the Russian physicist who first worked it out in detail, in 1934, Pavel Alekseyevich Cherenkov (he got a Nobel for his work, in 1958; because he’s Russian, it’s also sometimes called Cerenkov radiation).

Nothing’s faster than c, the speed of light … in a vacuum. In the air or water (or glass), the speed of light is slower than c. So what happens when something like a cosmic ray proton – which is moving way faster than the speed of light in air or water – hits the Earth’s atmosphere? It emits a cone of light, like the sonic boom of a supersonic plane; that light is Cherenkov radiation.

The Cherenkov radiation spectrum is continuous, and its intensity increases with frequency (up to a cutoff); that’s what gives it the eerie blue color you see in pictures of ‘swimming pool’ reactors.

Perhaps the best known astronomical use of Cherenkov radiation is in ICATs such CANGAROO (you guessed it, it’s in Australia!), H.E.S.S. (astronomers love this sort of thing, that’s a ‘tribute’ to Victor Hess, pioneer of cosmic rays studies), and VERITAS (see if you can explain the pun in that!). As a high energy gamma ray, above a few GeV, enters the atmosphere, it creates electron-positron pairs, which initiate an air shower. The shower creates a burst of Cherenkov radiation lasting a few nanoseconds, which the ICAT detects. Because Cherenkov radiation is well-understood, the bursts caused by gamma rays can be distinguished from those caused by protons; and by using several telescopes, the source ‘on the sky’ can be pinned down much better (that’s what one of the Ss in H.E.S.S. stands for, stereoscopic).

The more energetic a cosmic ray particle, the bigger the air shower it creates … so to study really energetic cosmic rays – those with energies above 10^18 ev (which is 100 million times as energetic as what the LHC will produce), which are called UHECRs (see if you can guess) – you need cosmic ray detectors spread over a huge area. That’s just what the Pierre Auger Cosmic Ray Observatory is; and its workhorse detectors are tanks of water with photomultiplier tubes in the dark (to detect the Cherenkov radiation of air shower particles).

However I think the coolest use of Cherenkov radiation in astronomy is IceCube, which detects the Cherenkov radiation produced by muons in Antarctic ice … traveling upward. These muons are produced by rare interactions of muon neutrinos with hydrogen or oxygen nuclei (in the ice), after they have traveled through the whole Earth, from the Artic (and before that perhaps a few hundred megaparsecs from some distant blazer).

ICAT: imaging Cherenkov Air Telescope
CANGAROO: Collaboration of Australia and Nippon (Japan) for a Gamma Ray Observatory in the Outback
H.E.S.S.: High Energy Stereoscopic System
VERITAS: Very Energetic Imaging Telescope Array System
UHECR: ultra-high-energy cosmic ray

This NASA webpage gives more details of how ICATs work.

Quite a few Universe Today stories are about Cherenkov radiation; for example Astronomers Observe Bizarre Blazar with Battery of Telescopes, and High Energy Gamma Rays Go Slower Than the Speed of Light?.

Examples of Astronomy Casts which include this topic: Cosmic Rays, and Gamma Ray Astronomy.


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

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.