Glossary of Telescope Terms:

Accessory Tray: Usually attached to the spreader for the tripod legs, this holds useful items while you are using the telescope.

(Actual) Field of view 'FOV' : The amount of sky visible in the eyepiece. Found by dividing AFOV by the magnification.

This is the amount of actual sky that the telescope shows. In the example this telescope shows just over the size of the Moon's disk, 35".

AFOV - Apparent Field of View: The width of the view in the eyepiece. This is not usually written on the eyepiece!

The Apparent width of the view when observed with the eye. In the example this appears to be about 40 degrees.  When you use an eyepiece you will notice different ones have different AFOVs, but that it depends on design and not necessarily the focal length of the eyepiece.

Alt-AZ, AZ Mounting: A simple 'up-down-left-right' mounting.  OK for low magnifications with wide angle telescopes.  Trickier to follow astronomical objects on high power than EQ mountings.  The Dobson mounting is an example of AZ.

In the example there is a Meade ETX70 on an AZ fork mounting. This is motorised and is used with a direction paddle to move the scope, however, being AZ it cannot easily follow the stars. (See Equatorial Wedge).

Apochromatic OG: A lens that has three elements to give superb colour correction.  Some are traditional glasses, some are ED glass. However, an ED scope is not necessarily apochromatic, usually only having two elements to the OG. (Another terminological inexactitude, used by greedy sellers to up the price of their telescope!)

AZ Mounting:  A mounting which operates in the Altitude (up-down) and Azimuthal (Left-Right) directions - Eg: Dobsonian.

Baffle: This takes the form of a tube, which stops stray light entering the eyepiece in a catadioptric telescope system. In a Maksutov, for example, you don't want light from the meniscus lens adding to the light from the spot, so there is a tube extending from the perforated primary to cut out this light.

Barlow lens: Used in conjunction with an eyepiece these optically multiply the telescope focal length and hence the magnification given by a particular eyepiece.  I would only recommend achromatic 2x Barlow lenses.

Bird-Jones Telescope: Not recommended. Using a spherically figured primary and a Barlow lens as a 'corrector', this is a poorly designed compromised optical system, best left alone!  Also called 'Catadioptric-Newtonian' telescopes.

Cassgrain Telescope:  A telescope type using two mirrors, one a perforated primary, to allow the light to focus behind. These are expensive telescopes with good capabilities, usually with high focal ratios.

When I bought this telescope, for £25, I thought it was a Schmidt-Cassegrain. However, on closer inspection and use, I noted that the front glass, that held the secondary, was plane, and not a corrector plate. This identified the telescope system as a sealed tube Cassegrain. It was a very nice instrument, very simply mounted.

Catadioptric Maksutov-Newtonian telescope: Design of telescope using a spherical main mirror and a meniscus corrector, but with a flat secondary and focuser at the side as in the Newtonian design. These are excellent telescopes, but very expensive!

Catadioptric-Newtonian telescope: Not recommended. Using a spherically figured primary and a Barlow lens as a 'corrector', this is a poorly designed compromised optical system, best left alone! 

Also called a 'Bird-jones' design.  You can distinguish one of these by looking at the physical tube length compeard to the optical focal length. In a Newtonian layout, if the latter is twice the former, it's a Bird-Jones! My advice is leave alone!

Cell: In any telescope, this is the device that holds the primary. In the Newtonian Telescope it holds the primary mirror and can be adjusted to allow precise collimation. 

Some expensive refractors have adjustable cells for the OG.  SCT and Maksutov telescopes are best adjusted by an expert - But these types shouldn't need adjusting from one decade to the next!

Counterweight: Used to off-set the weight of the telescope on a German type equatorial mounting.

The counterweight can adjust on the Dec balance bar to allow precise balance of the RA axis. This helps slow motions or motor to drive the RA axis precisely. It can also be adjusted, to off-set the weight of a DSLR camera, for example.

