Content on this page requires a newer version of Adobe Flash Player.

Get Adobe Flash player

Home Observing Astrophotography Telescopes Dark Skies Star Parties Solar System Light Pollution
A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

A Back to Top
Achromatic Achromatic is a term which applies to refractor telescope optics. An achromatic telescope uses two elements in its lens to minimize chromatic aberration. Most inexpensive refractors are achromats.
Alt-Az Alt-Az is an abbreviation for "Altitude-Azimuth".  This refers to a telescope mount which has one axis moving parallel to the horizon (Azimuth) and one perpendicular to the horizon (Altitude).
Aperture Aperture is the diameter of the light-gathering portion of a telescope.  In a reflecting telescope this is the size of the mirror.  In a refractor is is the objective lens that defines the aperture.  In small telescopes the aperture is often given in millimeters, while in larger scopes it is given in inches.
Apochromatic Apochromatic is a term which applies to refractor telescope optics.  An apochromatic telescope uses three or more lens elements, one or more usually possessing special properties, to eliminate chromatic aberration.  Apochromatic refractors are usually regarded as giving the best quality image but are by far the most expensive type of telescope for a given size.
Apparent Field of View Apparent field of view refers to how wide a field of view an eyepiece will give at a certain magnification.  The apparent field is independent of the telescope or magnification used.  The true field of view is determined by the magnification used and the apparent field of view.
Arcminute An arcminute is a fraction of a degree.  There are 60 arcminutes in a degree.  The apparent size of many deep-sky objects are measured in arcminutes.  For example, the globular cluster M13 is 17 arcminutes in diameter. The abbreviation for arcminute is a single hash mark: 17 arcminutes = 17'.
Arcsecond An arcsecond is a fraction of an arcminute.  There ar 60 arcseconds in an arcminute, or 3600 arcseconds in a degree.  The apparent size of small deep-sky objects, double stars, and planets are usually measured in arcseconds. For example Jupiter is about 45 arcseconds in diameter. The abbreviation for arcsecond is two hash marks:  45 arcseconds = 45".
B Back to Top
Barlow Lens A barlow lens is most easily thought of as a device which increases the effective magnification of a given eyepiece.  What really is happening is that a barlow acts to increase the effective focal length of a telescope.  Therefore a given eyepiece will provide a higher magnification. Most barlow lenses double the magnification of a certain eyepiece.  Others increase by a factor of up to 5 times.  A barlow lens is an inexpensive way to extend your eyepiece collection since, for example, two eyepieces and a barlow would be equivalent to having a total of four eyepieces.
C Back to Top
CCD Camera CCD stands for "charge coupled device".  A CCD camera uses the same technology as the popular digital cameras used for everyday photography.  Regular digital cameras, however, have some drawbacks which make them unsuitable for astrophotography.  Astronomical CCD cameras are cooled to remove electronic noise which, in a digital camera, would hide the image of faint celestial objects.  CCDs are becoming more popular than film photography because they are much more sensitive than film and therefore images can be taken much faster and more easily.
Celestial Equator The sky can be imagined as a sphere surrounding Earth.  A line projected out from the equator of Earth defines the celestial equator of the celestial sphere.  Standing on Earth's equator, the celestial equator would appear directly overhead.
Celestial Pole The sky can be imagined as a sphere surrounding Earth.  As Earth rotates the sky appears to move.  Standing at the north pole, the point directly above in the sky would be the North Celestial Pole, and all the stars would appear to revolve about this point.  On the opposite side of the sky is the South Celestial Pole.  By aiming an equatorially-mounted telescope's mount toward the celestial pole it can keep up with the rotation of Earth using a simple clock drive.
