About Telescopes
Ever wondered what is difference between Newtonian and a Dobsonian telescopes? What advantage does one have over the other? Which is right for you? Here we will try to demystify the large diversity of telescopes and put into simple language.
How to choose.
When selecting a telescope, consider which target objects you plan to explore as well as your level of experience. For viewing most night sky objects, you’ll want a telescope with as much aperture as possible. Keep in mind that as the size of the objective lens or mirror increases, so does the size of the telescope. So be sure to select a telescope that isn’t too heavy to manage or too complicated for you to set up – especially if portability is a necessity.
- Refractors: Refractors are less affected by atmospheric instabilities making them perfect for observing the Moon and planets. Perfect for first-time novice astronomers, small refractors are lightweight, portable and require very little maintenance. And, if you’re interested in viewing both land and celestial objects, refractors are equipped for both uses.
- Reflectors: Reflectors feature larger apertures for a wide range of viewing at an affordable price. Designed with the eyepiece located at the top of the tube, reflectors are more comfortable to use for viewing night-sky objects such as nebulae, the Moon, planets and galaxies. Reflectors tend to be heavier and larger than refractors.
- Eyepieces: Lower power eyepieces provide a wider field-of-view and a brighter image making them ideal for viewing the full Moon and planets, star clusters, nebulae and the constellations. To focus in on the finer details of the Moon and planets such as mountains, ridges and craters, use an eyepiece with higher magnification.
- Mounts: There are two basic types of telescope mounts, the altitude-azimuth (or altaz) and the equatorial. Altaz mounts are the easiest to maneuver and are ideal for first-time astronomers. Equipped with a manual control or motor drive that allows you to follow the movement of objects across the night sky, equatorial mounts are ideal for more experienced astronomers.
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Basic Workings
A telescope samples a small area of view, a field, and then magnifies it so that distant objects appear larger. Parallel light rays entering the telescope are focused to a single point, called the focus or focal point. These focused rays are then magnified with a very powerful lens, or more commonly a set of lenses, called an eyepiece, to give enlarged views of distant objects. The eyepiece acts in the reverse direction to the telescope lens, taking the focused rays and sending them to the eye as parallel rays.
Refractor Telescopes
There are three basic ways to bring light rays to a focal point. The earliest method used by telescope makers, was to bend the rays by passing them through one or more pieces of glass which had curved, polished surfaces. This method produces a type of telescope called a refractor. The refracting telescope design was originally used in spy glasses and astronomical telescopes but is also used in other devices such as binoculars and long or telephoto camera lenses.
Refractors have several advantages over other designs. They are enclosed so that dust and moisture doesn’t enter the optical tube.
They have fixed optics so that they don’t require routine collimation, which means that the optics don’t have to be aligned by the user. They do not have a central obstruction, which reduces the amount of light entering the tube and causes an alteration of the diffraction pattern. The resulting high-contrast, fine-resolution images produced are considered ideal for planetary viewing. A problem with refractors is that since many wavelengths of light are passing through glass, the uneven bending of the rays causes false colour, around bright objects. This must be counteracted with additional lenses and special glass. Since at least four lens surfaces usually have to be very accurately shaped, polished and coated, they are more expensive to produce than other telescope designs. An example of a modern day refractor is the Sky-watcher Black Diamond ED80mm Apo Refractor
Maksutov-Cassegrain Telescopes
The design is named after Russian optician Dmitri Maksutov who is credited with inventing it around 1941 to 1944 The Maksutov-Cassegrain telescopes design provides high powers and a narrower field of view perfect for lunar and planetary viewing and photography, other advantages are that they’re completely sealed from the environment making them quite robust and portable, see Sky-Watcher Maksutov-Cassegrains telescopes
The Maksutov-Cassegrain design provides inverted-image view, so they are not suitable for daytime land viewing.
Sky-Watcher 127mm Maksutov Cassegrain with Goto Pro5-mount
Dobsonian Telescopes
Dobsonian Telescopes are a Newtonian optical tube assembly (O.T.A) on a simple mount made popular by John Dobson in the 1960s. Dobson had ease of use and deep sky 
observing in mind with this design because of its light gathering power and simple mounting system. The advantage of Dobsonian telescopes are their low price vs light gathering ability. It is a very simple design and quite portable, ideal for the novice or first time user.
