Telescopes and Mounts

Telescopes are amateur astronomers tools. They come in a great variety of types. Mounts are these devices which allow a telescope's pointing and celestial objects tracking

Telescopes

Amateur astronomers telescopes are optical instruments working on a same basis. A primary light-gathering device collects light and brings it to a focus point where it is magnified for display by an eyepiece (an eyepiece is a small lens ensemble). Telescopes sort first according to what primary light-gathering device they are using: will it be a lens atop the instrument, the telescope will be called a "refractor" (as the lens is refracting the light); will the telescope use a mirror as primary optics, it will be called a "reflector" (as the mirror is reflecting the light). Due to light path, celestial objects images are most of the time reverted in some way (upside-down, left-right, etc). This is a normal characteristic of astronomical instrument. To use astronomy instrument for terrestrial purposes, you will have to use accessories correcting light path (newtonians telescopes may not be corrected this way)

All amateur telescopes are sorting based on this difference. Main amateur telescopes types are further differentiated into other categories however. These categories are more related to types which are actually found on the market, i.e. refractors, Schmidt-Cassegrain, newtonians, and Dobsonians

• refractors have a lens atop a tube. At the other extremity of the tube is found the focuser and the eyepiece. Refractors are renowned to be expensive. On another hand they are certainly the best instruments available as providing a sharp, high contrast and as they allow high magnifications. They are not good at deep-sky objects however and are best at planetary objects, Moon, and binary stars. Most simple and inexpensive models are good initiation tools
• Schmidt-Cassegrains ("Schmidt-Cassegrain Telescopes", SCTs) are catadioptric telescopes, that is a mix of refractors and reflectors: light is entering the tube through a correcting lens. It is reflected at the bottom of the instrument by a primary mirror unto a secondary which brings the light to the focuser and eyepiece through a hole in the primary. Schmidt-Cassegrains are expensive (althoug less than refractors) but are all-around, multi-purpose instruments. They are excellent for deep-sky visual observation because of very sharp images on a wide-field, and they are very good for Moon, planets, and binaries. They are the instruments better suited for deep-sky astrophotography
• Maksutov-Cassegrains are catadioptrics too but with a thicker correcting lens and a smaller secondary mirror than SCTs, they have a slight advantage about planets due to a lesser secondary mirror obstruction
• newtonians are the most basic reflectors (light enters an open tube, is reflected by a primary mirror to a secondary which sends light on the side atop the instrument) and were amateurs favourites during a long time due to their relatively low cost and their versatility. Newtonians are good a deep sky objects, good for deep-sky astrophotography, and reasonably good for planets and Moon. They are optically less good than SCTs however. Main unconvenience of newtonians is that they have to be collimated (regular alignment of the primary and the secondary mirror) and that due to their open tube they are subject to turbulence. Their coatings may degrade
• dobsonians are large newtonians. Small ones are initiation tools. These instruments are great light collectors and good at dark sky from non-light polluted locations. Their relatively low pricing is mainly due to their simple wooden mount. They cannot be motor driven, as their motion is hand-controled only

More categories may be found like Newtonian-Cassegrains (a correcting lens but a light egress like a newtonian) or more exotic species. A recent trend are short-focal/wide-field refractors (either achromat or higher-end apochromat) which are presented like good at deep-sky objects and planets at the same time (planets with a Barlow or short-focal eyepieces, as the Barlows are said to have nothing more in common with the former bad reputation of such accessories among the amateurs). The advantage of such instruments is their portability and their ease of set up. This is a new attitude compared to instruments of one or two decades ago when comet hunters were looking for such tools, or their binocular equivalent, planetary observers were preferring long-focal refractors and SCTs where the multi-purpose, high-end instruments

Mounts

Telescopes, whatever their type, are affixed onto a mount. Mount proper is the device which allows telescope motion and pointing. It is found atop of the tripod, or of the pedestal or pier. Mounts come in two flavours due to how they handle the dual-axis motion necessary to follow a celestial object moving along the sky apparent motion

• altazimutal (or altaz) mounts are equipping beginners instruments. Such mounts oblige to a dual axis motion to follow a celestial object. Due to night sky apparent motion and Earth's axis tilt, any sky object in a mid-northern or mid-southern sky is rising -or descending- at the same time it is moving East-West. As the name, the altazimutal mount necessitates an azimuth motion (East-West), and an altitude motion (high-low). Altazimutal mounts got a new interest lately as Meade or Celestron featured such mounts for their large SCTs. Such high-end altazimuths are fitted with dual-axis electric motions and tracking systems which makes them suitable for astrophotography

• equatorial (or polar, or German) mounts came before those high-end altazimuthals, and are still useful (and necessary for astrophotography). They allow too a simplier tracking as their main axis is tilted to the Polaris, thus parallel to Earth axis, instead of being vertical as the case in an altazimutal. Once an object acquired in altitude, the rotation of the main axis at the opposite of Earth's rotation is enough to follow its apparent motion. Due to the awkward configuration such equatorial mounts may attain, they feature counterweights at one end of the secondary axis. Such equatorial mounts usually may be fitted with motors on both axis. Main axis is moving at Earth's rotation rate; secondary axis is used for corrections. High-end altazimutals may be transformed into equatorials using a proprietary wedge which tilts the mount main axis.