Celestial Sphere The sky can be imagined as the inside surface of a sphere with Earth at the center.  This system makes the location of objects easy since all objects can now be given coordinates in the same way as places on Earth.  If we project a line out from Earth's equator we have a celestial equator dividing the sky in two.  At each pole of the sky, 90 degrees north and south of the equator, we place a celestial pole.  Lines of Declination in the sky are analogous to lines of Latitude on Earth, designating an object's position north or south of the celestial equator.  Lines of Right Ascension circle the sky perpendicular to lines of declination and designate an object's position east to west.
Central Obstruction Reflecting telescopes such as Newtonians and Schmidt-Cassegrains have a secondary mirror placed in the optical path of the scope to intercept light rays from the primary mirror and direct them to a convenient position for the eyepiece or camera.  This secondary mirror obstructs a small portion of the primary mirror and is thus known as a Central Obstruction.  The amount of light lost in terms of light-gathering area of the primary mirror is extremely minor (normally amounting to less than 10 %).  Central obstructions are usually blamed for the decrease in contrast noticed when viewing through a Schmidt-Cassegrain versus a refractor which has no central obstruction.  The truth is, high-quality refractors are normally made to much higher standards than other telescopes (and accordingly carry much higher price tags and occasionally very long backorders) and this accounts for most (if not all, in many cases) of the difference in optical quality between scopes.
Chromatic Aberration When light passes through a lens, such as in a refracting telescope, the various wavelengths of light, from red to blue, do not all focus to the same point.  This is the same effect that causes sunlight to separate into different wavelengths after passing through a raindrop to form a rainbow.  This is Chromatic Aberration.  Chromatic aberration is usually seen as a colored halo around bright stars and planets.  It is reduced by using an achromatic lens in a telescope.  It is effectively eliminated by using an apochromatic lens which causes all visible wavelengths to focus back to one point.
Clock Drive Earth rotates once in 24 hours.  This motion is apparent when viewing a celestial object through a telescope.  In just a matter of seconds an object can drift out of a telescope's field of view due to Earth's rotation.  A clock drive turns the telescope once in 24 hours about an axis (the polar axis) parallel to the axis on which Earth spins.  Thus, a celestial object may easily be tracked to keep it in the field of view. 
Coatings Glass does not inherently transmit all incoming light.  This can be seen by rolling down the window in your car part way.  Notice how much brighter the scenery is through the open part of the window than through the glass.  Anti-reflection coatings are deposited onto lenses to increase the amount of light passing through them.  Reflective coatings are used on mirrors in reflecting telescopes to increase the amount of light the mirror bounces back to the eyepiece or camera.
Collimation In order for a telescope to perform at its best, it must have all of its optical components - lenses, mirrors, focuser, etc. - aligned properly.  Aligning the optics of a telescope is know as collimation.  Refractors and Maksutov-Cassegrains come from the factory with their collimation permanently set and should not require adjustment. Newtonians and Schmidt-Cassegrains require occasional collimation.  Truss-tube Dobsonian telescopes must be collimated each time they are assembled.  Click here for more information on how to collimate your telescope.
Coma When referring to optics, Coma is a type of aberration.  (The word coma is also used to describe a part of a comet.)  Coma is usually short for the more specific term Off-Axis Coma.  When coma is present in an optical system stars at the center of the field appear as nice little pinpoints (assuming no other aberrations are present) while the stars near the edge of the field are elongated radially away from the center of the field.  This effect is seen especially in Newtonian telescopes with fast focal ratios and is minimized by using a coma-corrector lens ahead of the eyepiece or by using special eyepieces that diminish coma.
Computer-Controlled Telescope This is a general term referring to a type of telescope which uses a computer to help locate celestial objects.  Computer-controlled telescopes include scopes which use either digital setting circles or have goto systems.
Corrector Plate A corrector plate is the glass lens on the front of a Schmidt-Cassegrain telescope (SCT).  An SCT uses spherical mirrors which suffer from spherical aberration.  The corrector plate is used to eliminate this aberration.
