Terminology
ABERRATIONS: There are several optical designs used for telescopes. Remember that a telescope is designed to collect light and form an image. In designing optical systems, the optical engineer must make tradeoffs in controlling aberrations to achieve the desired result of the design. Aberrations are any errors that result in the imperfection of an image. Such errors can result from design or fabrication or both. It is impossible to design an absolutely perfect optical system. The various aberrations due to a particular design are noted in the discussion on types of telescopes. Below we will briefly describe specific telescope aberrations:
Chromatic Aberration -- usually associated with objective lenses of refractor telescopes. It is the failure of a lens to bring light of different wavelengths (colors) to a common focus. This results mainly in a faint colored halo (usually violet) around bright stars, the planets and the moon. It also reduces lunar and planetary contrast. It usually shows up more as speed and aperture increase. Achromat doublets in refractors help reduce this aberration and more expensive, sophisticated designs like apochromats and those using fluorite lenses can virtually eliminate it.
Spherical Aberration -- causes light rays passing through a lens (or reflected from a mirror) at different distances from the optical center to come to focus at different points on the axis. This causes a star to be seen
as a blurred disk rather than a sharp point. Most telescopes are designed to eliminate this aberration.
Coma -- associated mainly with parabolic reflector telescopes which affect the off-axis images and are more pronounced near the edges of the field of view. The images seen produce a V-shaped appearance. The faster the focal ratio, the more coma that will be seen near the edge although the center of the field (approximately a circle, which in mm is the square of the focal ratio) will still be coma-free in well-designed and manufactured instruments.
Astigmatism -- a lens aberration that elongates images which change from a horizontal to a vertical position on opposite sides of best focus. It is generally associated with poorly made optics or collimation errors.
Field Curvature -- caused by the light rays not all coming to a sharp focus in the same plane. The center of the field may be sharp and in focus but the edges are out of focus and vice versa.
AIRY DISK BRILLIANCE FACTOR: When you view a star in a properly focused telescope you are not going to see an enlarged image since stars, even at high power, should look like points of light rather than disks or balls. This is simply because stars are very, very far away. But, if you magnify a star’s image by about 60x per inch of aperture and look carefully you may be able to see rings around the star. This is not the star’s disk you are seeing but the effect of having a circular aperture in your telescope and due to the nature of light. Under close inspection, when the star is at the center of the telescope’s field of view, this highly magnified star image will show two things; a central bright area called the airy disk, and a surrounding ring or series of faint rings called the diffraction rings. The airy disk becomes smaller as you increase the aperture. Airy disk brilliance (the brightness of a point-source stellar image) is proportional to the fourth power of aperture. In theory, when you double the aperture of a telescope, you increase its resolving power by a factor of two and boost its light gathering ability by a factor of four. But more importantly, you also reduce the area of the airy disk by a factor of four resulting in a sixteen-fold gain in stellar image brilliance.
APERTURE: The diameter of a telescope’s primary lens or mirror. This is the single most important factor in choosing a telescope. The prime function of all telescopes is to collect light. At any given magnification, the larger the aperture, the better the image will be. The clear aperture of a telescope is the diameter of the objective lens or primary mirror specified in either inches or millimeters (mm). The larger the aperture, the more light it collects and the brighter (and better) the image will be. Greater detail and image clarity will be apparent as aperture increases. For example, a globular star cluster such as M13 is nearly unresolved through a 4" aperture telescope at 150 power but with an 8" aperture telescope at the same power, the star cluster is 16 times more brilliant, stars are separated into distinct points and the cluster itself is resolved to the core.Considering your budget and portability requirements, select a telescope with as large an aperture as possible.
CAMERA ADAPTER: This accessory is used during astrophotography. It is one of several accessories needed to successfully capture images on film of the night sky.
CATADIOPTRIC TELESCOPE: A telescope that combines lenses and mirrors in its optical system. The Schmidt-Cassegrain is the most common catadioptric design among amateurs.
CELESTIAL COORDINATES: The system of determining the location in the sky of an object. Similar to the Earth system of latitude and longitude. Extend the Earth’s latitude and longitude grid into space as a background for the constellations and you have a map to determine each object’s celestial coordinates. There are two components to an object’s celestial coordinates, right ascension and declination.
CELESTIAL SPHERE: The imaginary sphere surrounding the Earth, with the stars and other astronomical objects attached to it.
