What’s Out Tonight?

A general ASTRONOMY site to get you started exploring the night sky

Telescope, Optics & Mount Terms

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1.25 inch diameter eyepieces
This is the standard and most common eyepiece barrel diameter for telescopes. It is in inches, and it is a world-wide standard. 

2-inch diameter eyepieces
A larger barrel diameter for eyepieces than the standard 1.25-inch barrel diameter. Two-inch diameter eyepieces are not necessary for 99% of amateur viewing. Their only advantage is that they can provide the widest possible fields-of-views. However, they are usually expensive, and you can get incredible 1.25-inch diameter barrel eyepieces for a lot less money. 2-inch eyepieces are big and heavy and do not easily fit in your pockets like 1.25 inch eyepieces. Not all focusers/telescopes can accept the 2-inch barrel size. 

12 volt battery
Almost all amateur telescopes/mounts, worldwide, will run/operate on 12-volt batteries. This also includes many accessories, like the Digital Setting Circles and astrophotography equipment. 

45° diagonal holder for eyepiece
Used on refractor telescopes to provide a normal upright image but it is not common for astronomical use. 

90° diagonal holder for eyepieces 
Allows more comfortable viewing through a refractor telescope or Scdmidt-Cassegrain Telescope (SCT). It is not used on Newtonian Reflectors. 

A type of refractor telescope that uses two lenses in the front of the scope to bring images to a focus. An achromat has one lens made of flint-type glass and the other made of crown-type glass. It was invented in the early 1700s and is still in use today. Long focal length achromats of around f/10 or more provide good imagery. As the focal length gets shorter, the image will have more and more color fringing either with blue or red.

T​he two lenses of the Achromat

Alt-Az mount 
The type of mount used on binoculars at tourist attraction. This mount is a simple mount that moves up and down vertically and swivel around to any compass point. This type of mount can be manual or motorized/computerized. Many refractors are sold with alt-az mounts. 

Altitude is the vertical up and down movement that takes you from the horizon (or lower) to the zenith (very top of the sky). Azimuth is a 360°‚ horizontal circular movement that takes you to any compass point.

An optical system for a refractor that consists of either 3 or 4 lens elements. These types of refractors provide the highest quality imagery, are free of any color fringing and thus are the most expensive. When you here “APO” you know the optical system is the best. Now, there are companies that use two lens elements for their front objectives but special glass is employed for the two lenses that provides better color correction than the ordinary achromat. Technically, these scopes are not apochromat but they come close.

Apparent Field-of-View
See Field-of View.


A shape of a lens that is not spherical. Most lenses have spherical concave or convex shapes for their surfaces. Aspherical surfaces can correct for optical defects that cannot be achieved with spherical lenses. Aspherical shapes are much more difficult to make (and test) than the simple spherical shape, however, a single aspherical lens can often replace a few or several spherical lenses.

T​he three lenses of the Apochromat

A 45° diagonal that is used to erect the image (provide a normal upright image) from a refractor telescope, mainly for terrestrial viewing.

A Schmidt-Cassegrain Telescope (SCT) that has a 
90° diagonal for more comfortable viewing. These are also used extensively on refractor telescopes.

2-inch diameter barrel eyepiece (far left) compared to a family of the more common 1.25-inch diameter barrel eyepieces.

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A barlow is used in conjunction with an individual eyepiece to double or more the magnification of the eyepiece. An eyepiece is inserted into the barlow and the barlow is then inserted into the eyepiece holder or focuser. 

Bar Clamps
See Dovetail Bars and Clamps

A telescope that uses a combination of mirrors and “lenses.” The Schmidt-Cassegrain Telescope (SCT) and Maksutov-Cassegrain Telescope are examples. 

Cheshire Collimation “Eyepiece”
A special type of “eyepiece” that is inserted into the focuser and used to collimate Newtonian Reflectors. 

C hromatic Aberration
An inherent optical “defect” of lenses where all colors of light do not focus at the same place causing color fringing around objects. This is often seen as a blue-color fringing in achromat refractors with shorter focal lengths. Or, an orange fringing in cheap binoculars. If you increase the focal length of an achromat refractor to about f/10 or so, the color fringing basically disappears. In general, reflector telescopes do not suffer from chromatic aberration.