D Back to Top
Dark Frame Astronomical CCD cameras are cooled to reduce the amount of electronic noise in the system, but some noise still exists which can wash out faint detail when imaging celestial objects.  A dark frame is an image taken with the CCD's shutter closed.  This image records only the electronic noise.  Then a picture is taken of the night sky which records both the object in the sky as well as the noise.  Then the dark frame is subtracted to leave only an image of the subject.
Declination If the sky is imagined to be a sphere surrounding Earth, lines of declination can be drawn in the sky which are analogous to lines of latitude on Earth.  An astronomical object's declination tells how far north or south the object lies from the celestial equator.
Degree A degree is a unit of measure.  The sky can be imagined as a sphere around Earth.  Like all circles, this sphere measures 360 degrees around.  The degree is a convenient unit telling how large something is, how far away one object is from another, how high something is above the horizon, etc.  The apparent size of constellations, very large deep-sky objects, and other large scale objects such as the moon and bright comet tails, are given in degrees.  For example, the Andromeda Galaxy is 3 degrees in diameter, and the moon is 1/2 of a degree across.  The symbol for a degree is the same a a temperature degree, a small circle:  1 degree = 1°.
Dew Shield In humid climates it is possible for dew to form telescopes whose optics are exposed to the air.  Both Schmidt-Cassegrain telescopes and refractors are prone to having dew form on their optics.  Dew will obscure the view through the telescope and its formation is prevented by placing a dew shield around the front of the telescope.  In extremely humid conditions dew heaters are also employed to fight off moisture by keeping the temperature of the optics just above the ambient air temperature.
Diagonal A diagonal is a mirror (or sometimes a prism) placed in the optical path of a telescope which reflects incoming light at a 90-degree angle in order to bring the light to a convenient position.  In a Newtonian the diagonal is a mirror located within the telescope tube and brings the light to the eyepiece on the side of the tube.  On a Schmidt-Cassegrain or refractor, the diagonal bends the light so that the eyepiece is at a right angle to the optical axis of the telescope.
Digital Setting Circles Digital setting circles (DSCs) are a type of computer control used to help locate celestial objects.  DSCs use optical encoders to read out the position of the telescope and give information to the user on how to move the telescope to reach the desired location in the sky.  DSCs differ from Goto telescope systems in that they do not move the telescope automatically, they only tell the operator how to move the scope to point to an object.  The term comes from the use of setting circles to find objects in the sky.
E Back to Top
Equatorial Mount An equatorial mount has one axis parallel to Earth's rotation axis.  All the stars in the sky appear to revolve around us once in 24 hours due to Earth's rotation.  An equatorial mount equipped with a clock drive, which rotates the mount once in 24 hours, can track celestial objects across the sky.  Click here to learn more about types of telescope mounts.
Eyepiece The eyepiece is the part of the telescope into which you look.  The eyepiece magnifies the image collected by the telescope.  Click here to learn more about eyepieces.
Eyepiece-Projection Photography When a camera is placed on the eyepiece-end of a telescope (prime focus photography) the image of the subject is magnified.  But for small targets such as planets and small details on the moon, this magnification is not enough.  By placing an eyepiece between the telescope and camera, using a special adapter, it is possible to increase the magnification enough to photograph small objects.  This technique is also used for taking CCD images of the planets and moon.
Eye Relief Eye relief is a term used in reference to eyepieces.  The eye relief of an eyepiece is the distance your eye can be from the eyepiece and still see the entire field of view.  Long eye relief makes viewing more comfortable, especially for eyeglass wearers, and many eyepieces incorporate special optical designs to increase their eye relief.
F Back to Top
Fastar Fastar is a feature of certain models of Celestron telescopes which allows a CCD camera to be placed at the front of the telescope.  The telescope's secondary mirror is removed and replaced with a special lens assembly which holds the camera.  The two biggest advantages to this system are:  the time required to take  pictures is significantly reduced because the focal ratio is decreased to around f/2;  and, a wide field of view is obtainable with a relatively inexpensive CCD camera which would otherwise give a very narrow field of view.