CHROMATIC ABERRATION: A defect of optical lenses, chromatic aberration is the dispersion of light such that different wavelengths—and therefore different colors—are refracted, or bent, at different angles. The result is colored halos
around images seen through the optical lens.
COLLIMATION: The proper alignment of the optical elements in a telescope. Collimation is critical for achieving optimum results. Poor collimation will result in optical aberrations and distorted images. Not only is the alignment of the optical elements important but even more important is the alignment of the optics with the mechanical tube -- this is called opto/mechanical alignment.
COMA: A defect of optical mirrors, coma is the reflecting of light from the outer regions of a mirror to a different focal point than light reflected from the central region of the mirror. The result is objects appearing fuzzy toward the edge of your
field of view in an eyepiece.
CONTRAST: Maximum image contrast is desired for viewing low-contrast objects such as the moon and planets. Newtonian and catadioptric telescopes have secondary (or diagonal) mirrors that obstruct a small percentage of light from the primary mirror. Some of the literature on amateur astronomy would lead you to believe that image contrast is severely reduced with Newtonians or catadioptrics because of this obstruction, but this is not the case. (It would be if more than 25% of the primary mirror surface area was obstructed.) To calculate the secondary obstruction, use the formula (pi)r² for the primary and secondary mirrors which gives you the surface area of each. Then calculate the percentage of obstruction. For example, an 8" telescope with a 2¾" secondary obstruction has an 11.8% secondary obstruction:
primary 8" = (pi)r² = (pi)4² = 50.27
secondary 2¾" = (pi)r² = (pi)1.375 = 5.94
percentage = 5.94 is 11.8% of 50.27
Seeing conditions (or air turbulence) is the single most important factor that adversely affects image contrast when seeking planetary detail through a telescope. Instrument problems that can also adversely affect contrast in order of decreasing importance are: optical figure, collimation, optical smoothness, baffling, and a small increase in central obstruction. Note that the increase in central obstruction is rated as the smallest contributor adversely affecting contrast.
DECLINATION: Similar to latitude, an object’s declination is its angle north or south from the celestial equator as measured in degrees, minutes and seconds. The north celestial pole, an extension of the Earth’s pole, has a declination of 90
degrees north. The south celestial pole is at 90 degrees south declination. Any object located along the celestial equator has a declination of 0 degrees.
DECLINATION MOTOR: A declination motor allows the telescope to be electronically driven through this axis. It is of most use when doing astrophotography and ccd-imaging. Long exposures of deep-sky objects require that the telescope tracks during the exposure. Since no drive system is manufactured to perfection, it is necessary to guide during the exposure, making subtle corrections in both axis of motion.
DIFFRACTION LIMITED (RAYLEIGH CRITERION): A diffraction limited telescope has aberrations (optical errors) corrected to the point that residual wavefront errors are substantially less than 1/4 wavelength of light at the focal point. It is then acceptable to be used as an astronomical telescope. In compound optical systems, the individual components must be better than 1/4 wavelength for the wavefront error at the focal point to be at least 1/4 wavelength. As the wavefront number gets smaller (1/8th or 1/10th wavelength), the optical quality is progressively better.
EXIT PUPIL: The exit pupil of a telescope is the circular beam of light that leaves the eyepiece being used and is measured in mm. To calculate exit pupil, divide the aperture (in mm) by the power of the eyepiece being used. For example, an 8" aperture telescope (203mm) used with a 20mm eyepiece is working at 102 power and has an exit pupil of 2mm (203/102= 2mm). Or, you can calculate the exit pupil by dividing the focal length of the eyepiece (in mm) by the focal ratio of the telescope.
FIELD OF VIEW: The amount of sky that you can view through a telescope is called the real (true) field of view and is measured in degrees of arc (angular field). The larger the field of view, the larger the area of the sky you can see. Angular field of view is calculated by dividing the power being used into the apparent field of view (in degrees) of the eyepiece being used. For example, if you were using an eyepiece with a 50 degree apparent field, and the power of the telescope with this eyepiece was 100x, then the field of view would be 0.5 degrees (50/100 = 0.5). Manufacturers will normally specify the apparent field (in degrees) of their eyepiece designs. The larger the apparent field of the yepiece (in general), the larger the real field of view and thus the more sky you can see. Likewise, lower powers used on a telescope allow much wider fields of view than do higher powers.