The process of aligning the optics in a telescope to provide the optimum image quality. Newtonian Reflectors usually require the most frequent alignments—some special “eyepieces,” like the Cheshire (and others) are helpful with this process. Refractors will usually retain their alignment better than reflectors and can often be adjusted with the push-pull screws (see Push-Pull) around the circumference of the cell holding the objective lenses, however, some refractors do not have provisions for this adjustment. New owners of Newtonian Reflectors are often daunted by the collimation process but this quickly dispels after a few collimations. 

Collimation Screws or Bolts Often refers to the three screws/bolts/ wingnuts/knobs that control the tilt of a reflector’s primary and secondary mirrors used for optical collimation of the telescope. 

Double Concave, left.
Double Convex, right.

An optical “defect” in reflector telescopes that causes images off axis to be elongated and flared. It is especially noticeably and prevalent in short focal length Newtonian Reflectors with focal lengths of around f/4 or shorter. Newtonian reflectors of around f/8 or longer do not exhibit coma. 

Coma Corrector
An eyepiece type lens that corrects for coma in short focal lengths Newtonian Reflectors (especially around f/4). Regular eyepieces are inserted into the coma corrector and the eyepiece/corrector “unit” is then inserted into the focuser. 

A shape like a bowl. 

A roundish bulging shape like a dome. 

Corrector Plate
The front glass plate of a Schmidt-Cassegrain Telescope (SCT) is not technically a lens, that is, it does not bring light to a focus. This plate of glass (and it is very thick) does have a aspherical optical shape that corrects for spherical aberration because the two mirrors are spherical and thus easier to manufacturer. 

Counterweight shaft
Used on German Equatorial Mounts (GEM) mounts to balance the telescope “around” the polar axis. 

Specific to German Equatorial Mounts (GEM). Round metal weights that can be moved on a round metal shaft (part of the Declination axis) to balance the telescope around the polar axis.

Crayford eyepiece focuser
A eyepiece focuser design invented in the early 1970s that does not use rack and pinon gears to move the focus tube. Instead it uses the friction of a perpendicular round rod against the focus tube (with tension springs)—turn the round rod (with a knob on its end) and it moves the focus tube up and down. At the time in the early 70s and afterwords, it was more difficult to make precision rack and pinion gears. This is not the case anymore with computer-controlled machining. Although versions of the Crayford focuser are still in use, the preferred focusing mechanism is the rack and pinion especially for astrophotography.

Nice drawing showing the concept of the Crayford eyepiece focuser.

Severe Chromatic Aberration—the blue and orange color fringing.

The traditional Barlow doubles the magnification of any inserted eyepiece but there are barlows that will magnify up to 5x. ​The 15mm eyepiece will act as a 7.5mm eyepiece but keep all of its other optical properties.

Severe Coma. Stars at the very center are pinpoint but quickly appear comet-like just a short distance away from the center. This is an optical “defect” of very short focal length Newtonian Reflectors 
(around f/4).

A German Equatorial Mount (GEM), showing the counterweight shaft and counterweight that balances the telescope around the polar axis.


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Declination Axis
The axis on a equatorial mount that moves the telescope in declination, which is akin to latitude. The North Celestial Pole has a declination of +90° while the celestial equator has the declination of 0°. See German Equatorial Mount.

Dew Cap or Dew Shield 
Refractor telescopes and Schmidt-Cassegrain Telescopes (SCT) that have a “lens” at the front end of the scopes can be directly affected by dew and stray light. Most refractors have a dew or light shield cap that extends outward from the objective. Some of these can be detached when not in use or slid backwards onto the telescope tube when not in use. Dew shield can delay the formation of dew but they cannot prevent it. SCTs do not normally come with a dew shield so it must be purchased separately—often a flexible material that is wrapped around the front of the telescope. Dew shield can also limit stray domestic lights and indirect Moon light that can decrease the contrast of the view through the eyepiece. Newtonian Reflectors do not need dew or light shields because the primary mirror is located at the back or end of the telescope tube. 

Dew Heater
In humid climates, dew formation on the front lens of a refractor or the corrector plate of a Schmidt-Cassegrain Telescope (SCT) can be an serious impediment to observing. Dew shields can help retard the formation of dew but they cannot prevent it. The only solution is to purchase a warming strip that circles the circumference of the lens or corrector plate—these are removable and do require electrical power (12 volts DC). 

