# Telescope

A telescope (of the Greek tele meaning “far” and skopein meaning “to look at, to see”) is an instrument Optique which makes it possible to increase the apparent size of the objects observed and especially their luminosity. Its role of amplifier of light being as, even more important as its optical Enlargement, it makes it possible to see not easily perceptible or invisible specific celestial objects with the naked eye.

The telescopes are mainly used in Astronomie, because their adjustments make them favourable only with the observations very distant objects.

## Telescope and glasses

A linguistic precision is essential here because of possible confusion in the use and the translation of the word telescopes, in particular during the consultation of documentations or notes in English language. Indeed, in English, the word telescope indicates two types of instruments:
• one, the “refracting telescope”, indicates the Telescope with its objective made up of a whole of lenses,

• the other, the “reflecting telescope”, indicates the telescope itself with its objective made up of a mirror. This article treats only this last instrument.

## History

Precursor of the telescope, the glasses of approach was designed in Italy towards 1586; its invention is most probably due to the Italian optician Giambattista della Porta. But it is in 1609 that the astronomer Italy N Galileo presented the first Telescope. Its German fellow-member Johannes Kepler improved the principle of it, by proposing an optical formula with two lens S convex.

In a telescope, a concave Miroir is used to form the image. In 1663, the Scottish mathematician James Gregory was the first to propose the formula of the telescope with a growth due to the secondary. Nevertheless, Mersenne, had anticipated a system to him in which the primary education and the secondary were parabolic, the pupil of exit was located on the secondary, which was used thus as eyepiece. But the field was very weak.

The English mathematician and physicist Isaac Newton built a first version in of it 1671. In this type of instrument, reflected light by the concave Miroir primary must be caused to a position of observation, in lower part or on the side of the instrument. Henry To drape, one of the very first astronomers American to build a telescope, will use two centuries later a prism with total reflection instead of the plane mirror of the telescope of Newton.

The pioneer was the telescope 2.54 m in diameter of the Observatoire of the Mount Wilson, in California: remained famous to have served in the Years 1920 with work of the American astronomer Edwin Hubble, its use ceased 1985 with 1992 under the effect of financial pressures.

Design of the Telescopes Keck mark an important innovation: the reflective surface of the mirror of each of the two telescopes is made up of a mosaic of thirty-six hexagonal mirrors, all directional individually thanks to three Vérin S. It is equivalent to a primary Miroir of 10 m Diamètre, without having the weight of them. Techniques known as of Optique activates make it possible to play on the Vérin S to optimize the profile of total reflective surface.

On its side, the Broad Very Telescopes (VLT) European Southern Observatory (ESO), is composed of four telescopes, having each one a mirror of 8.20 Mr. It is located at the Chile, at the top of the Cerro Paranal, to 2600 m of altitude. It was equipped in 2002 with the system with adaptive Optique NAOS enabling him to be twice more precise than the Space telescope Hubble.

It is also possible to today use in the optical field the principles of the Interférométrie to improve the resolution. It is the principle used by both Kecks, but especially by the VLT whose four mirrors, distant to the maximum of 130 m, have the same theoretical resolution as only one mirror 130 m in diameter. The sensitivity is however not improved, and the technique of interferometry remains rather special, often used in very particular cases.

## Components

The instruments of astronomical observation generally consist of two optical systems complementary: the objective and the eyepiece.

### Objective

In a telescope the objective is a concave mirror, generally parabolic. With difference of ices used in life everyday, face reflective is located ahead, so that the light does not cross glass which is used only as support with a film of Aluminum of some hundredths of Micromètre S. the light being simply reflected and not refracted, contrary to what occurs in a telescope, the achromatism of the telescopes is total.

The light is then focused in a point called Foyer image. The convergent beam can be returned towards an eyepiece using a second mirror which is plane in the case of a telescope of Newton. This small mirror inevitably causes an obstruction, i.e. a loss of luminosity what is not serious, but also a light loss of contrast without gravity if it does not exceed 20%.

### Eyepiece

The Oculaire is the part of the instrument which makes it possible to increase the image produced by the objective on the level of the hearth-image; an eyepiece is anything else only a sophisticated Loupe. The development is done by regulating the distance between the objective and the eyepiece. A telescope is theoretically a instrument afocal, i.e. it is possible to make coincide the hearth-image of the primary education mirror with the hearth-object of the eyepiece.