FOCAL LENGTH: The length of the light path from the primary lens or mirror to the focal point of the telescope. This is the distance (in mm.), in an optical system, from the lens (or primary mirror) to the point where the telescope is in focus (focal point). The longer the focal length of the telescope, generally the more power it has, the larger the image and the smaller the field of view. For example, a telescope with a focal length of 2000mm has twice the power and half the field of view of a 1000mm telescope. Most manufacturers specify the focal length of their various instruments; but, if it is unknown and you know the focal ratio you can use the following formula to calculate it: focal length is the aperture (in mm) times the focal ratio. For example, the focal length of an 8" (203.2mm) aperture with a focal ratio of f/10 would be 203.2 x 10 = 2032mm. Most amateur telescopes have focal lengths that range from 500mm to 3,000mm. Most refractors and Newtonian reflectors have focal lengths approximately equal to the length of their telescope tube. Catadioptric telescopes fold the light path back upon itself making the tubes two-to-three times shorter than their focal lengths.
FOCAL RATIO (PHOTOGRAPHIC SPEED OR F/STOP): Like cameras and lenses, telescopes are given a speed rating. Focal ratio is the telescope’s focal length divided by its aperture. An 80mm refractor with a 900mm focal length will have a focal ratio of 11. This is written as f/11. Although a fast telescope requires shorter exposure times for photography (an f/8 system requires four times the exposure of an f/4, for example) it does not produce brighter images for observing than a slower focal ratio telescope of the same aperture. Many people equate focal ratios with image brightness, but strictly speaking this is only true when a telescope is used photographically and then only when taking pictures of so-called "extended" objects like the Moon and nebulae. Whether a telescope is used visually or photographically, the brightness of stars (point sources) is a function only of telescope aperture-the larger the aperture, the brighter the images. When viewing extended objects, the apparent brightness seen in the eyepiece is a function only of aperture and magnification, it is not related to focal ratio. Extended objects will always appear brighter at lower magnifications. Telescopes with small (sometimes called "fast") focal ratios do, however, produce brighter images of extended objects on film, and thus require shorter exposure times. Generally speaking, the main advantage of having a fast focal ratio with a telescope used visually is that it will deliver a wider field of view. Sometimes referred to as an RFT (rich field telescope), an f/4 to f/6 telescope provides a wide field, low power view and requires shorter exposure times during astrophotography and CCD imaging, hence the term "Fast". "Slow" scopes are f/12 and longer and are sometimes called NFT’s (normal field telescopes). NFT’s provide narrow field, high power images.LIGHT-GATHERING POWER (LIGHT GRASP): This is the telescope's theoretical ability to collect light compared to your fully dilated eye. It is directly proportional to the square of the aperture; an 8-inch aperture telescope has four times the light gathering power of a 4-inch aperture telescope. You can calculate this by first dividing the aperture of the telescope (in mm) by 7mm (dilated eye for a young person) and then squaring this result. For example, an 8" telescope has a light gathering power of 843. ((203.2/7)² = 843). Reflectors, refractors and catadioptrics of equal aperture have equal light-gathering power.
LIMITING MAGNITUDE: Astronomers use a system of magnitudes to indicate how bright a stellar object is. An object is said to have a certain numerical magnitude. The larger the magnitude number, the fainter the object. Each object with an ncreased number (next larger magnitude number) is approximately 2.5 times fainter. The faintest star you can see with your unaided eye (no telescope) is about sixth magnitude (from dark skies) whereas the brightest stars are magnitude zero (or even a negative number). The faintest star you can see with a telescope (under excellent seeing conditions) is referred to as the limiting magnitude. The limiting magnitude is directly related to aperture, where larger apertures allow you to see fainter stars. A rough formula for calculating visual limiting magnitude is: 7.5 + 5 LOG (aperture in cm). For example, the limiting magnitude of an 8" aperture telescope is 14.0. (7.5 + 5 LOG 20.32 = 7.5 + (5x1.3) = 14.0). Atmospheric conditions and the visual acuity of the observer will often reduce limiting magnitude. Photographic limiting magnitude is approximately two or more magnitudes fainter than visual limiting magnitude.
MAGNIFICATION (POWER): The focal length of the telescope divided by the focal length of the eyepiece. A 10mm eyepiece in telescope with a 900mm focal length yields 90x magnification. In a 1,200mm focal length telescope, that same 10mm eyepiece produces 120x magnification. The longer a telescope’s focal length, the more magnification it will provide with any given eyepiece .There are practical upper and lower limits of power for telescopes. These are determined by the laws of optics and the nature of the human eye. As a rule of thumb, the maximum usable power is equal to 60 times the aperture of the telescope (in inches) under ideal conditions. Powers higher than this usually give you a dim, lower contrast image. For example, the maximum power on a 60mm telescope (2.4" aperture) is 142x. As power increases, the sharpness and detail seen will be diminished. The higher powers are mainly used for lunar, planetary, and binary star observations.
