|
A telescope is an optical instrument that brings distant objects closer for examination. It usually consists of a tube containing the optics and a mounting with a tripod to point it at it's target.
There are two applications for a telescope: spotting terrestrial objects during the day, (generally using one of a Refractor design) and studying astronomical objects at night. Since the astronomical telescope is used mainly at night to observe faint objects, it is advantageous to have a larger diameter objective to ensure greater light transmission. The types of TelescopesThe Newtonian ReflectorThis is the most common type of reflector telescope using a concave objective mirror (primary mirror) at the back of the tube and a secondary mirror, which directs the light to the eyepiece. Orientation of the image is not important for astronomy purposes, so the image is left uncorrected (i.e. upside down) The Newtonian reflector represents excellent value for money, as typically they offer more aperture for a given price, than other types of telescope.The RefractorRefracting telescopes have an objective lens at the front of the tube. The light exits out through the back of the tube to the eyepiece. Since many observations are made high in the sky, a right-angled diagonal can be used to avoid neckstrain. This also provides an upright image, making them suitable for terrestrial observations.The CatadioptricTelescopes using a combination of both mirrors and lenses are called catadioptrics. There are many different designs. Usually a full aperture lens is used to correct aberrations in a compound reflecting telescope. The corrector lens also increases the performance of the instrument as air currents are eliminated. A main advantage of a catadioptric system is a relatively long focal length in a shorter, more compact tube, allowing for greater portability.The Cassegrain ReflectorThis type of telescope uses two mirrors with no correcting lens element. Normally a concave objective (primary) mirror reflects light to a small secondary mirror, which then reflects light back through a hole in the objective to the eyepiece.Telescope MountingsThe Alt-Azimuth mounting is a simple system that moves in two directions - sideways and up and down - and is sometimes provided with slow motion cable controls to accurately point the telescope. This type of mounting is ideal for terrestrial observations and can also be of use for basic astronomical observations.The Equatorial Mount is designed to correct the rotation of the Earth with one motion during astronomical observations. The Earth's rotation causes objects to rise in the East, follow a circular path across the sky and set in the West. When using an astronomical telescope at high power, it will only take approximately 30 seconds for the object to move out of the field of view. The Equatorial mounting allows the telescope to track the objects and keep the image in the centre of the eyepiece. When the equatorial mount is correctly aligned, tracking can be achieved either by turning a gear manually or with a motor drive, that tracks objects automatically. |