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Refractors
Spotting scopes
Galilean telescope
Keplerian telescopes
Newtonians
Cassegrain

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Refractor telescopes

All refracting telescopes use the same principles. The combination of an objective lenses and some type of eyepiece is used to gathered more light than the human eye could collect on its own, focus it, and present the viewer with a brighter, clearer, and magnified virtual image. The objective in a refracting telescope refracts or bends light.

The refractor type of design is the most popular telescope in the world after binoculars. The reason for its popularity is easy to understand, they are rugged and maintain alignment better than almost any other telescope and they are adaptable for all sorts of activities such as sighting scopes, spotting scopes, terrestrial viewing, and astronomical viewing.

One supposed advantage of a refractor over reflectors is the lack on a central obstruction in the light path (no secondary mirror obstructing the light source). However, any loss to a secondary mirror is more than made up by the fact that large mirrors can be supported from the back side. Large lenses must rely on their rims for all their support. No successful refractor greater than 1 meter has ever be built. Large unsupported glass lenses bend under their own weight resulting in distorted images. However, most commercial refractor telescopes vary between 60mm up to 150mm.

Refractors must chose glass for its optical qualities. Unfortunately, the best optical qualities, the best strengths and the best temperature insensitivity are not found in a single type of glass.

The diameter of the objective lens determines how much light can be gathered to form an image. It is usually expressed in millimeters.
Focal ratio (or "f/ number"):
f/ = (Objective Focal Length)/(Objective Diameter).
The smaller is the focal ratio, the more concentrated is the light in the focal plane and the easier it is to see faint extended objects like nebulae.


Achromatic lens
An achromatic lens or achromat is a lens that's designed to limit the effects of chromatic and spherical aberration ('false' colors that appear around an image). Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus in the same plane.
The most common type of achromat is the achromatic doublet, which is composed of two individual lenses made from glasses with different amounts of dispersion. Usually one element is a concave lens made out of flint glass, which has relatively high dispersion, while the other, convex, element is made of crown glass, which has lower dispersion. The lens elements are mounted next to each other, typically cemented together, and shaped so that the chromatic aberration of one is counterbalanced by the chromatic aberration of the other.

Apochromatic lens
An apochromat, or apochromatic lens (apo), is a photographic or other lens that has better color correction than the much more common achromat lenses. Chromatic aberration is the phenomenon of different colors focusing at different distances from a lens. In photography, it produces soft overall images, and color fringing at high-contrast edges, like an edge between black and white.
Apochromatic refractors have objectives built with special, extra-low dispersion materials. They are designed to bring three wavelengths (typically red, green, and blue) into focus in the same plane. The residual color error (secondary spectrum) can be up to an order of magnitude less than that of an achromatic lens. Such telescopes contain elements of fluorite or special, extra-low dispersion (ED) glass in the objective and produce a very crisp image that is virtually free of chromatic aberration. Such telescopes are sold in the high-end amateur telescope market. Apochromatic refractors are available with objectives of up to 5 m in diameter, but most are between 70 and 150 mm.


Lens Coatings
The optical elements of the binocular are coated to reduce internal light loss and glare, which in turn ensures even light transmission, resulting in greater image sharpness and contrast. Choosing a binocular or telescope with good lens coatings will translate to greater satisfaction with the product you ultimately select. Lens coatings range in quality as follows: coated -- fully coated -- multicoated -- fully multicoated. Coated lenses are the lowest quality and basically will not result in a product that will satisfy you. Fully coated lenses are quite economical and can work well for you, depending on your needs. Multicoated lenses are very good choices. Fully multicoated lenses give the best light transmission and brightest images, and are therefore the most desirable.

Chromatic aberriation
<-Chromatic aberration. This happens due to an optical phenomenon called chromatic aberration. Refractive index of a glass is sensitive to wavelength of the light. This means that all colors do not focus on the same plane. Violets and Reds, which are on the edge of visible spectrum would get focused at different plane than others and hence create these colored edges (also called fringe colouration).