Field Rotation Field rotation results when a telescope in improperly polar aligned.  The most extreme example is a telescope which is mounted in an Alt-Az configuration such as a gotoSchmidt-Cassegrain telescope.  Field rotation is seen in photographs or CCD images as stars trailed around the center of the picture.  Imagine taking a picture of an object near the eastern horizon with the camera oriented horizontally on an Alt-Az mount.  When the subject reaches the meridian in the south it will have turned so that it is oriented vertically, but the camera is still oriented horizontally.  The result is star trails around the center of the picture.  If the telescope were mounted equatorially then the camera would correctly track the subject across the sky since the telescope and Earth now rotate about the same axis.  By placing an Alt-Az telescope on a wedge, field rotation can be eliminated.
Filter A filter is a specially-designed piece of glass placed in a telescope's optical path to enhance the view.  Filters are also used with CCD cameras to obtain color images.  Click here to learn more about filters.
Finderscope A finderscope is a small, low-power telescope with a cross-hair in it which is used to locate celestial objects.  Finderscopes usually operate at magnifications between 5 and 10 times, and have apertures ranging from about 24mm to 60mm.  They are usually mounted on the side of the telescope.  An object is centered first in the finderscope and then in the telescope, which itself usually has too small a field of view to easily locate objects.  Many people, however, prefer the use of a device like a Telrad, with which is usually easier to find celestial targets.
Focal Length The focal length is the length of the path through a telescope which incoming light follows.  In a Newtonian this is the distance from the primary mirror to the eyepiece.  In a refractor it is the distance from the objective lens to the eyepiece.  Schmidt-Cassegrains (SCTs) are a special example in which the effective focal length is actually longer than the path the light follows.  The reason for this is that the secondary mirror in a Schmidt-Cassegrain has the effect of multiplying the focal length of the primary mirror by a factor of about 5.  Therefore a Schmidt-Cassegrain can be much shorter than its effective focal length, which is part of the reason SCTs are so popular.  The magnification of a telescope is determined in part by the focal length of the scope.
Focal Ratio The focal ratio of a telescope relates the scope's focal length to its aperture.  The focal ratio of a telescope is simply the focal length divided by the aperture (making sure both measurements are in the same units - focal lengths are usually given in millimeters, but apertures are often given in inches).  Therefore, a telescope with a diameter of 100mm (4 inches) and a focal length of 1000mm has a focal ratio of 10, usually written f/10 (pronounced f-ten).  The focal ratio is important in determining the photographic speed of a telescope.  With a camera attached to the eyepiece-end of a telescope (prime-focus photography), the scope has effectively become a super-telephoto camera lens.  The "faster" the focal ratio of the telescope, the shorter the exposure time will be.  Smaller numbers mean faster focal ratios, for example, f/5 is faster than f/10.  Often a focal reducer is used to make the telescope operate at a faster focal ratio for photographic and CCD-imaging purposes.
Focal Reducer A focal reducer is used primarily for photographic or CCD-imaging purposes.  A focal reducer decreases the effective focal ratio of the telescope.  This allows images to be taken much faster.  This also gives a wider field of view since the focal length of the telescope is decreased.  Some focal reducers may be used visually to obtain a wider field of view, but others are intended for photographic/CCD use only.
Focuser A focuser is the part of a telescope which allows adjustment for the fact that different eyepieces and cameras focus at different places in the optical path of a telescope.  It also allows for each observer to adjust for his or her own eyesight.  On a Newtonian the focuser sits on the side of tube, near the front of the telescope, and racks a drawtube in and out carrying the eyepiece or camera.  On a refractor, the focuser adjusts a drawtube at the back of the telescope, in line with the optics.  A Schmidt-Cassegrain focuses by adjusting the position of the primary mirror.  This gives the advantage of close focus for terrestrial use, and of a wide range of focus travel for a variety of eyepieces, cameras, focal reducers, barlow lenses, etc., which may not reach focus on other telescope designs. 
