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Telescopes
This tutorial serves as an introduction to telescopes and points to consider when choosing a telescope.

Types of Telescopes

There are 3 main types of scopes commonly used: refractors, reflectors and catadioptrics.

Refractor - The layman's idea of a telescope, it uses a lens to gather light.

Pros

  • Highest quality images for a given aperture assuming all other things equal
  • Once collimated, they are very hard to knock out of alignment again
  • Shorttubes can be very portable and offer rich-field views
  • Easy to use and their closed tube design prevent "accidents"

    Cons

  • At apertures above 4", the scope is liable to be long and cumbersome
  • Scopes cost a lot more for the same size aperture compared to reflectors
  • Achromatic scopes demonstrate chromatic abberration (false colour) when viewing bright objects
  • Apochromatic scopes (APO) don't show chromatic abberration but cost alot more.

    Reflectors - Newtonian reflectors (named for their inventor Isaac Newton) use mirrors to gather light.

    Pros

  • Cheapest of all designs for a given aperture
  • Dobsonians (dobs) which are reflectors on a special altitude-azimuth mount, are known for their ease of use, stability, affordability and can also be built at home by ATMs (amateur telescope makers)
  • Free from chromatic abberration

    Cons

  • Need to be collimated more often. Frequency depends on the quality of workmanship.
  • Open tube design more liable to let dirt and dust in
  • Central obstruction (varies in size) reduces contrast of image a little

    Catadioptric Scopes - These hybrid scopes are a combination of refractors and reflectors as they use both lenses and mirrors. Popular examples would be the Schmidt-Cassegrain (SCT, pictured below) and Maksutov-Cassegrain (Mak) designs.

    Pros

  • Compact and portable as the "light path" is folded resulting in a stubby tube
  • Well-adapted for astrophotography
  • Some catadioptric scopes' designs have closed tubes

    Cons

  • Central obstruction
  • High cost setups

    Note:Astrophotography is an expensive word and requires a lot of money for sophisticated equipment and while you can get good pictures with experience (and money), don't expect anything Hubblesque.

    Aperture vs. Power

    Aperture is the most important factor in a telescope. The aperture (diameter of the objective) determines how much light is gathered and thus how much detail the image has. By using different eyepieces, you can vary the magnification. There is a limmit to the power you telescope can handle before the image degrades badly. A rough estimate is 50x per inch depending on quality and viewing conditions.

    Avoid "toy" telescopes advertised by their power, for eg. 600x. They are often sold with weak parts, wobbly mounts, poor optics etc.

    A variety of powers is ideal. Lower powers are for finding your way around and large objects. Medium power resolves clusters and high powers reveal details on the planets. However, at higher powers, vibrations are more pronounced and it is hard to centre your target.

    Portability

    Buy the biggest aperture you can afford AND carry/transport.

    Telescope Mounts

    The telescope mount usually accounts for about half the cost of a telescope system. The best optics in the world are useless without a good, sturdy mount. The two basic types of mounts are the altazimuth and the equatorial.

    The altazimuth (alt-az) mount has two motions, altitude (vertical) and azimuth (horizontal). They are easier to use and cheaper but to keep objects in view, you must constantly adjust the mount. Such mounts are thus not suitable for astrophotography.

    A special type of alt-az mount is very popular for reflectors. If well-made, it moves smoothly with nudges, yet stays firmly on target. Such setups are commonly referred to as Dobsonians (dobs).

    The equatorial mount allows you to add a motor to track objects automatically. Such mounts are much more expensive, heavier and more complicated to use.

    Eyepieces & Barlows

    Eyepieces/barlows come in 3 sizes - .965", 1.25" or 2" eyepieces. Your scope can only accept one size unless an additional adapter is used. 1.25" is the standard now and .965" should be avoided (unless you have serious budget constraints). 2" eyepieces cost a lot more than 1.25" eps. Most eps have a focal length in mm printed on like 25mm. By dividing the focal length of the scope by this figure, you get the magnification.

    Eps have different designs and different price ranges. Budget eyepieces would be Kellners. These are the lowest quality you can accept for a start. Popular designs are Plossls and Orthoscopics which are a step up in quality at reasonable prices.

    Barlows allow you to increase your range of available powers. For example, with a 2x barlow, your eyepiece which normally gives 30x can also give 60x.

    Finder Scopes

    A finder is usually a small, low power refractor which is attached and aligned with the main scope. If aligned properly, targets centred in the finder can be seen under higher magnification in the main scope. Common finders are 6x30 and 8x50.

    Some people find the new reflex finders like the Telrad much easier to use than finder scopes. However, they work best in dark skies where there are already enough stars visible to the naked eye to act as "guideposts".