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CigTech's Astronomy Page

Astronomy for Beginners: A Work in Progress.
Author: CigTech

How Telescopes Work:
The purpose of a telescope is to collect light. Not to magnify the image as commonly thought. The larger the telescope's main light-collecting element, whether it be lens or mirror, the more detail is collected. Importantly, it is the total amount of light collected that ultimately determines the level of detail in the object you are viewing. Although magnification (or power) is useful, it has no inherent effect what so ever in determining the level of detail visible through a telescope.

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 either instrument.

Types of Telescope: 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) employs 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. A quality 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.

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.

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.

The Eyepieces:
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, which determines the operating power of the eyepiece. (See How to Calculate Power)

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:
By inserting 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 variety of telescope accessories are either supplied as standard equipment or available optionally, depending on the telescope model.

Diagonal Mirrors: When observing an object 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.

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:

(a) a diagonal mirror (b) a viewfinder

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. A 45° erecting prisms enable this correct image orientation and also result in a comfortable 45°-observing angle.

Telescope Mounts:
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 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.

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 GOTO, and watch as the telescope automatically moves to the object and centers it in the telescope's field of view.

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.

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), surrounded 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.

Resolving power is the ability of a telescope to separate two closely-located starpoints.

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.

What You Can See Through a Telescope:
Boy, how many times have I heard this one question. The only correct answer to this question is a 6 question:
1. What type of scope do you own?
2. What is the f/stop for the scope you own?
3. What power range are you going to be using?
4. What eyepieces are you going to be using?
5. What is the Object Lens/Mirror size?
6. What do you want to see?
Most people who have never looked through a quality, moderately priced telescope have no real idea of how much can be observed. Common perceptions are that a telescope capable of showing the rings of Saturn, for example, costs "thousands of dollars," or that reading an automobile license plate from one mile requires a telescope out of a spy novel. Such perceptions could not be more wrong. The extremely wide range of celestial objects observable through amateur telescopes can be categorized into the following groups:

1. Objects in the Solar System:
The Moon, Planets and Comets: Because of their relative proximity to Earth, the Moon and major planets have long been primary sources of interest to amateur astronomers. Each of the major planets from Mercury to Neptune is visible through any telescope; only the outermost and faintest planet, Pluto, requires a telescope of 10"-aperture or larger to be seen.

2. Objects in our Milky Way Galaxy:
Star Clusters: Positioned throughout our Milky Way galaxy are various types of star clusters, including loose stellar associations (open star clusters) and tightly packed ball-shaped star groupings (globular star clusters). The Pleiades, an open cluster of 6 or 7 stars easily visible to the unaided eye, becomes a glittering sight in any telescope, with hundreds of stars now visible through the eyepiece. The Pleiades (M45)

Double and Multiple Stars: Of the 100 billion stars in our galaxy, roughly half of them are multiple stars consisting of two or more stars linked in a common gravitational field and revolving about a common center. The famous star Mizar, located at the bend of the handle of the Big Dipper, resolves into two stars of about equal brightness.

The Big Dipper is in the constellation Ursa Major.

Variable Stars: Many stars are not fixed in their brightness levels, but periodically change in brightness, some in days, others over a period of years. In the easily-located constellation of Perseus, the star B (beta) Persei, or Algol, changes brightness rapidly every three days, dimming from moderately bright to moderately faint in a period of only about four hours. Algol's fascinating variability in brightness is easily studied in any telescope model.

Nebulae: Gas clouds, called nebulae, are scattered throughout our own galaxy, the Milky Way. These clouds typically are illuminated by nearby stars. Nebular objects are further grouped into sub-types as tenuous, amorphous clouds (diffuse nebulae) and highly structured clouds of gas (planetary nebulae). Use a beginning to intermediate Meade telescope (e.g., 2.4" Refracting or 4.5” Reflector) to observe such spectacular objects as the Great Nebula in Orion, a diffuse nebula that fills the field of view at low powers. Within the Orion Nebula a grouping of four stars, called the "Trapezium" because of its trapezoidal orientation, illuminates the nebula and can be easily viewed with any Meade telescope. Larger telescopes allow the observation of dozens of additional stars embedded in the nebula.

3. Deep-Space:
Galaxies: Subdivided into spiral and elliptical types, galaxies are large islands of billions of stars located throughout the visible universe. From a practical point of view the number of galaxies in the universe is virtually uncountable. The Andromeda Galaxy, seen without a telescope only as a fuzzy spot, blooms into a large elliptically shaped field of apparently glowing gas (actually a grouping of more than 100 billion stars) through any telescope. With the ETX or 4.5” Reflector the galaxy's structure starts to become visible.

It should be emphasized that the above listing only hints at the breadth of celestial objects within the view of any telescope. The larger the telescope's main lens or mirror, the more detailed, the more resolved, the brighter will be all of the objects observed, from the Moon, to the planets, to deep-space galaxies. But even through the lens of the smallest telescope, the 60mm, the night sky is transformed into a universe never before seen with the unaided eye. A telescope of any aperture size permits the user to uncover some of the most stunning sights in all of nature and to observe these sights as most people never thought possible.

How to Calculate Power:
The magnification, or power, at which a telescope is operating, is a function of the focal length of the telescope's main (objective) lens (or primary mirror) and the focal length of the eyepiece employed. The focal length of the objective lens is the distance between the lens and the point at which it brings light rays to a focus; this focal length (in millimeters, or mm) is printed on a label affixed to the optical tube of most telescopes. The focal length of each eyepiece (which typically ranges from 4mm to about 40mm) is printed on the upper surface of the eyepiece. To calculate power, multiply the objective lens/mirror size by the f/stop then divide it by the eyepiece size.

Example: A telescope with an objective lens/mirror 114mm and has a f/stop = 8, when this telescope is used with a 25mm eyepiece, will have a power of 114 * 8 / 25 = 36.48 power (written as "36.48X").

To find the max useable power for a telescope, use the formula: Objective lens/mirror size times 20 to 25.

Example: A telescope with an objective lens/mirror size 114mm (4.5”), would have a max power range of 90x to 112.5x. Take the objective lens/mirror size in inches (4.5”) and multiply it by 20 to 25.

A Word about "Power": When buying a telescope one of the least important factors to consider is the power, or magnification, of the instrument. The key to observing fine detail, whether on the surface of the Moon or on a license plate one mile in the distance, is not power, but aperture - i.e., the diameter of the telescope's main (objective) lens or primary mirror. The eyepiece employed determines the power at which a telescope is operating. All telescopes include one or more eyepieces as standard equipment, and optional eyepieces are available for higher or lower powers. Within reason power is useful, [but the most common mistake of the beginning observer is to "overpower" the telescope and to use magnifications which the telescope's aperture and typical atmospheric conditions can not reasonably support.] The result is an image, which is fuzzy, ill defined and poorly resolved through no fault of the telescope. Keep in mind that a smaller, lower-power, but brighter and well-resolved, image is far superior to a large, high-power, but dim and poorly resolved one.

© CigTech 2003