The eyepieces are interchangeable, which makes it possible to modify the characteristics of the instrument. They consist of lens S which introduce aberrations corrected more or less well according to the models. More the current is the eyepiece of Plössl today, the eyepiece of Huygens and of Ramsden made up of two lenses are abandoned today. The diameter of the eyepieces is standardized, it is thus possible indifferently to use them on any type of instrument, including with a telescope. The American standard of 1" 1/4 (31,75 mm) is most current. But eyepieces of 2" (50,8 mm) are increasingly popular for the long focal distances, in spite of their higher price.

### Mounting

Mounting is the moving part, that which makes it possible to direct the instrument. There exist three types of mountings: ; The azimuth Mounting It is the basic mounting, made up of a vertical axis and a horizontal axis. It is of a catch in easy hand but is not adapted to the prolonged observations. It is not generally used that on the telescopes of less than 60 Misters It comprises a major defect which is the rotation of the image making it unsuitable with the photographic installations.

; The equatorial Mounting The use of this mounting is made practical because of the rotation of the celestial Sphère. It makes it possible to follow the same star while making swivel the instrument on only one axis, the other being parallel to the axis of rotation of the ground. For that, it has four axes of which two make it possible to regulate, one says to put in station , mounting. Two others being used to direct the instrument according to the celestial coordinates given by the variation and the Right ascension. This mounting requires to control the bases of astronomy but it offers finally a better comfort of use (see the detailed article equatorial Monture). It is the mounting generally used on the telescopes.

; The Mounting altazimutale Like azimuth mounting, it consists of a vertical axis and a horizontal axis. But, like equatorial mounting, it allows the follow-up of a star because it is been driven by an engine on each one of its axes. It is thus controlled by a computer integrated in the telescope, or outside, with automatic positioning on a star (Function known as “Go to”) without erection, but after indication of the position of two stars at the beginning of meeting of observation. This mounting is generally used on the Schmidt-Cassegrain telescopes of 8 inches (203 mm) or more.

The most important characteristic of these last years is the increase in the electronic capacity of mountings (altazimutales and equatorial allemandes in particular): these mountings allow the guidance by a autoguidor or a Caméra of astronomy double sensor without having to use external computer. One can even control a telescope with a computer via some Logiciels of astronomy, one can also do it by Internet.

### Accessories

In addition to the elements already described and obviously essential to the use of a telescope, various accessories make it possible to widen the field of use of an instrument.

; Enquiring This sight, small generally réticulée glasses, must be correctly regulated: it must be parallel to the tube of the instrument. To check it, aim at the terrestrial object most distant possible like the roof from a house and look at if the center of the reticle corresponds to the center of the field of view of the telescope. Its goal is to facilitate the pointing towards a zone of the sky thanks to its broader field of view, which makes it possible to be located more easily among stars.

; Tripod Element whose great importance should not be neglected, it accommodates mounting and supports the instrument. For this reason, it must be adapted to support the weight of the unit. Various models available, are carried out out of aluminum or steel, all aiming at the same goal: to balance and stabilize the unit to avoid to the maximum the risks of rocker of the instrument (whatever its position) and to absorb the vibrations.

; solar Filter In general is also provided a solar filter which one screws with the eyepiece. These filters should not be used alone because they are located close to the hearth and are found confronted with an intense heat which can make them burst. Their use must imperatively be associated with a Hélioscope with Herschel which disperses heat, this accessory is on sale in the specialized shops. To specialize in the observation of the Sun, rather than a filter of eyepiece, it is preferable to use filters out of metallized glass which are placed in front of the objective, they are more expensive but surer. Moreover they offer a more stable image and make it possible to make photography without risk for the apparatus. (see Observation of the Sun)

; Reference bent It allows a more comfortable observation and avoids having recourse to not very comfortable postures during the observation, mainly towards the zenith. Its use is not necessary with a telescope of the Newton type because of its construction. The bent references can be made up of a mirror or a prism with total reflection.