G Back to Top
Goto A Goto telescope is a type of computer-controlled telescope.  Specifically, it is a telescope which can automatically move itself across the sky to locate a celestial object.  This differs from a telescope which uses digital setting circles in that a goto telescope has high- speed motor drives which can slew the telescope across the sky automatically. 
Guidescope Guiding a telescope to eliminate tracking errors is necessary for long-exposure astrophotography.  A guidescope is a second, smaller telescope, usually a 60-80mm refractor, mounted on the main instrument which is used to view a star on which the whole system is guided.  Guidescopes are best suited for use when photographing through a refractor and will not usually provide satisfactory results when used with a Schmidt-Cassegrain or Newtonian.  With these types of scopes it is best to use an off-axis guider.
M Back to Top
Magnification The magnification of an optical system is simply the number of times an image is enlarged when viewed through the optical system.  However, magnification can often be a source of confusion, since it is not the magnification that defines the "power" of an astronomical telescope.  Instead is it the amount of light the telescope can gather, determined by its aperture, that defines the "power".  Click here to learn more about magnification.
Magnitude The magnitude scale is used to compare the brightness of celestial objects.  Objects with smaller magnitudes are brighter, for example, an object of 1st magnitude is brighter than an object of 2nd magnitude, and so on.  The magnitude scale is based on ancient Greek astronomy, which assigned stars to six magnitudes from 1 to 6.  Modern science allows us to be much more accurate and so there are divisions within each magnitude; for example, a star can be magnitude 3.14.  We can also use telescopes to see much fainter now.  Magnitude 6 is usually considered the faintest visible to the unaided eye (although from very dark sites with very good eyes it is possible for experienced observers to glimpse 8th magnitude stars).  An 8-inch aperture telescope will show stars as faint as 15th magnitude, and the Hubble Space Telescope can image galaxies as faint as 30th magnitude, 4 billion times dimmer than the average person can see with the unaided eye!  The star Vega was (more or less) arbitrarily selected to define magnitude 0.  Thus it is also possible to have negative magnitudes since the Sun, four stars, four planets, and Earth's moon (and occasionally a very bright comet) all appear brighter than Vega.  Sirius, for example, is the brightest star in the night sky at magnitude -1.46.
Meridian The meridian is the imaginary line in the sky dividing east from west.  The meridian runs from north to south through a point directly overhead.  An object is at its highest point in the sky when is crosses the meridian (with the exception of objects near the celestial pole which can also cross the meridian at their lowest point when passing under Polaris, the north star).
Mirror Cell A mirror cell holds the primary mirror of a telescope in place.  This term is usually applied to Newtonian telescopes.  A mirror cell has some form of adjustment for collimating the optics of the telescope.
N Back to Top
Newtonian Click here to learn more about Newtonian telescopes.
O Back to Top
Objective The term objective is used to refer to the lens of a refracting telescope.  The objective lens of a refractor is simply the main lens at the front of the telescope.  The aperture of a refractor is determined by the diameter of the objective lens.
Obstruction See Central Obstruction
Off-Axis Guider Guiding a telescope to eliminate tracking errors is necessary for long-exposure astrophotography.  A guidescope is often used when photographing through a refractor.  Unfortunately, this method does not usually work with a Newtonian or Schmidt-Cassegrain because of flexure, or shifting of the guidescope relative to the main instrument.  A Schmidt-Cassegrain presents the worst problem since it has a movable primary mirror for focusing.  The mirror can shift relative to the guidescope, and even when the scope appears to have been perfectly guided, the picture comes out with trailed stars instead of nice little pinpoints.  An off-axis guider sits ahead of the camera in the optical path.  It lets most of the light from the telescope fall onto the film in the camera, but a small amount of light at the edge of the incoming light path (off axis) is picked off by a small prism and sent up to a guiding eyepiece.  Any shift of the primary mirror is corrected for since the guidestar is being viewed through the photographic instrument.
