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.