; Lens of Barlow The Lentille of Barlow lengthens the focal distance to which the hearth-image is formed. It is used in complement of the eyepiece, of which it makes it possible to multiply the enlargement by the coefficient (generally 2, but also 3) which characterizes it. Those sold with the telescopes are often of bad quality. The lens of Barlow must be made up of a doublet or achromatic triplet not to deteriorate the image and its use must be held for the Moon and planets. Its interest is to avoid using the lenses of very short focal distances which are expensive and fragile (less than 5 mm). There it is mainly used to reject the resulting focal plan apart from mounting and of being able to place, since it is accessible, a photographic plate, a sensor CCC.

; Reducing of focal distance This instrument with the opposite effect of the lens of Barlow, i.e. it shortens the focal distance from the hearth-image. For that, it must be placed between the objective and the hearth. By decreasing the f/D report/ratio (see further), it increases the luminosity on all the field of vision. Its use is reserved for the Photographie with the hearth (silver or numerical) and makes it possible to decrease the exposure times or to increase contrasts.

; lunar Filter or solar Filter This filter is used at the time of the observation of the the Moon or the Sun. These two celestial objects are very luminous and can create a dazzling. One thus uses a filter which consists of two Polariseur S. While swivelling them one can modify the transparency of the filter to optimize it with the type of observation that one wishes to make.

## Characteristics and properties

### Features

; The diameter The diameter of the objective, in fact the primary education mirror, is the most important characteristic of the instrument because the majority of the optical properties of the instrument depend on it. The larger it is, the more it authorizes strong enlargements and makes it possible to observe remote star S. The diameter is generally expressed in millimetre for the instruments commercial, sometimes in inches (1" = 25,4 mm). As opposed to what think the beginners, a telescope of large diameter is not enough to make a good instrument of observation, many other conditions relative as well to quality as with stability must be held.

; Focal length It can be a question of the focal length of the primary education mirror or that of the eyepieces. The focal length of the instrument strictly speaking corresponds to that of the objective, it is expressed in millimetre or must be calculated starting from the f/D. report/ratio.

; The f/D report/ratio The focal distance ratio/diameter is the report/ratio of the focal length of the primary education mirror and its diameter, expressed of course in the same unit. A weak f/D report/ratio gives a compact instrument, therefore stable and easy to handle and transport. Nevertheless, the precision of collimation believes like (D/f) ². In other words, a telescope open to f/D=5 will be twice more difficult to collimate than a telescope open to f/D=7. A report/ratio equal to or higher than f/D=5 is very satisfactory; beyond f/D=10, the instrument has a limited field but a weak obstruction, which is favorable out of planet gear. Moreover, the eyepieces being able to be of longer focal distance, the retreat of eye and thus comfort will be better. To make Astrophotographie a f/D of 4 will be acceptable especially if one improves the field with a corrector of Ross with two lenses.

### Optical properties

; Resolution The To be able of resolution is the capacity of an optical system to reveal the details, it gains in smoothness with the diameter of the objective. The resolution measures the smallest angle separating two points which one manages to see like distinct one from the other, is approximately 1 Minute of arc for the human eye. One can extremely simply calculate it by dividing 120 by the diameter of the instrument expressed into mm, For example, a telescope 114 mms in diameter has a resolving power of approximately 1" (120/114), a telescope of 200 mm has a resolving power of 0,6". However, atmospheric turbulences, the stability of the instrument and the quality of the objective often prevent from reaching the theoretical limit of resolution.

One can determine the size T details which an instrument by the relation can solve:

$T = \ tan \ frac P \left\{3600\right\} \ times D$
Where D is the distance from the star which one wishes to observe, and P (second of arc) resolution. For example, a telescope of 200 mm (P = 0,6"), on the Moon (D = 392000  will be able to distinguish; km), of the details of 1,14  km (T).

; The enlargement It corresponds to the relationship between the apparent diameter of the image on the outlet side of the eyepiece and the apparent diameter of the real object. It can be calculated by dividing the focal length of the primary education mirror by that of the eyepiece. The enlargement reveals additional details only insofar as it makes it possible to overcome the weak resolution of the eye. Beyond the limit of resolution of the instrument, the enlargement does not reveal any more other details but the defects of the image and induces a reduction in clearness. A contrario , a weak enlargement makes it possible to observe a broad field of the sky, which can be made profitable if the instrument has a sufficient clearness, or for the observation of the the Moon and the Sun. A weak enlargement requires an instrument of short focal distance, preferable with the use of eyepieces of long focal distance which can involve a loss of clearness (for more precise councils, to see Observation of the sky and Astrophotographie).

