Happy 400th birthday to the telescope!

At the point in The Hitchhiker's Guide to the Galaxy when Arthur Dent finds out what infinity looks like, Douglas Adams was moved to write that "infinity itself looks flat and uninteresting. Looking up into the night sky is looking into infinity - distance is incomprehensible and therefore meaningless."

Of course, the night sky is probably not infinite, as the universe appears to be expanding in a heavily studied war between the momentum of expansion and the force of gravity. This is probably why it looks so gigantic and gives a better indication of what infinity is like, rather than infinity itself.

Those who love to gaze at the night sky can celebrate the 400th anniversary of the invention of the telescope, which is this week.

The telescope has come a long way. The earliest working telescopes were invented in the Netherlands by spectacle makers Hans Lippershey, Zacharias Janssen and Jacob Metius in 1608. Galileo heard about this and made his own, greatly improved version in the following year. Galileo usually gets all the credit because he told the public about it and presented his prototype to the chief magistrate of Venice.

These early telescopes were refracting telescopes. Refracting telescopes create an image using an objective lens, which gathers the light coming from the object under study and focuses the rays. (Interestingly, the word "lens" comes from the Latin for "lentil," presumably because a double-convex lens looks like a lentil.)

Galileo's design used a convex objective lens and a concave eyepiece lens and magnified things by about 30 times, albeit with defects due to flaws in the lens shapes. But this was good enough for him to see the Moon's craters as well as some of Jupiter's moons.

Soon afterwards, the advantages of using parabolic mirrors as objectives instead of lenses became apparent. This improvement reduced spherical aberration, which distorts the image as a result of increased light refraction as light rays strike a lens. The use of parabolic mirrors instead of lenses also eliminated chromatic aberration, the failure of a lens to focus colors to one point.

In the 1660s, Sir Isaac Newton joined the parabolic mirrors school of thought and proposed that chromatic aberration occurred as a result of different refractive indices in lenses for different wavelengths of light.

He then invented the first reflecting telescope, which used a concave primary mirror and a flat diagonal secondary mirror, which reflected the image at 90 degrees to the eyepiece (allowing the image to be viewed without obstruction from the objective). This also proved his theory of color and got him admitted to the Royal Society of London, which is still rather a big deal.

Reflecting telescopes elucidated much about space, probably most notably the spiral form of the galaxies, which was discovered using a 72-inch telescope nicknamed the "Leviathan of Parsontown."

The problem with the giant reflecting telescopes that became popular in the 1800s was the poor reflectivity resulting from the mirrors tarnishing and having to be repolished, which could change the curve of the mirror, which was a great bother.

In 1857, Léon Foucault solved this by a method for silver layer deposition on the mirrors, which was much more reflective and didn't tarnish as fast. Most importantly, even if the silver mirrors needed to be removed, re-deposition did not change the shape of the substrate.

This discovery led to even larger telescopes and better methods for metal layer deposition, such as thermal vacuum evaporation. In the 1990s, adaptive optics (AO) emerged, which reduce atmospheric distortion by measuring distortions in a wavefront of air with lasers and then making up for them in a system of actuators applied to a deformable mirror. This took a long time to occur because it's hard to calculate the compensation times without computers.

The advent of computer technology spawned construction of telescopes that could use wavelengths other than visible light. Modern telescopes parse wavelengths from radio to gamma-rays.

The two internationally-owned and operated 26-foot Gemini telescopes on Hawaii's Mauna Kea Observatory and in the Andes in Chile use AO technology as well as infrared. Together, they provide almost total coverage of the northern and southern skies, can be run remotely and yield extremely high-quality images. The observatory includes a sputtering chamber that can apply silver coatings on the telescopes' mirrors.

Mauna Kea is also home to the Keck Observatory, whose 10-meter, 8-story, 300-ton telescopes have mirrors composed of 36 hexagonal parts that function as single sheets of glass. The Keck optical/infrared telescopes have helped scientists discover galaxies, study supernovas and find planets in other systems.

Another modern telescope is the appropriately named Very Large Array (VLA) near Socorro, New Mexico, which has 27 radio antennas that weigh 230 tons each and are each 82 feet in diameter. The VLA is part of the National Radio Astronomy Observatory. This telescope allows investigations of radio galaxies, quasars, pulsars, gammay ray bursts, the planets and black holes, and can communicate with spacecraft.

The beloved Hubble Space Telescope, a collaborative effort between NASA and the European Space Agency, was carried into orbit by the space shuttle Discovery in April 1990. The advantages of Hubble's location in space include the ability to take extremely sharp images, like the Ultra Deep Field image, the most detailed visible light image made of very distant objects in the universe (it looks back about 13 billion years and contains about 10,000 galaxies).

The Hubble telescope was also instrumental in determining the rate of expansion of the universe. The Hubble telescope will function until 2013 after the repairs planned for next month, after which its successor, the James Webb Space Telescope (JWST), will be launched.

The JWST, as a result of redshift and low temperatures of intended sources to be studied, can only observe in infrared, unlike Hubble, which observes visible light and ultraviolet. Its main mission has several parts: to search for light from the first stars and galaxies to form in the universe, to study the formation and evolution of galaxies and stars, and to investigate planetary systems and the origins of life. JWST is due to be launched in June 2013 from French Guiana.