P Back to Top
PEC PEC stands for Periodic Error Correction.  All telescope drive systems have small errors in their gears from the limited machining tolerances possible when manufacturing them.  Since these errors repeat each time the main gear goes around, it is possible to correct for these errors during one revolution of the gear and then playback the correction from the PEC chip in the telescope's drive.  This feature is helpful when guiding long-exposure astrophotos and is a feature found only on telescope mounts with the capability of doing long-exposure photography.
Piggyback Photography By placing a camera on the top of a telescope and photographing through the camera's lens while letting the telescope's mount track the rotation of Earth, it is possible to obtain wide-field photographs of the night sky.  This is piggyback photography, and it is the easiest way to get started in deep-sky astrophotography.
Pixel The term pixel is used when referring to CCD cameras.  A CCD camera had a chip in it which detects the incoming light from the telescope.  This chip is divided into thousands of individual squares, or pixels.  Each pixel gathers light and the image is displayed on a computer screen with each CCD pixel matching up with a pixel on the computer screen.  The size of a CCD's pixels in part determines the resolution of the camera.
Polar Alignment In order for a telescope on an equatorial mount to properly track the sky to compensate for Earth's rotation, the mount must be correctly polar aligned.  For a telescope with a simple clock drive, polar alignment is as easy as pointed the mount's polar axis toward Polaris, the north star.  For photography and CCD imaging the polar alignment must be more accurate.  Visit the Polar Alignment page for more information on this subject.
Primary Mirror All reflecting telescopes have a primary mirror which gathers incoming light and sends it either to a secondary mirror or to a camera placed at the focus of the primary mirror.  In the case of Newtonians and Schmidt- Cassegrain telescopes it is the diameter of the primary mirror which determines the aperture of the telescope.  Some less common designs use an oversized primary mirror in which case the diameter of the opening at the front of the scope determines the aperture.
Prime Focus Photography When the eyepiece (and diagonal, in the case of refractors or Schmidt-Cassegrains) is removed from a telescope and replaced with a camera body (with lens removed) it is called prime-focus photography.  This essentially turns the telescope into a super-telephoto lens.  This method of photography is best for the moon or for terrestrial subjects such as wildlife.  By using a telescope with a very accurate clock drive and an off-axis guider or guidescope, long-exposure astrophotos can be taken as well.
R Back to Top
Radial Guider See Off-Axis Guider
Reducer/Corrector See Focal Reducer
Refractor A type of telescope that uses lenses to gather light. Click here to learn more about Refractor telescopes.
Resolution Resolution refers to the smallest detail visible through an optical system.  This can mean detail seen visually, or detail captured with film or a CCD camera.  Visual resolution is determined in theory by the aperture of the telescope;  the larger the aperture, the finer detail that can be seen.  In practice, the resolution is limited by the atmospheric conditions, or seeing.  Photographic or CCD resolution is determined in theory by the focal length of the telescope and the inherent resolution of the film (determined in part by the grain of the film - grainy, fast film will have lower resolution than fine-grained, slow film) or the size of the pixels in the CCD chip.  Again, in practice the true resolution is determined by the seeing and by the tracking accuracy of the telescope.
Right Ascension If the sky is imagined to be a sphere around Earth, then everything in the sky can be given coordinates similar to the way all locations on Earth have coordinates determined by latitude and longitude.  Lines of declination are analogous to the lines of latitude.  Lines of Right Ascension are analogous to lines of longitude, and like longitude they have their zero-point at an arbitrary point in the sky.  Longitude of Earth is starts in Greenwich, England;  Right Ascension starts at a point in the sky where the Sun is located on the first day of spring.  Lines of Right Ascension are divided into 24 hours, since the whole sky appears to rotate in that period due to Earth's motion.  Right Ascension increases from east to west, thus an object with a right ascension of 1 hour reaches the meridian before an object with a right ascension of 2 hours (in fact, exactly one hour before).  Facing north, objects appear to rise on your right (east), hence the name Right Ascension.  Right Ascension is often abbreviated R.A.