Skywatcher 10 Inch Dobsonian telescope
A variation of the Dobsonian telescopes is the collapsible Dobsonian 
telescope which requires no reassembly for the ultimate in convenience. They are excellent for transporting to your favorite observing site because the tube assembly collapses down in size, which is also an attractive storage feature!
Skywatcher Collapsible Dobsonian Telescope 12inch SWFLEXDOB12
Dobsonian Telescope design provides inverted-image view, so they are not suitable for daytime land viewing.
Newtonian telescope
As the name suggests this design was created by Sir Isaac Newton and comprises of two mirrors the concave primary mirror and the flat diagonal secondary mirror.
The advantages of the Newtonian telescopes design is cost, they are usually less expensive than other comparable quality telescopes because the light source does not pass through expensive objective lenses. Light is reflected from the concave primary mirror to the secondary mirror then through the eye piece (The only lenes in the telescope) They are much simpler in design, Newtonian telescopes have no lenses or corrector plates to cause chromatic aberration 
like refractor telescopes and are usually a little more portable. Newtonian telescopes can suffer from coma, put simply its sharp in the center and not so sharp around the edges Newtonian telescopes can also be less contrasty and suffer more from the knocks and bumps that occur from transportation of the scope. It’s necessary to collimate your Newtonian’s mirror from time to time but it is a simple exercise. Sky-Watcher Newtonian type Telescopes are supplied with instructions on how to to this.
Skywatcher SW135 Reflector Telescope
Glossary of Terms for Telescopes
- Alt-azimuth Mount: This is a simple, sturdy mount which can be adjusted up-and-down or side-by-side to aim and hold the telescope on a distant object.
- Barlow Lens: A lens placed before the eyepiece to increase power of the eyepiece. 2x Barlow doubles, 3x triplets, etc.
- Clock Drive: Used with equatorial mounts, it is a special motor, which drives the mounts gearing so that the telescope will be synchronized with celestial motion at the earth’s rotation.
- Equatorial Mount: This is a more sophisticated, versatile mount which can be adjusted 360-degrees to facilitate viewing a moving object across the night sky. Alt-azimuth Mount: Can be adjusted both side-to-side (parallel to the horizon) as well as up-and-down.
- Erecting Eyepiece: An erecting prism or lens system inserted between the objective and eyepiece lenses to invert upside-down images for terrestrial viewing.
- Eyepiece or Ocular Lens: The lens closet to the eye. The eyepiece forms the light transmitted form the objective lens into a visible image.
- Fixed: Permanently attached and not intended to be removed (also occurs in microscopes).
- Focal Length: Focal length is the distance between the point where incoming light meets the first optical element and the point where it passes through the eyepiece to create a magnified image. Focal length determines the telescope’s magnification and the longer the focal length, the more powerful the telescope.
- Interchangeable: Refers to multiple eyepieces that attach to the telescope and provide varying magnifications.
- Finderscope: A small, low power scope attached to the telescope and aligned to its optical axis. Facilitates locating specific celestial objects.
- Moon Filter: Enhances contrast and removes glare when observing the moon.
- Objective Lens: The primary light-gathering element of a telescope, located at the opposite end of the eyepiece. May be a mirror or a lens.
- Power: The power, or magnification rating, is the result of a mathematical calculation which divides the focal length of the telescope with the focal length of the eyepiece to determine how much closer the telescope makes distant objects appear. A 200 power telescope makes a distant planet appear 200 times closer – or 200 times larger than it really is.
- Refractor Telescopes: A refractor telescope collects light though a large objective lens and directs it through the telescope to the eyepiece for magnification. Invented by the famous astronomer Galileo, refractor telescopes are noted for bright, high-quality images with excellent resolution.
- Reflector Telescopes: Reflectors use a large concave mirror to collect and focus light back to a diagonal mirror which redirects the light to the eyepiece for magnification. Reflectors deliver the most magnification in a compact design because they achieve a longer focal length through the use of mirrors.
- Solar Projection Screen: Because observing the sun can be harmful to the eye, a white screen, positioned directly in line with the eyepiece, is used to capture the projected image of the sun for solar observation.
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