Diagonal Mirror 
Two definitions. 1. The secondary and smaller mirror in a Newtonian Reflector that bends the light from the primary mirror 90° to the focuser near the top of the tube. The diagonal mirror is held in place by a single rod/stock or a “spider” having three to four veins. 2. A diagonal mirror is also a holder for an eyepiece that bends the imaging light from the telescope 90° for more comfortable viewing. These are used on refractors and Schmidt-Cassegrain Telescopes (SCT). 

Diagonal Stalk
A single rod of metal, often brass painted black, that holds the 45° angled diagonal mirror that sits below the focuser of Newtonian Reflectors. This is the simplest method to suspend a diagonal mirror in the center of a Newtonian Reflector telescope.  

Digital Setting Circles (DSC)
A control box that can be used to point a telescope to objects in the sky. Often DSC are used on a manual telescope that is pushed by hand to locate objects. The telescope or mount has to be equipped with encoders (see Encoders) on both moving axes for a DSC to work. As the telescope is moved, the encoders measure the movement of the mount/telescope. The DSC has an accurate clock that knows the position of the telescope (using the movement of the encoders) once the telescope has been aligned to a few stars. The DSC box has a digital readout and buttons to access catalogues of all the celestial objects in the sky including the planets and Moon. Usually needs a battery to operate. 

D ob
Abbreviation for a Dobsonain style (simple alt-az mount) Newtonian Reflector. 

A Newtonian Reflector in a simple alt-az mount. In the late 1960s, John Dobson made many larger Newtonian Reflectors on simple alt-az mounts to show the public celestial objects. The design of his simple alt-az mount was NOT of a type, at the time, used by the amateur community (but the amateur community did use alt-az mounts) and was a combination or integral mount and stand. This mount/stand combination allowed amateurs to make larger and larger Newtonian Reflectors for personal use, up to 30 inches or greater diameter mirrors. 

Generally refers to the achromat objective used in refractors to bring light to a focus and consists of two different types of glass that are either cemented together or having a small air space between them.

Dovetail Bars and Clamps
Used to attach/clamp telescope to mount. Rings are bolted to the dovetail bar which is tightened into the dovetail clamp. These work well. See picture.

Telescope rings are attached to a dovetail bar which is inserted and tightened into the dovetail clamp.

The Nexus control box for Digital Setting Circles (DSC). It “walks” you through aligning the scope to a few stars, then provides arrows and countdowns to move the telescope to any object selected from it many catalogues, including our Planets and Moon. Your telescope axes must be equipped with optical encoders for a DSC to operate.

A Dew Shield on a Schmidt-Cassegrain Telescope (SCT). It prevents or delays the fromation of dew on the front Correcor Plate and keeps stay light out increasing the contrast of images.

Two typical Dobsonian (Dob) telescopes. Remember Dobs are Newtonian Reflectors on simple integral alt-az mounts. The telescope on the left is a common design for mirror with diameters of 10 inches or less. The design of the telescope on the right will start with mirror sizes of 12-inches and up and over 36 inches in diameter.

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Encoders are used in conjunction with Digital Setting Circles (DSC) to measure the rotational movement of the telescope/mount in both axes. The encoders are internal or external to the telescope’s mount axes. They are simple devices consisting of a disk engraved with many vertical lines around a perimeter that are optically counted as the mount is moved. This allows the DSC to calculate the location that the telescope is pointed. The DSC and encoders are often powered by batteries (not alway a 12 volt battery). See DSC.

A lens or lenses in a small and short tube that is used to magnify the image brought to a focus by a telescope. Eyepieces of different focal lengths are used to change the magnification of telescopes. 

Top:  The housing for a typical encoder needed for the Digital Setting Circles (DSC). The round shaft ​is connected to an axis by various means. Bottom:  The inside of an encoder.

f-ratio, Focal Ratio
This is a fairly important concept in regards to telescope focal length. Example: A telescope with a focal ratio of f/8 means that the focal length of the telescope is 8 times the diameter of the objective lens or primary mirror. If you have a 6-inch diameter Newtonian Reflector with an f/8 focal ratio, then the focal length of the telescope is 6-inches x 8 = 48 inches. The focal ratio often indicates traits of an optical system or telescope. Some common focal ratios are f/4, f/5, f/6, f/8, f/10

There are two aspects of field-of-view. One relates directly to the optical design of an eyepiece (Apparent). The other relates to how much of the sky you actually see (True) when looking through an eyepiece. See pictures below.

