• Introduction
• Types of Telescopes
• The Eyepiece
• Telescope Mountings
• Resolution, Resolving Power and Diffraction Images

• Diagonal Mirrors: When observing objects nearly overhead through refracting or mirror-lens telescopes, the diagonal mirror (or in some cases, diagonal prism) permits a comfortable observing position. The diagonal mirror diverts light out to a right-angle to the telescope's main tube.
• Viewfinders: Most telescopes have rather narrow fields of view. As a result, finding and centering an object in the telescopic field can be difficult unless a viewfinder is used. The viewfinder is a small, low-power, wide-field telescope, usually equipped with internal crosshairs for easy object-sighting. With the viewfinder aligned parallel to the main telescope, objects first located in the viewfinder are then also in the main telescope's field.
• Erecting Prisms: Astronomical telescopes image objects in an upside-down and reversed-left-for-right orientation. This orientation is of no consequence in astronomical observing, but for terrestrial observing a normal right-side-up image orientation is highly desirable.

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Telescope Mountings
Once an object, whether terrestrial or astronomical, is located and centered in the telescope's field of view, the telescope's mechanical mounting permits the observer to track, or follow, the object as it moves across the landscape or sky. Types of telescope mountings include the following:

Altazimuth Mountings. The simplest type of telescope mount allows the telescope to be moved up and down (in vertical, or altitude) and left to right (in horizontal, or azimuth). The altitude-azimuth (altazimuth) mounting thus permits the observer to follow objects by simple motions of the telescope in vertical and horizontal. Slow-motion controls, sometimes operated through flexible cables, can facilitate these motions. The altazimuth mount, owing to its simplicity and relatively lower cost, is widely used with telescopes in both land-viewing and astronomical applications.

Equatorial Mountings. Although celestial objects are essentially fixed in their positions in the sky (on the celestial sphere, the imaginary spherical surface on which all astronomical objects are located), they appear to move in an arc across the sky as the earth rotates underneath the sky once every 24 hours. From an astronomical point of view, therefore, the task of the telescope mounting is to compensate for the Earth's rotation and allow the observer to track the moon, planets and stars.

This task is made vastly easier by the equatorial mounting, the type of mounting incorporated into most larger or more advanced telescopes. By aligning one axis of the equatorial mount to the Earth's rotational axis (a simple process which involves pointing one telescope axis to the North Star), the observer can track astronomical objects by turning one control cable, instead of the two simultaneous motions required with the altazimuth mount. If a small motor is attached to the equatorial mount, this tracking can be performed automatically (called a motor drive).

Computer-Controlled Telescope Mountings. In 1992, Meade Instruments announced a revolutionary telescope mounting concept that soon became the largest-selling telescope mounting in the world among serious amateur astronomers. The Meade LX200 computer control system permits the telescope to be mounted in an altazimuth orientation, while motors, directed by an internal microprocessor, on both telescope axes follow astronomical objects with extreme precision. The LX200 system further allows the observer to input an object's catalog number or celestial position to a handheld keypad, press GO TO, and watch as the telescope automatically moves to the object and centers it in the telescope's field of view.

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Resolution, Resolving Power and Diffraction Images
These terms form a basic part of the jargon associated with optics and telescopes, a jargon that even the most novice telescope user can understand.

Resolution is a qualitative expression of how much detail can be observed through a given telescope. Telescopes are said to be of high resolution if they are manufactured to optical standards that permit a level of visible detail consistent with the aperture and optical design of the instrument

Stars (as opposed to the moon, planets or terrestrial objects, for example) are among the most difficult of objects for a telescope to image and bring to a sharp focus because stars are point sources of light: from the astronomer's point of view stars, consist of light energy packaged in an infinitesimal area, or point.

Surprisingly perhaps, the telescope forms images of stellar point sources as finite-sized discs having real diameters. In other words, although nature sends a point-size beam of light to the telescope, the observer looking through the telescope sees not a point-size image, but a tiny disc (called the Airy disc) with faint rings of light surrounding it. This telescopic image of a star, consisting of the Airy disc and its surrounding rings of light, is called the diffraction image. See the diagram at right.

The concept of the diffraction image is important because it allows the telescope user to rate the quality of the telescope's optical system.

One such rating is determined by the telescope's ability to clearly separate, or resolve, two starpoints (i.e., two Airy discs) located very close to each other. The larger a telescope's aperture, the greater its ability to show two adjacent stars as separate, distinct images, rather than as one overlapping image. This ability is called resolving power (see diagram at right). If a telescope's optical quality permits it to resolve starpoints to the theoretical limit of its aperture capabilities, then the telescope is said to be diffraction-limited.

From information provided by Meade.com.

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