Do not believe manufacturers who advertise a 375 or 750 power telescope which is only 60mm in aperture (maximum power is 142x), as this is false and misleading. Most of your observing will be done with lower powers (6 to 25 times the aperture of the telescope [in inches]). With these lower powers, the images will be much brighter and crisper, providing more enjoyment and satisfaction with the wider fields of view. There is also a lower limit of power which is between 3 to 4 times the aperture of the telescope at night. During the day the lower limit is about 8 to 10 times the aperture. Powers lower than this are not useful with most telescopes and a dark spot may appear in the center of the eyepiece in a Catadioptric or Newtonian telescope due to the secondary or diagonal mirror's shadow.
MAGNITUDE: A logarithmic scale measuring an object’s brightness. Each change of five magnitudes is equivalent to a change in brightness of 100. Each increase of one magnitude is equal to a decrease in brightness of approximately 2.5. Each decrease of one magnitude is equal to an increase in brightness by the same amount.
MESSIER, CHARLES (1730-1817): A French astronomer who prepared one of the earliest catalogues of nebulous objects with the assistance of associate Pierre Mechain. The entire Messier list of 103 deep-sky objects, since expanded to 109, is visible in a 3-inch telescope.
NEAR FOCUS: This is the nearest distance you can focus the telescope visually or photographically for close terrestrial work.
REFLECTING TELESCOPE: A telescope that uses mirrors in its optical system.
REFRACTING TELESCOPE: A telescope that uses lenses in its optical system.
RESOLUTION: This is the ability of a telescope to render detail. The higher the resolution, the finer the detail. The larger the aperture of a telescope, the more resolution the instrument is capable of, assuming the telescope optics are of high quality.
RESOLVING POWER: For telescopes this is referred to as "Dawes limit." It is the ability to separate two closely-spaced binary (double) stars into two distinct images measured in seconds of arc. Theoretically, to determine the resolving power of a telescope divide the aperture of the telescope (in inches) into 4.56. For example, the resolving power of an 8" aperture telescope is 0.6 seconds of arc (4.56 divided by 8 = 0.6). Resolving power is a direct function of aperture such that the larger the aperture, the better the resolving power. However, resolving power is often compromised by atmospheric conditions and the visual acuity of the observer.
RIGHT ASCENSION: Similar to longitude, an object’s right ascension is its angle along the celestial equator as measured in hours, minutes and seconds. The sky is divided into 24-hours of right ascension, each hour being 15 degrees wide. Right ascension increases as you move east.
RIGHT ASCENSION MOTOR: If a telescope mount is motorized, this is the axis that is driven. The motor drive moves the telescope opposite the Earth’s rotation at the same rate. The effect is that stars and other celestial objects remain centered in the eyepiece.
SEEING: One of two standard measurements of atmospheric conditions while observing. Seeing refers to resolution and the detail visible in objects. On nights of good seeing, Jupiter will show lots of detail—bands, ovals, festooning—and structure in galaxies, nebulae and clusters will also be apparent. On nights of bad seeing, Jupiter appears to be a fuzzy ball with all but the largest features hidden and deep-sky objects present virtually no detail.
STAR DIAGONAL: This accessory fits in the telescope’s focuser and redirects the light path allowing the eyepiece to be placed in a more comfortably accessible position. A standard star diagonal has a 90° offset. The view through a star diagonal can be somewhat confusing as the image will be correct top-to-bottom but flipped left-to-right. A star diagonal with a 45° offset will provide a correct left-to-right and top-to-bottom image allowing you to use the scope for daytime observing, too. An Amici prism is a a 90° diagonal designed to provide a correct image.
TRANSPARENCY: The second standard measurement of atmospheric conditions, transparency refers to darkness of the sky and the faintest magnitude visible to the naked eye and telescope. It’s possible that conditions of good seeing and poor transparency co-exist. Humidity and haze, which will inhibit transparency, often makes for good seeing when doing planetary observing.
T-RING: This accessory allows a camera body to be mounted to a camera adapter
on a telescope.