Traditionally, a finderscope is a small refractor telescope attached to the telescope having low power and a wide field-of-view to help point a telescope. Without a finderscope, it is difficult to point a telescope to objects in the night sky. Although many telescopes are still sold with traditional  finderscopes, most people prefer the reflex finder for aiming a telescope scope because they are easier, faster and “natural” providing a red dot or bullseye projected onto the night sky (see Reflex Finder). A downfall of most traditional finderscopes is that the slight  magnification (5x to 10x) causes confusion matching the image to the night sky and many provide inverted images, which really adds to the confusion.

Focal Length 
The length it takes for a lens or mirror to come to a focus. Often measured and expressed by an f-ratio number like f/8 or f/10 (see f-ratio in the column to the left). You do need to know the actual focal length in millimeter to calculate the magnifications for eyepieces.

Focus Knob
A knob on the back end of the Schmidt-Cassegrain Telescope (SCT) or Maksutov (Mak) that is turned to focus the telescope. The 90° eyepiece holder on SCTs or Maks is in the center of the back and the focusing knob if off to its side.

Eyepieces are inserted into focusers which usually have two knobs that are turned to move the eyepieces either up or down for Newtonian reflectors, or in and out for refractors to focus the telescope. The preferred mechanism for focusers is a rack and pinion. Another type of focuser, called the Crayford (see Crayford) does not use gears but uses a roller that puts pressure on the side of the focus tube. 

Fork or Yoke Mount 
A type of mount that has two “arms” and the telescope is placed between the two arms so it can pivot or move up and down in altitude. Some fork or yoke mounts, like those used in some models by Celestron and Meade have a single arm but the two arm models are stronger (and more expensive). A fork mount can be found on both alt-az and equatorial mounts. However, they are not used on GEM mounts.

The arrow points to a traditional finderscope, a small refractor with a low magnification and large field-of-view, sometimes with crosshairs. One problem with these is that the view is usually upsidedown which can cause confusion when aiming the telescope. Most amateurs prefer a reflex-type finder that projects a dot or crosshair onto the night sky without any magnification.

The focusing knob on a Schmidt-Cassegrain Telescope (SCT). It is not always below the 90° eyepiece holder—often it is to the side.

An equatorial fork or yoke mount.

TRUE FIELD-OF-VIEW (TFOV) .  Here are three views through eyepieces with the same Apparent Field-of-View. All three eyepieces have the same optical design but different focal lengths to change the magnification. The image of the Orion Nebula on the left is at a lower magnification and the one on the right a higher magnification. So, the actual or TFOV decreases with the images going from left to right or as we increase the magnification. For another example, examine the two images of Orion at the left above the explanation of Apparent Field-of-View. The left image has a larger True Field-of-View than the right image.

APPARENT FIELD-OF-VIEW (AFOV) .  The optical design of eyepieces dictates the “window” that you get to view through. Both images of the Orion Nebula are at the same magnification. However, the image on the left, a view through an eyepiece, has a bigger or larger Apparent Field- of-View, that is, a bigger “window” that you get to look through. Eyepieces with large Apparent Fields-of-View cost more. Apparent Field-of-View is measured in degrees. Plössl eyepieces (Plössl is the name of an optical design for an eyepiece), the basic eyepieces, have an Apparent Field-of-View of 50° and the largest AFOV for 1.25 inch barrel diameter eyepieces is about 82°. The 82° are expensive but they are nice to look through.

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Abbreviation for German Equatorial Mount. 

German Equatorial Mount (GEM)
A popular telescope mount invented around 1823 that mimicks the movement of celestial objects. It has two axes. The “stationary” Polar axis points to a celestial pole which is close to Polaris in the northern hemisphere and allows east/west movement of the telescope. The other Declination axis is perpendicular to the polar axis and allow north/south movement of the telescope. The GEM is distinguish by a shaft with weights “opposite” the telescope. 