Collimating a refractor
Read here to perfectly collimate your refractor

Spotting scopes

Identical to a refractor type telescope with achromatic or apochromatic lenses but with a build-in diagonal or prism for upright images and mounted like a basic telescope (you could call it an improved binocular). A spotting scope is a portable telescope, optimized for the observation of terrestrial objects.
Mostly mounted in a ball and socket: It has a ball shaped end that can rotate freely in the socket mount.
The magnification of a spotting scope is typically on the order of 20X to 75X. Other common features include:

Galilean telescope


This is what you usually think of as a telescope: it has a lens at one end, and another on the other side where you look straight through. This is sometimes called as a "Galilean" telescope, as it is of the same design that Galileo used (although a Galilean telescope is a specific kind of refractor, one with a simple convex objective lens (like from a magnifying glass) and a simple concave eye lens.

Although the relative small opening of this type of telescope lets in little light compared to a larger apeture, there are advantages to having this small opening. For the hand-ground plain glass lenses of Galileo’s time, the center portion of the lenses were often excellent, as good as typical inexpensive glass lenses of today, but the lenses of his time were of much poorer quality toward the edges. So, he was using only the high quality center portion of his lenses. Therefor a field-stop (aperture stop) can do miracles in improving the image). Also, the lenses of his time as well as most inexpensive lenses of today are spherically ground. That is, the curve ground in has a constant radius. Ideally, a parabolic shape would be used, and that can be found today by spending sufficiently large amounts of money. Most reasonably priced lenses even today are spherically ground. The spherical curve leads to an optical defect called spherical aberration. It is unavoidable for this type of lens shape. The longer the focal length and the smaller the clear aperture that lets in light, the less of an effect the spherical aberration will have.

Keplerian telescope

The Keplerian Telescope, invented by Johannes Kepler in 1611, is an improvement on Galileo's design. It uses a convex lens as the eyepiece instead of Galileo's concave one. The advantage of this arrangement is the rays of light emerging from the eyepiece are converging. This allows for a much wider field of view and greater eye relief but the image for the viewer is inverted. Considerably higher magnifications can be reached with this design but to overcome aberrations the simple objective lens needs to have a very high f-ratio (Johannes Hevelius built one with a 45 m (150 ft) focal length). The design also allows for use of a micrometer at the focal plane (used to determining the angular size and/or distance between objects observed).
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Reflector telescopes

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Also called the Newtonian telescope and it's the first and the simplest of the reflecting telescopes (1668). Sir Isaac Newton was surely one of the greatest physicist and to this day and the greatest Euclidean geometer of all times. It was during his mathematical studies when he realized that a parabolic surface would form a focused image at least as well as a lens. To prove his point, he formed a small reflecting telescope out of a tin/copper alloy.

Speculum (tin/copper alloy) takes a very good shine and works well as a mirror. Unfortunately, since it is largely silver, it tarnishes easily. After re-polishing a few times the carefully manufactured surface is eroded degrading the telescope. It wasn't until later you could make the mirror out of glass and deposit a very thin layer highly polished aluminium. Aluminum is almost as shiny as silver. In the violet end of the spectrum aluminum actually outshines silver. It combines very readily with other chemicals to form almost totally inert molecules.
Today most telescopes use aluminum as the reflective metal. Pure aluminum when it "tarnishes" forms aluminum oxide, a totally transparent and very tough surface coating. The first layer of aluminum atoms is oxidized protecting the shiny layers below.

Mirrors eliminate the risk of chromatic aberration but may still produce other types of aberrations: In general, on axis they may produce spherical aberration, in which case the outer and inner zones of the telescope do not share a common focus. This was the construction flaw in the Hubble Space Telescope mirrors. Spherical aberration can be eliminated with aspheric (non-spherical) mirrors.

The Newtonian scope has been a favorite of amateur astronomers for more than a century. Easy to construct, simple to operate and less costly than any of its brethren, the Newtonian is a winner. The two most popular forms of the Newtonian are the Dobsonian and bowling ball mount designs. Essentially the same, these mounts keep the Newtonian low to the ground and stable.
Another reason why most large telescopes are reflectors is the cost of production. Mirrors are cheaper to manufacture than lenses. Mirrors need only one surface formed rather than four surfaces (all sides of the achromatic lens). The glass can be translucent or even opaque. In fact, some reflectors were made entirely of metal.

