Types

The role of a telescope is to increase the apparent size and the brightness of celestial objects, some of which are hardly noticeable or even invisible to the naked eye.

Components

1. Objective (or optical tube) ] It consists of a tube, generally made of aluminum or steel, containing lenses and/or mirrors. Their role is to focus the collected light towards a point, the focal point. The inside of the tube is black to avoid reflection effect. Alone, the objective does not give directly usable image by the eye. For this, a second element is necessary: the eyepiece.

2. Eyepiece (see “Eyepieces“)

3. Mount ] It is an element whose importance should not be underestimated. It supports the instrument and can make it point to the celestial objects. The mount is usually attached to a tripod (except for the Dobson) that allows you to adjust the height of the telescope and ensures its stability. It must be adapted to the weight of the instrument to avoid as much risk of tipping the instrument and vibrations.

Optical Principles

The telescopes are sometimes classified based on optical components they contain: refractor telescope (lentils), reflecting telescope (mirrors), catadioptric telescopes (lens + mirrors). But we think it’s better to be distinguished according to their way of focusing light. there exist three main optical systems:

1. Refracting telescope
2. Newtonian telescope
3. Cassegrain telescope

A distinction is made between the telescope in French (based on the principles of reflection) and telescope (based solely on principles of refraction), unlike English where they respectively referred to as “reflecting coating telescope” and “refracting telescope.”

REFRACTING TELESCOPE

► Design

A refracting telescope (or refractor telescope) consists of an optical tube to the input of which is a main lens which collects the light and focuses it towards the eyepiece. This restores the parallelism of light rays and enlarges the visible image.

The main drawback simple telescopes, that is to say having a single main lens is chromaticism, which focuses the different colors at different points. Terraced lens systems allow to partially correct this optical aberration:

The achromatic doublet is a lens doublet designed to limit the effects of chromatic and spherical aberrations. The achromat corrects the focal lengths of the two light beams of wavelengths (red and blue) to better converge to the same focal point. It also corrects spherical aberrations on a single wavelength. Compared to a single telescope of the same diameter, it offers a real increase in quality in the images. For the residual defects remain acceptable, the focal length must be high (F / D ratio equal or greater than 10).
The apochromatic triplet (or apochromat) is an assembly of three lenses. While achromat limited to the correction of focal lengths of two beams of different wavelengths, the apochromat enables it to converge blue, red and green on the same plan. The residual color dispersion is much lower than that produced by an achromat, aperture and focal distance equal. The apochromat also corrects spherical aberrations on two wavelengths.

There is also an optical formula called superapochromat, which is an improved version of apochromatic triplet lens with which the polishing quality is high. Such instruments may approach perfection but are very expensive.

► Benefits

1. Robust, require no maintenance to preserve the alignment of optical components.
2. sealed, thus more stable to temperature changes than other types of telescopes.
3. Very little maintenance (cleaning) due to the seal.
4. Easy to use, because it requires very few settings (for the kids).

► Disadvantages

1. More than a Newton or Cassegrain telescope of the same diameter.
2. Heavier and long telescope a reflector or catadioptric having the same diameter.
3. Prone to chromatic aberration. (Unless apochromatic)

► Area of Application

The refractor telescope generally suitable for planetary observation (Moon and planets) to those of terrestrial landscapes and less than deep sky (nebulae and galaxies, star clusters, …) because of its often too small field and its relatively low opening. The apochromatic premium eyewear are well suited to astrophotography.

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NEWTONIAN TELESCOPE

► Design
Newtonian Telescope is open composed of an optical tube in which are located the parabolic concave primary mirror that collects light and returns to a secondary plane mirror 45 ° much smaller, suspended in front of the tube by a fasteners called Spider system. Since secondary mirror, the image is reflected at a right angle to the eyepiece, located on the side of the tube, where the image will be enlarged. Parabolic mirrors generate optical aberration called coma, which deforms the star field edge by providing a form that resembles a comet, and thus reduces the usable field.

► Benefits

1. Save Money on the same diameter refractor telescope.
2. Bright, as has a large diameter relative to the length of the instrument (small focal ratio)
3. More light because the tube is open. The light is not partially restrained by a lens or a glass plate at the entrance of the tube.
3. Comfort of observation through the eyepiece location on the side of the tube.
4. No chromatic aberration.

► Disadvantages

1. Open Goal that requires regular alignment of its optical components (collimation).
2. The more fragile than a refractor or a catadioptric telescope.
3. Offers perfect picture on the axis (center field), but is subject to coma soon as you move away from it. This aberration is inherent in the very principle of Newton, is even stronger than the opening of the instrument is great.
5. Last temperature long enough tube.
6. Sensitive to turbulence.
7. Evil suitable for earth observation.

► Area of Application

Newton telescopes generally suitable for observing deep sky.

VARIATIONS

1. Schmidt-Newton

Same combination as the Newton, with a Schmidt plate at the entrance of the lens. It reduces the off-axis coma aberration of about 40% and allows for a closed tube, which confers greater stability to air inside thereof. Apart from this improvement, it has the same advantages and disadvantages as a classical Newton.

2. Maksutov-Newton

It is also of the same combination as the Newton, but with a meniscus at the entrance of the optical tube. It is more efficient than a conventional deep sky Maksutov and very suitable for astrophotography.