(Pronounced Go To) A mount that is motorized and computerized to find and follow any chosen celestial object. Usually, they have a hand controller to move the telescope and locate/track any object chosen from various catalogues stored in the hand controller. Many GOTO mounts need to be aligned to one to three stars for them to find and track objects—a process the hand controller walks you through. Usually, if you turn off or move the GOTO mount, you will have to align the mount again before it will auto find/track chosen objects from the hand controller. Almost always works on 12 volts DC.


The Great Dorpat Refractor, on one of the original German Equatorial Mounts made by Joseph von Fraunhofer.

The Schmidt-Cassegrain Telescope (SCT) often is configured as a GOTO telescope. It is the mount that is GOTO and not the actual telescope. The mount of this SCT is an alt-az fork mount with two arms. The hand controller is housed in one arm and can be detached but it is cabled to the mount.  ​This telescope has to be aligned to three stars before it will find and track objects selected from the hand controller. You do not have to know the names of the three stars—just pick and points the telescope to any three relatively bright stars spread across the night sky. The hand controller guides you through the process,

Hand Controller 
Something like a TV remote that allows control of the telescope/mount. Most hand controllers have a display and buttons to access catalogues of celestial objects including the planets and Moon. Most hand controllers are attached to the mount by a cable.

Light Shield
See Dew Shield 

Maksutov-Cassagrain Telescope (Mak)
A reflector telescope that is very similar in design and resembling the Schmidt-Cassegrain Telescope (SCT), however the front lens element is a meniscus (a single lens where one side is convex and the other is concave). Maks are popular in the 3 or 4-inch diameter size and are relatively inexpensive.


A Meniscus lens has one convex and one concave surface. 

Mirror Cell
For reflector telescopes, the “structure” that holds the primary mirror in place which generally has three adjustment screws/knobs to align the mirror to the optical system.

The shape of a meniscus.

A telescope mount allows the telescope to move and point to objects in the sky. The are two basic types, the alt-az and equatorial. The mount is usually attached to a pier or tripod. The Dobsonian type Newtonian Reflectors have an integral mount/stand.

Newtonian Reflector
Isaac Newton made the very first reflector in 1668 that was 2 inches in diameter and with a focal length of 6.25 inches. His design is still very popular today and was named after him.

Isaac Newton’s original reflector.

Usually refers to the front lens of a refractor telescope that brings light to a focus. However, it can also refer to the primary or main mirror of a Newtonian Reflector or Schmidt-Cassegrain Telescope (SCT).

An eyepiece design that was popular in the 1900s and at the time considered the premium eyepiece design. It is optically similar to the Plössl which is the entry level eyepiece of today. See Plössl for the limitations of this design.

The light path of the Newtonian Reflector. The primary mirror or objective is shown with its adjustable mirror cell and the three “screws” extending out the back that are used for collimation. The 45° diagonal mirror is in the center of the tube and below the eyepiece and focuser. The diagonal is suspended in the middle of the tube by a four vane “spider.” Diagonal mirrors usually have three “screws” for adjustment.

The German Equatorial Mount (GEM) is one type of equatorial mount that mimics the movement of celestial objects in the night sky. It is characterized by a weight or counterweight(s) on a counterweight shaft (red arrow). This counterweight shaft is part of the Declination axis that moves the telescope in Declination which is equivalent to latitude (yellow arrow). The Declination axis rotates about the Polar axis (blue arrow) which points to a celestial pole—close to Polaris in the northern hemisphere. The mount shown is a non-motorized manual GEM. It sports a dovetail clamp at the top to hold the telescope. For amateurs, GEM mounts are the preferred mount for astrophotography. For visual observation, the advantage of a manual GEM is that you just have to move/nudge the mount in one axis (Polar) to keep an object in view. 

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The preferred geometric shape of a Newtonian Reflector telescope mirror. You would think that the shape of this mirror should be a part of a perfect sphere, but a spherical shape does not focus all of its light to the same “point.” There is good focus at the center of images but fuzzy focus around the perimeter of the image. A parabola shape, which is deeper at its center than a spherical shape will focus better across an area of the image. 

