The purpose of a telescope is not to magnify, as commonly thought, but to collect light. The larger the telescope's
main light-collecting element, whether lens or mirror, the more light is collected. Importantly, it is the total amount of
light collected that ultimately determines the level of detailin a distant landscape or in the rings of Saturnvisible
through the telescope. Although magnification, or power, is useful, it has no
inherent effect whatever in determining the level of detail visible through a telescope.
The planet Saturn. This image shows Saturn approximately as it appears under good atmospheric conditions through the Meate ETX telescope at a power of 100x.
Example: Two telescopes, one with a main lens of 2" diameter (or aperture) and one with a main lens of 4" diameter
are focused on the planet Jupiter. Both telescopes are set to use a power of 100 times (written as 100X). In the 2"
telescope Jupiter's largest cloud belts are clearly observable; but in the 4" telescope the same cloud belts are seen to
take on added structure and color, and smaller cloud belts are now visible that could not be discerned in the smaller
instrument. It is the larger telescope's advantage in light-collecting capability that permits it to present more detail,
more information, to the eye than is possible through the smaller telescope, irrespective of the powers employed on
All telescopes fall into one of three optical classes. The relative advantages of each of these
telescope designs will be made clear below.
In the refracting telescope (a) light is collected by a 2-element objective lens and brought to a focus at F. By contrast the reflecting telescope (b) uses a concave mirror for this purpose. The mirror-lens, or catadioptric, telescope (c) employes a combination of both mirrors and lenses, resulting in a shorter, more portable optical tube assembly. All telescopes use an eyepiece (located behind the focal point, F) to magnify the image formed by the primary optical system.
Refracting Telescopes use a large objective lens as their primary light-collecting element. Meade refractors, in all
models and apertures, include achromatic (2-element) objective lenses, in order to reduce or virtually eliminate the
false color (chromatic aberration) that results in the telescopic image when light passes through a lens. Example:
Meade Model 230.
Reflecting Telescopes use a concave primary mirror to collect light and form an image. In the Newtonian type of
reflector, light is reflected by a small, flat secondary mirror to the side of the main tube for observation of the image.
Example: Meade Model 4500.
Mirror-Lens (Catadioptric) Telescopes employ both mirrors and lenses, resulting in optical configurations that
achieve remarkable image quality and resolution, while housing the optics in extremely short, highly portable optical
tubes. Example: Meade ETX.
With the telescope's primary optics (objective lens, primary mirror, or a combination of lenses and
mirrors) having formed an image at the telescope's focus, the purpose of the eyepiece (consisting of two or more
small lenses mounted in a metal barrel) is to magnify this image. Eyepieces are available in a wide range of optical
configurations, barrel diameters, and focal lengths. It is the focal length of the eyepiece, in conjunction with the focal
length of the main telescope, that determines the operating power of the eyepiece. (See How to Calculate Power)
Eyepieces of varying focal lengths are used to obtain different powers.
Eyepieces are typically available in focal lengths between 4mm (high-power) and 40mm (low-power). Note that an
eyepiece's optical type (MA: Modified Achromatic; PL: Plössl; SP: Super Plössl, etc.) has no effect on power, but
does affect such characteristics as the field diameter seen through the telescope, color correction of the image, as well
as image sharpness.
The Barlow Lens: Inserted into the telescope in front of the eyepiece, the Barlow lens effectively multiplies the focal
length of the main telescope. A 2X Barlow lens doubles the main telescope's effective focal length, thereby doubling
the power of each eyepiece used with the Barlow.
A 2X Barlow lens doubles the power of every eyepiece with which it is used.
Diagonal Mirrors, Erecting Prisms, and Viewfinders: A variety of telescope accessories are either supplied as
standard equipment or available optionally, depending on the telescope model.
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. All Meade refractors and mirror-lens telescopes include
a diagonal mirror or prism for this purpose. Examples: Model 230, and Model 203SC/500.
The diagonal mirror (a) diverts light to a 90 degree angle for comfortable observing; the wide-field viewfinder (b) facilitates object - location.
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. Meade 45° erecting prisms enable this correct image orientation and also result in
a comfortable 45° observing angle.
The 45° Erecting Prism (shown here attached to the Meade ETX Astro Telescope) results in a correctly-oriented image for land observing.
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.
Example: Meade Model 390.
The Meade Model 390 includes an altazimuth mount, for horizontal and verticle tracking.
With the equatorial mount of the Meade Model 395, tracking astronomical objects is simplified.
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.
These motor drives are available for most Meade equatorially mounted telescopes. Example: Meade Model 4500.
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.
The 8" LX200's computer automatically locates over 64,000 celestial objects.
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.
|Resolution, Resolving Power, and Diffraction Images|
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.
Diffraction image of a star: at high power the image of a starpoint appears in even a perfect telescope as a disc ( the Airy disc), surronded by faint rings.
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.
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. 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.
Resolving power is the ability of a telescope to separate two closely-located starpoints.