; Clearness Clearness increases with the diameter of the objective, it is theoretically proportional to the sectional surface of the telescope, decreased by the obstruction of the secondary mirror. One can calculate an approximate factor by dividing the square diameter of the objective to that of the Pupille (approximately 6 mm in the black). For example, if a telescope has a diameter of 114 mm, it will collect 361 times more light than the eye (1142/62). However, the luminosity of the images also depends on the enlargement, except for the stars which always provide a specific image. The diffuse stars, such as the Nebulous S or the Galaxy S, must thus be observed with instruments having a weak f/D report/ratio to be able to apply weak enlargements. The human eye is hardly any more used like direct “sensor”. The old photographic plate is replaced by electronic sensors to which the output is currently close to 100%.

## Types of telescopes

A telescope uses an optical formula which, by the form and the provision of the mirrors, seeks to obtain images of best possible quality, as well in smoothness as in luminosity, to reveal the maximum of details.

One distinguishes two type of telescopes:

• the telescope reflectors

The first uses mirrors to collect and focus the light on the eyepiece (like those of Newton type), whereas the second type is seen associating a thin lens, the correct blade, laid out in front of the tube to increase the field of vision (used in particular by the Schmidt-Cassegrain formula).

### Newton type

This type of telescope was developed by Isaac Newton. It uses a primary education mirror parabolic and a plane secondary mirror. It is the oldest assembly, it is currently used in many constructions of amateurs because of his moderate cost. In a more general way, it is the plane secondary mirror, tilted with 45°, which characterizes the assembly Newton (which can be declined on other types of telescope); it makes it possible to return the focal image to 90° of the optical axis close to the opening of the tube, which returns the position of more comfortable observation. The parabolic mirrors generate a Aberration, known as of coma; it deforms stars in edge of field, which reduces the useful field.

### Cassegrain type

He was proposed at the 17th century by the priest and Physicien French Laurent Cassegrain. It is the prototype of the systems with two mirrors concave/convex. It is composed of a parabolic concave primary education mirror and a hyperbolic convex secondary mirror. In the Cassegrain assembly, contrary to the assembly Newton, the primary education mirror is bored in its center and the observer is placed behind this one. Cassegrain present at identical opening the same coma as Newton, which will limit in theory the field of clearness. Nevertheless this type of telescope will be not very open (F/15-F/30) and in practice that will not constitute a limitation. Taking into account the primary education which is parabolic like Newton, this one could be also used in Newton if it is not too open (F/4), which makes to an instrument potentially general practitioner of it.

### Schmidt-Cassegrain type

It is an alternative of the type Cassegrain, very appreciated among the amateurs, which uses a Objectif catadioptrics. This hybrid assembly takes again the principle of the spherical primary education mirror by associating it with a Lame of Schmidt to correct the aberration of sphericity. It is a general-purpose instrument and which provides luminous and clear images on almost the whole of the field. It with the disadvantage of being very expensive because of the difficulty of designing the blades of Schmidt.

### Maksutov-Cassegrain type

It is another alternative of correctly corrected Cassegrain. The primary education is concave spherical and the secondary is convex spherical, the aberration being corrected by a meniscus (a thicker concave lens on the edges). The main advantage of this type of telescope is its facility of realization by indutrial resource, because it is only made up of surfaces spherical, therefore easily realizable by machines and with homogeneous results (what is not always the case with other types of telescope).

### Telescope Ritchey-Christian

Cassegrain gives an image deprived of spherical aberration; the Ritchey - Christian invented about 1910, thanks to a primary education and a hyperbolic secondary, gives a focal image also deprived of coma. There then remains the astigmatism and the curve of field, which is cancelled if the primary and secondary curves are equal and opposite. Taking into account its qualities, it is the optical formula most used in the modern professional observatories, formula with which is generally associated a corrector with more or less complex quartz field in order to correct the residual aberrations.

The four principal telescopes 8,2 meters in diameter of the Broad Very Telescopes (VLT) use these optical criteria, just as the Space telescope Hubble.