A type of design for the lenses of a refractor that consists of two groups of two lenses each—a total of 4 lenses. One group is at the front of the telescope and the second group is farther down the tube. Provides excellent color correction. This type of lens arrangement is often referred to as apochromatic, indicating high quality imagery. 

The lenses of a Petzval refractor.

Usually a 6-inch to 10-inch or so diameter aluminum or steel tube about 4 feet tall that is often permanently anchored to the ground. The mount and telescope are attached to the top of the pier. There are pier type tripods (tri-piers) that are very common for telescope use—a central pier is supported by three legs. 

A tripod called a tri-pier.

The name of a popular optical design for an eyepiece (there is a whole family with many different focal lengths to change the magnification of the telescope) that can usually be purchased at a reasonable price. It provides excellent imagery with a “comfortable” field-of-view. The biggest downfall of the Plössl design (as well as the orthoscopic which is a similar design) is that the eye relief, that it, the distance you must place your eye above the eyepiece to see images gets very short as the focal length of the eyepiece decreases. For example you really have to press your eye against a 6 millimeter focal length Plössl (or orthoscopic) to look through it and this is not very comfortable. There are much more expensive eyepieces that provide greater eye reliefs and wider field-of-views but I do not recommend these more expensive eyepieces for first-time telescope buyers until they get some experience observing. The Apparent Field-of-View for Plössls is about 50°, a measure of the window you look through—see Field-of-View. 

The lens groups of a Plössl and Orthoscopic eyepieces.

Polar Axis
Refers to an equatorial mount. This is one of its two axes. The polar axis is aligned to and points to a celestial pole—near Polaris in the northern hemisphere. This axis, is in effect, stationary and the other axis, the declination axis, rotates around the polar axis.

Refractor telescopes attributed to Galileo. The big oval flourish below the scope houses his objective that he used to observe and write about this findings.

Usually refers to the main mirror in a reflector telescope that collects light and brings it to a focus. Reflector telescopes almost always incorporate a few mirrors to bring light to a convenient focus so “primary” is used to indicate the main mirror. 

Most often refers to screws (bolts) on the edge of a lens cell used for collimation. A lens cell holds the objective in place and a good one has three sets of two screws/bolts spaced 120 degrees apart that can be adjusted to align the focus point of the lens to the center of the telescope tube. One set of push-pull screws is two side-by-side bolts/screws that can be turned against “each other” to move and tighten/secure the position of the front lens in minute increments for alignment. You need to see one and play with it to get the best feel for the genius of such a simple “system.” You will also find push-pull systems used for Newtonian Telescope mirror cells but strong springs and wingnuts or knobs work best for adjusting this primary mirror. 

Rack and Pinion
Gears that are often used on focusers for telescopes, especially refractors and Newtonian Reflectors. The rack is a straight gear and the pinion is a round gear. See picture below. 

Reflector Telescope
A telescope that primarily uses a concave mirror to bring light to a focus for magnification. The most common are the Newtonian Reflector and the Schmidt-Cassegrain Telescope (SCT).

A Dobsonian-type Newtonian Reflector. The “stand” at the very bottom with the yellow “D” is the rocker box—an integral alt-az mount/tripod. 

Reflex Finder
A zero magnification pointing device used to aim telescopes that projects a red dot or reticle onto the night sky. The red dot or reticle is optically virtual and does not actually get projected beyond the device, like a laser beam. Because of its natural and ease of use, this type of finder is very popular and usually preferred over the traditional finderscope, which is a small low magnification refractor telescope. Small reflex sights are used on handguns and these can be modified for use as finders on telescopes. See Telrad.

Refractor Telescope
A telescope that uses a lens or lenses to focus light for magnification. This was the first type of telescope and was invented in 1608 using eyeglass lenses having magnifications of about 3x. Galileo quickly discovered how to increase the magnification. 

Right Ascension Axis or Polar Axis
The axis on an equatorial mount that points to a celestial pole which is near Polaris in the northern hemisphere. On motorized equatorial mounts, this axis moves at the same rate as the objects in the night sky, allowing equatorial mounts to “correctly” follow celestial objects. 

Rocker Box
A rocker box is the integral alt-az mount/stand that is specific to the Dobsonian Newtonian Reflector. The real rocker boxes are on large Dobsonians (12-inch diameter and larger) and are very low to the ground—see picture.

