User:LouScheffer/Sandbox

Telescopes (and the optically similar microscopes) are based on the science of optics and the technology of mirrors and lenses, and the history of the telescope is interwined with advances in these fields. Crude optics and crude lenses had been known since antiquity, but the scientific study of optics started in the 10th and 11th centuries by scholars such as Alhazen and Ibn Sahl, and lenses good enough for telescopes were developed by the spectacle makers of 13th century Italy and Germany. . The combination of two lenses in a single device (compound optics) was first used by Giambattista della Porta in 1589 and the telescope and microscope soon followed. Although there is fragmentary evidence for earlier designs the first historically accepted telescopes appear in the Netherlands in 1608. The invention of the telescope is generally credited to three individuals: Hans Lippershey and Zacharias Janssen of Middelburg (who also invented the microscope), and Jacob Metius of Alkmaar.

Galileo greatly improved upon these designs the following year and is generally credited with being the first to use a telescope for astronomical purposes. Galileo's telescope used Hans Lippershey's design of a convex objective lens and a concave eye lens and this design has come to be called a Galilean telescope. Johannes Kepler proposed an improvement on the design that used a convex eyepiece, often called the Keplerian Telescope. Theory was of little help in the development of refractors. Isaac Newton, in a famous mistake, thought achromatic lenses impossible, delaying for decades the development of the achromatic lens. Aspheric lenses, though understood early on, could not be constructed until the 19th century. The 20th century addition of anti-reflective coating leads to the refractive telescopes of today.

For reflecting telescopes, which use a curved mirror in place of the objective lens, theory proceeded practice. The potential advantages (primarily lack of chromatic aberration) and optical theory were known by 1632, including the Cassegrain and Gregorian configurations. , but the technology of fabricating and testing the required mirrors was the limiting factor. Isaac Newton developed pitch lap for grinding spherical mirrors and has been generally credited with constructing the first practical reflecting telescopes, the Newtonian telescope, in 1669. Other important developments in reflecting telescopes were paraboloidal metal mirrors over 100 years later, silver coated glass mirrors in the 19th century, long-lasting aluminum coatings in the 20th century , segmented mirrors to allow larger diameters, and active optics to compensate for gravitational deformation. Finally, recent reflecting telescopes use sophisticated optical designs to simultaneously optimize both light gathering power and field of view, while minimizing off-axis optical aberrations.

A mid-20th century innovation was catadioptric telescopes such as the Schmidt Cassegrain, which uses both a lens and mirror as primary optical elements. The lens, though aspheric, is very weak and serves mainly to correct the spherical aberration of the mirror. This allows very compact optical configurations, provides support for the secondary, and keeps dust out of the optical path.

The resolution of Earth-bound telescopes is determined by seeing and not the diffraction limit. One way around this problem is a space based telescope such as the Hubble space telescope. However, space telescope are extremely expensive and of limited size, so adaptive optics for ground-based telescopes has been developed as well.

Until the mid 20th century, all telescopes were optical, and were called simply 'telescopes'. However, starting with the radio telescope, new devices to record emanations of different forms from the heavens were called telescopes as well. Now the phrase 'optical telescope' (a retronym) is used for a telescope that gathers visible light, to distinguish it from telescopes of other types, ranging from other electromagnetic bands (radio through gamma ray) as well as other forms such as neutrinos, cosmic rays, and gravitational waves. The ancient Greeks knew of the focusing properties of parabolic mirrors

As of 2008, telescopes of less than about 10cm in diameter are typically refractors. Larger telescopes are typically reflectors.

Here are some potentially missing points, in no particular order:
 * Reflecting telescopes were guided by theory, but the mirror technology is hard. Note that the optics used in the Hubble were understood in 1632, and the mirror mis-manufactured in the 1980s.  The Hale (Palomar) 5 meter was the last big parabolic mirror, since Hale thought the project risky enough already.  All big telescopes since then are Ritchey-Chrétien telescope or more sophisticated yet.
 * In comparison, theory did not help much with refractors, and in fact set them back. In particular, Newton thought an achromatic lens was impossible, and this discouraged research for many years.
 * On the other hand, Newton invented the pitch lap, the key to polishing reflectors for the next few hundred years.
 * In many cases there are three contributors, the person who first discovered it, the person who popularized it, and the person who made it work. Cavalieri (1632) and Mersenne (1636) invented the Cassegrain and Gregorian reflectors, Gregory (1663) and Cassegrain (1672) made them popular (all before Newton's reflector) but could not make them work, but later Hooke (1674, Gregorian) and (???, Cassegrain)  figured out how to make the mirrors and make them practical.
 * Many modern telescopes are optimized for etendue as well as diameter. For many research programs, field of view is as important as mirror size (since in a large angle investigation or survey, one can be traded for the other.)
 * Modern telescopes have no need to accomodate a human observer at the focus, though this was a requirement for many years.