The biggest drawback to the Newtonian design is the placement of the eyepiece. In smaller scopes, this isn't much of a problem, but in larger scopes, the eyepiece may wander all over, requiring a step ladder and contortions.

Two lesser drawbacks are vibration and collimation. Newtonian have the eyepiece or instrument package at the front at the end of a long lever arm. Heavy counter weighting is required at the back to balance these tools. On a regular basis, and surely when the reflector has been moved around, the telescope needs to be collimated to ensure its optimum performance.


Read here about perfect collimation or there

 

Cassegrain telescopes

Is a type of reflecting telescope with a folded optical path achieved by two mirrors – a large concave paraboloidal primary with a central hole and a small hyperboloidal convex mirror mounted on the large front corrector plate. Light strikes the primary mirror, which reflects the image back to the smaller convex secondary mirror, which in turn reflects the magnified image through the center hole and on to the eyepiece. The design was conceived in about 1672 by the Frenchman Guillaume Cassegrain. This type of scope does not have a corrector plate/lens. The advantage of this design is not in the least the portability because it can be made relative small.
Classic Cassegrain

Modified Cassegrain

In an alternate scheme, called a modified Cassegrain, a small optical flat placed immediately in front of the primary brings the light out to the side of the telescope tube and eliminates the need for a perforated primary. Like the Gregorian and Newtonian telescopes, the paraboloid-hyperboloid combination of the Cassegrain is free from spherical aberration.

The Dall-Kirkham telescope is a variant of the Cassegrain that uses a concave ellipsoidal primary mirror and a convex spherical secondary. Invented independently by the English amateur telescope-maker Horace Dall (1901-1986) and the American Alan Kirkham, it is particularly suited to planetary observation where good resolution is more important than wide field of view.

Magnification
The rule used by amateur astronomers is 50X per inch of aperture or 2 X objective aperture in mm. This is for quality optics, as the optical quality goes down so the ability for the scope to give clear images at higher powers. So with this information we can conclude that the highest power a 60mm refractor from a department store can resolve is 125X.

What kind of eyepieces to use
Look for a low power eyepiece to view nebula, open star clusters, and galaxies through. This should be in the 35X to 50X range. The next one you will want is an eyepiece that is in the 75X to 100X range. This would be to look at details in globular clusters, small parts of nebula. Then eyepieces that will give you 50X par inch aperture. This is the high power eyepiece used for planetary viewing when the conditions are correct.


More info on eyepieces found here: eyepieces

Mount
The two basic types are alt-azimuth and equatorial. Alt-azimuth mounts are simpler, easy to use, and cheaper than equatorial mounts. You set the horizontal and vertical coordinates of the object when sighting it, and then lock it in. You must readjust the horizontal and vertical coordinates as the object moves out of the field of view due to the Earth's rotation.
Horizontal mount or Dobsonian mount, which has one axis pointing at the zenith, controlling the azimuth angle, and another axis horizontal, controlling the elevation angle. Such a mount requires computer-controlled motor adjustment, since the rotation axes of the mount and Earth differ. A Dobsonian mount for photography is not useful, since the star field also rotates as the mount tracks the stars. Therefore, you'd need three computer-controlled motors as opposed to just one for the equatorial mount.
Equatorial Mount (EQ): The equatorial mount also has two perpendicular axes of rotation -- right ascension and declination. However, instead of being oriented up and down, it is tilted at the same angle as the Earth's axis of rotation. When properly aligned with the Earth's poles, equatorial mounts can allow the telescope to follow the smooth, arc-like motion of a star across the sky. Also, they can be equipped with:
- setting circles - allow you to easily locate a star by its celestial coordinates (right ascension, declination)
- motorized drives - allow you or your computer to continuously drive the telescope to track a star.
The two axes of the GEM are known as Right Ascension (RA) and Declination (DEC). When the mount is polar aligned, moving the telescope in RA is all that's necessary to track a celestial object. Movement in both axes will likely be required to place an object in the eyepiece of the telescope, but once found, movement in RA alone will keep the object in view.

Note: You do need an equatorial mount for astrophotography.
The World’s Easiest Equatorial Mount Instruction Manual for Setting Up Telescopes/Reflectors
More in detail explanations of setting up a EQ mount

 

 

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