A parabolic mirror is deeper in the center than a spherical mirror. Additionally, a parabolic mirror will focus light to a single point but the geometry of a sphere does not allow it. As a note, all mirrors focus the different wavelengths or colors of light equally so there is no color or chromatic aberration with reflector telescopes.

The two “screws” that make up 1/3 of the push-pull system to collimate the front objective of a refractor telescope. The D155mm refers to an objective diameter of 6-inches.

A rack and pinion. Notice that the gears are slanted or helical which makes for a better meshing of the gears.

A small reflex-type finder that can fit on small telescopes. Notice the small red-dot in the right photo. It is easy and fast to use this type of finder to point the telescope.

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Schmidt-Cassegrain Telescope (SCT)
A reflector telescope that has a folded optical system to make it a very compact (short in length) unit. It utilizes a corrector plate up front that also seals the telescope from the environment. Its primary and secondary mirrors are spherical making them easy to manufacturer however the front corrector plate has an unusual shape to eliminate spherical aberration. The front corrector plate is not a lens because it does not focus light. 

Secondary (mirror) Generally refers to a second and smaller mirror in a reflector telescope that is use to further focus the optical system and/or redirect the light from the primary mirror to a more accessible location for focus. In Newtonian Reflectors the secondary mirror is a very flat piece of glass that is inside the telescope tube, below the focuser and redirects the light from the primary to the focuser. In a Schmidt-Cassegrain Telescope (SCT), the secondary is positioned in the center of and behind the front corrector plate reflecting light back to the primary mirror and out through a hole in the center of the primary mirror. 

Setting Circles
Divisional scales on both axes of equatorial mounts that can be used to locate objects using the celestial coordinate system. It is common to find setting circles on the cheapest and basically worthless German Equatorial Mounts (GEM). These two scales use the same increments that are used for plotting the position of all celestial objects—called Right Ascension (akin to longitude) and Declination (akin to latitude). However, even though you will find setting circles on most GEM mounts, no one uses them anymore to find objects (well, I do have a friend who likes to use them for his amusement). They were used much more to find celestial objects before the advent of motorized/computerized mounts. 

Setting circles on a German Equatorial Mount (GEM).

Spherical Aberration
An optical “defect” of spherical optical surfaces that can be present in both spherical telescope lenses and spherical telescope mirrors where the rays coming from the edges of the optical surface do not focus on the same plane as the rays from the center of the optical surfaces. 

Spherical Shape
Almost all lens elements used in astronomical telescopes and eyepieces are spherical in shape, some concave and some convex. Newtonian Reflector telescope mirrors have the shape of a paraboloid. 

A holder for the 45° tilted secondary mirror of a Newtonian Telescope that “sits” below the focuser. Generally, a spider has 3 or 4 arms or veins (sometimes curved) that span the inside of the telescope tube and suspend the diagonal mirror at the tube’s center. 

A three vane spider that holds the secondary mirror in a Newtonian Reflector telescope.

A very popular reflex finder that provide an illuminated target-type reticle for pointing the telescope. It has zero magnification. This is a very popular finder that is easy to use but it is large and cannot be attached to small telescopes. 

A three-legged stand that the telescope mount is attached to—the same design used for camera tripods but usually sturdier. 

True Field-of-View 
See Field-of-View

Tube Rings
Are used to attach a telescope to a mount. Usually the tube rings open to accept the telescope, allowing balancing and removal of the telescope. Some tube rings, usually used for finderscopes, have three adjusting screws that suspend the little scope in the middle of the rings. 

A platform that holds a fork mounted Schmidt-Cassegrain Telescope (SCT) that can be tilted to ones latitude creating an equatorial mount.

Yoke Mount
See Fork or Yoke Mount

A Schmidt-Cassegrain Telescope (SCT) on a wedge that changes the standard alt-az mount to an equatorial mount.

A one arm, computerized and motorized GOTO Schmidt-Cassegrain Telescope (SCT). It has a secondary mirror behind the front corrector plate.

The Telrad (top) is an exceptional reflex-type finder that provides a nice bullseye (bottom) for pointing a telescope. Unfortunately it is very big and cannot be attached/used on smaller telescopes.