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CASSEGRAIN TELESCOPE

► Design

The Cassegrain telescope consists of a parabolic concave primary mirror and a convex hyperbolic secondary mirror. The primary mirror is always pierced at its center and the eye is located behind it. Light enters the tube, is reflected first on the primary mirror and the secondary mirror, and finally converge in the drilling of the primary mirror towards the eyepiece.

► Advantages

1. Versatile enough.
2. More compact than a telescope or a Newton with its mirrors (long focal length in a short tube). the length of the tube is generally less than 2 times its diameter.
3. No chromatic aberration.

► Disadvantages

1. More than a diameter of the reflector.
2. As Newton, it gives a perfect image in the axis, but it is marred by off-axis coma.
3. The field curvature is more pronounced than on the glasses and Newton.

► Area of Application

Versatile: Suitable for all types of observations of celestial and terrestrial objects.

VARIATIONS

1. Schmidt-Cassegrain

This is a variant of the Cassegrain type comprising, at the front of the tube, a glass closure blade, called Schmidt plate. This has the effect of compensating for the spherical aberration created by the primary mirror.

Advantages
1. Versatile
2. Provides bright and clear images in virtually all of the field.
3. Solid
4. Compact, easy to carry
5. The optical tube is closed, we find the same benefits with a telescope: Stable Images rare collimation, very little maintenance.

Disadvantages
1. More expensive
2. Not very bright because of the presence of the secondary mirror in the optical axis
3. coma and astigmatism as soon as one moves away from the axis.

Area of Application
Observation of planets, deep sky observation and even observation of nature.
Perfect for astrophotography.

2. Maksutov-Cassegrain

This is another variant of the Cassegrain. The Maksutov-Cassegrain has a concave spherical primary mirror, a convex spherical secondary mirror and a meniscus front corrector (concave lens thicker at the edges) which follows the same curve of the primary mirror.
The light rays passing through the meniscus is reflected on the primary mirror and the secondary convex mirror, and passes through the eyepiece.

Advantages
1. High quality image
2. Same as the Schmidt-Cassegrain
3. Allows high magnifications.
4. This telescope perfectly corrects aberrations on the axis.
5. The rear surface of the meniscus has an aluminized area to reflect light to the rear of the optical tube
6. The coma aberration is reduced

Disadvantages
1. Scope of restricted vision.
2. Setting long temperature.
3. This type of telescope is limited to small diameters, the concave lens correction becoming massive and costly beyond.
4. The meniscus is sensitive to moisture.
5. Suffering of vignetting when one moves away from the optical axis.

Area of Application
Perfect for astrophotography.

3. Ritchey-Chrétien

The Ritchey-Chrétien has a concave hyperbolic primary mirror, a convex hyperbolic secondary mirror, and an open tube (without closing corrector plate) in the front and even on the sides of the tube.

Advantages
1. Corrected image of the coma over the entire field.

Disadvantages
1. The open pipe has the disadvantages described above (see Newton telescope above).
2. Present astigmatism off the optical axis.

>> There are Ritchey-Chrétien telescopes with a closing corrector plate. The closed optical tube corrects astigmatism present in conventional Ritchey-Chrétien and supports the secondary mirror, which limits diffraction and distortion. In addition, the multilayer coating of the closing blade allows good light transmission. It has the disadvantage of being against often very expensive.

Area of Application
Instrument well suited for deep sky photography.

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Telescopes Specialized

SOLAR TELESCOPE

The proximity of the sun makes it very easy to observe with a suitable instrument. But beware: viewing the sun with a telescope is even more dangerous than the naked eye. Indeed, the energy concentrated at the focus of a telescope pointed at the sun can ignite a piece of paper! It is therefore necessary to use either a filter (FILTERS) which is placed on the lens telescope (never on eye) or an instrument exclusively for solar observation.

► Coronographe
telescope designed to study the sun’s corona hiding the solar disk through a screen. obscures the sun with a metal disc with the same apparent diameter as the image of the bright central part of the star (photosphere) seen through the observation instrument.

► H-alpha solar Telescope
bezel with integrated filter isolates the emission line of ionized hydrogen (h-alpha) and get rid of the glare of the photosphere. many training courses that reflect the chromosphere of the activity are visible simultaneously: tasks, rash, filaments, spicules and prominences.

► Solar telescope Ca-K
Bezel that lets this time to observe the sun in the emission line of ionized calcium (ca-k), and so view the faculae, tasks and structure “orange peel” deeper levels of chromosphere. These instruments are often more suited to photography.

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ASTROGRAPH

An astrograph is a telescope designed for astrophotography. highly unsuitable for visual observation and planetary imaging, astrographs are usually used to make wide-field surveys of the night sky. the astrographs tubes are often made of carbon fiber and covered, inside, with a black felt intended to prevent internal reflections. this construction provides a rigid tube two times lighter than a conventional tube of the same size and which offers a high thermal stability. therefore, no change in the development, nor any unwanted bending does come compromising performance, even if the temperature changes significantly. with its short focal ratio, it reduces the exposure time while getting a good signal / noise ratio. the aberrations due to newton optical system are corrected with the addition of an optional wynne corrector. in addition, it is possible to modify the focal ratio by replacing this correction with optional relays: either a reduction or a barlow lens with correction. therefore, a single optical tube offers the possibility of obtaining a polyvalent astrographe with minimal constraints.

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