### Telescope of Schmidt

The Schmidt camera is a photographic room of large opening designed for the Astrophotographie. It is based on a spherical primary education mirror and a deformed blade especially produced to compensate for the spherical aberration. The luminosity of the catches is exceptional thanks to a very weak f/D report/ratio (approximately f/2).

His report/ratio of opening returns it perfectly adapted for the photograph to large field, but it is necessary to compensate for the focal image which is a portion of sphere; it was used a long time for the systematic studies of great portions of the sky. The commercial availability of sensors CCC widens its possibilities considerably. Nevertheless, its length which is equal to the radius of curvature makes him prefer today other formulas optics shorter with three mirrors, giving a plane field and not curved, allowing smaller and more economic cupolas.

### Telescope with x-rays

In addition to the visible light, telescopes also study in the x-rays. But because of the fact that the terrestrial atmosphere reflects such rays, these instruments are embarked on board satellites.

### Reflecting telescopes liquid

A very particular alternative is the liquid reflecting telescope: the rotation of a tank of mercury deforms the interface liquid-air in a paraboloid ideal at a relatively reduced cost. It allows naturally only one observation the zenith. This type of telescope was imagined as of 1850 by Ernesto Capocci, and into practice put by Henry Key in 1872. An instrument of this type with a mirror 6 m in diameter was started in 2005.

## Practical

The present section treats only astronomical telescopes.

### Purchase

Before throwing itself on the small instrument of the optician of the district, it is useful to check some points:
• First of all, to seek the instrument with the diameter more adapted according to your experiment: do not buy a too heavy instrument and too powerful to start, a good 114/900 or 150/900 for example will make the deal of the beginner. The first figure corresponds to the width in millimetre of the telescope and the second quantifies the focal distance of the instrument, which comes the length of the telescope and which indicates the installed capacity of the telescope. These indications will variront according to the type of telescope used, that is to say Cassegrain, Newton or refracting telescope.
• the sales points based on the enlargement are lures: one can obtain, in theory, any enlargement with any instrument. The true limiting factor of an instrument is generally the quantity of light which it can collect, i.e. its diameter; keep in mind which one often seeks to observe of the objects far from luminous but not necessarily very small.
• Vérifiez that mounting is stable, and that the instrument is equipped with a correct researcher and eyepieces, of Plossl type for example.
• Finally, it is always useful to get information in a club of astronomy or near one impassioned.

### Construction

The construction of an optical instrument is realizable today rather easily if one does not miss patience and of some to know to make concerning manual work. It is generally divided into two stages:
• the realization of the optical parts;
• the mechanical realization of telescope (structure, mounting, etc).

For the realization of the optical parts it is necessary to be armed with patience and perseverance, but especially to have the possibility of working in a clean room free from suspended dust.

For the implementation of the mechanical part, a little ingeniousness will be useful, and the access to tools of machining will constitute more notable.

Like any project, it is important to define it as a preliminary well and to ask council near experts, in the clubs of astronomy for example.

The development of optics is a rather long stage, which requires at least a hundred work hours to obtain a good Miroir. The time necessary depends on many parameters, inter alia diameter of the primary education mirror, f/d report/ratio, and of course of the experiment. To cut then to polish a mirror is an art and the precision is reached only after several stages: roughing and cutting with abrasives, polishing, and finally deformations and repeated tests of optics. It is however possible and even current to obtain optical parts of a quality much higher (lambda/15 on the wave instead of lambda/5 classically) than that which one can find in the trade of the Astronomie amateur and that for an often lower price.

The instrument generally produced by the amateurs is the telescope of Newton, which requires to cut then to polish a parabolic primary education mirror. The current and reasonable diameters lie between 150 and 300 mm with a f/d report/ratio higher than 5. More exotic optical formulas can be carried out when a certain experiment is had.

Jean Texereau studied these techniques through several articles published in the review Astronomy as from 1939. Its work gave access to instruments differently out of carried astronomers amateurs of the time. Its book the construction of the telescope of amateur is universally regarded as a reference book. Today the commission of the instruments of the astronomical Société of France, from which he was the Secretary a long time, continues its work by accommodating any astronomer amateur eager to produce his own mirror of telescope to the workshop of optics of the Sorbonne in Paris.