S Back to Top
Schmidt-Cassegrain Click here to learn more about Schmidt-Cassegrain telescopes.
Secondary Mirror Most reflecting telescopes have a secondary mirror which reflects light from the primary mirror to a convenient position for the eyepiece or camera.  In a Newtonian the secondary mirror is a diagonal, sending the light out to the side of the tube.  In a Schmidt- Cassegrain the secondary mirror is a convex mirror places in the center of the optical path which sends the light back through a hole in the primary mirror to a convenient focus point behind the primary.
Seeing Seeing refers to the steadiness of Earth's atmosphere.  If the seeing is good then the air is stable and fine detail is visible through a telescope.  If the seeing is poor then there is significant turbulence in the atmosphere and is is difficult to resolve fine detail.  Objects with very fine detail such as double stars and the planets are best viewed on nights of good seeing.
Setting Circles Each object in the sky has coordinates describing its position on the celestial sphere.  These coordinates are similar to longitude and latitude on Earth and are called Right Ascension and Declination.  Setting circles are used on equatorially-mounted telescopes to locate an object by its coordinates.  Setting circles are often inaccurate and can be inconvenient since the right ascension and declination of each object must be known.  Many observers prefer to use other methods such as starhopping or using a computer-controlled telescope.  Click here to learn more about how to find celestial objects.
Slewing Goto telescopes which can move automatically by using high-speed drive motors are said to be "slewing" when the move across the sky.
Slow-Motion Controls Many telescope mounts have slow-motion controls used to make fine adjustments for centering an object in the field of view.  Certain types of  telescope mount use a hand-controller to move the scope using small motors.
Spherical Aberration A parabolic mirror will focus incoming light to a single point.  A spherical mirror will focus incoming light to different points causing stars to no longer appear as nice round points of light.  This distortion is called spherical aberration.  Parabolic mirrors are, however, more expensive to make than spherical mirrors.  Schmidt-Cassegrain telescopes use spherical primary mirrors, but a glass corrector plate is placed at the front of the telescope to correct for spherical aberration.
T Back to Top
T-Adapter A T-Adapter fits on the eyepiece-end of a telescope (with the eyepiece removed) and is used to attach a camera body to the telescope for photography.
Telrad A Telrad is a device used to point the telescope to a desired location in the sky.  It appears to project a red bull's-eye pattern onto the sky showing where the telescope is aimed.  Unlike a conventional finderscope, a Telrad has no magnification and does not invert the image, therefore the Telrad is much faster and easier to use.  On a small telescope where a Telrad will not fit, a Star Pointer or similar device can be used which appears to project a small red dot onto the sky.
Transparency Transparency is a term referring to the clarity of Earth's atmosphere.  On a hazy night very little starlight can penetrate the atmosphere and the transparency is said to be low.  On an exceptionally clear night the sky is said to be very transparent.  Dim targets and faint detail within objects are best seen on nights of good transparency.
T-Ring A T-Ring is used to attach a camera body to a T-Adapter.  Since each camera manufacturer uses a different mount for its own cameras, a T-Ring specific to the type of camera being used must be attached in order to mount the camera to the T-Adapter.
True Field of View The true field of view is the angular size of sky seen through a telescope.  The true field of view is determined by the apparent field of view of the eyepiece used and the magnification.  Visit the eyepiece page to learn more about field of view.
W Back to Top
Wedge A wedge is use to turn an Alt-Az mounted Schmidt-Cassegrain telescope into an equatorially- mounted telescope.  The wedge attaches between the tripod and telescope base and places the fork arms at the correct angle for polar alignment.  This arrangement in necessary to eliminate field rotation.
Zenith The zenith is the point in the sky directly overhead from your location.  Directly opposite the zenith, below your feet, is the "nadir".
Site Map Observing Telescopes Books Dark Skies Star Parties Favorite Objects