Dec Axis: This is the axis of an EQ mount that the telescop attaches to. This axis gives North and South movement along the lines of declination. (Also see: RA)

Dew Cap: A small shroud on the front of a refractor to stop dew forming on the cold glass of the Object Glass.

Dobson Mounting: A type of AZ mounting using simple plywood box sections. It is typified by having teflon pads to give low friction movement of the Azimuthal and Dec axes. The Az is two flat surfaces with a bolt for an axel, and the dec axes are made of a stout tube running in a cemicircular, teflon lubricated channel.

They are very smooth in opearton and sturdy giving many years faultless use.  It was designed by John Dobson as a simple, effective and cheap way of mounting very large Newtonian telescopes.

It is common for telescope manufacturers to call almost any altazimuth mounting a Dobson mounting. Not correct but very prevalent in the world of telescope sales!  The telescope in this pic is correctly described as Dobson mounted.

Dovetail: This is a metal wedge that allows easy attachment to its mounting. This can attach to the scope directly, allowing easy attachment to different mountings.

The dovetail bar can also attach to tube rings, into which fits the telescope tube allowing the telescope to be rotated during use which allows easy access to the eyepiece on a Newtonian or refractor.

Draw Tube: This is the tube into which the eyepice fits. It is moved, usually by geared focusing wheels, to allow focusing.

Drive Motor: A device that moves the RA axis to follow the sky.  More complicated ones have variable drive speeds.

DSLR Camera: A Digital Single Lens Reflex camera using a CCD to collect light instead of film. 

In this example, it is Gareth Jackson's Canon D600

ED Refractor:  Using "Extra-low Dispersion" glass, an object glass designed to give superior colour correction. (See: O.G.)

The example is a SkyWatcher 100ED-PRO. Which I bought from CashConverters for £89.  Look up the new price of a 100ED OTA if you dare!  (See my guide to buying second hand)

Equatorial Head: A device that automatically follows the rotation of the heavens, turning around the Right Ascension axis.

The example is a German Type Equatorial.  It is an EQ2 from SkyWatcher.

Equatorial Wedge: A device designed to attach a fork mounting to tripod at the correct angle to allow EQ observations.

Erecting Adapter: This can be an actual eyepiece or an aditional element containing a cheap lens.

They give erect images when used with Newtonians.  Also called an Erecting Eyepiece (It's not an eyepiece!), they can also be used with a refractor when placed before the diagonal in the optical train.

Eyepiece: This is the small lens that fits into the draw tube to present the image to the eye and magnify the image.

The focal length of the eyepiece is usually printed on the top or side. The view through the eyepiece gives the AFOV. The AFOV divided by the system Magnification gives the actual field of view in degrees.  There are many different designs containing different lens trains, including: Erfle, Galilean, Herschillian, Huygens, Kellner, Monocentric, Nagler, Orthoscopic, Plössl, and Ramsden.

Exit Pupil: The size of the column of light exiting the eyepiece.

The exit pupil size can be found by dividing the eyepiece focal length by the focal ratio of the primary.  You need to keep this figure to 7 or less for best results with your telescope, especially on DSOs.

Field of view 'FOV' : The amount of sky visible in the eyepiece. Found by dividing AFOV by the magnification.

This is the amount of actual sky that the telescope shows. In the example this telescope shows just over the size of the Moon's disk, 35".

Finder: A small, wide-field telescope attached to the main tube to allow easy location of astronomical objects.

For best results, and to be able to line up on objects too faint for the eye to see, an achromatic magnifying optical finder is best. In this example a straight through 6x30mm finder is seen in its mounting bracket. To be most effective, especially for DSO observations, your finder should have a diameter of at least 20% of the main scope. (Also see 'RDF')

NB: Telescopes up to 150mm can use 30mm finder. Over 150mm should use a 50mm.

Flat: 'The Flat', is a term used to describe the secondary mirror in a Newtonian telescope.  This mirror is perfectly flat and reflects all light rays equally from all parts of its surface. It is used to change the dirction of the cone of light from the primary mirror out through the side of the telescope tube.

The flat (secondary mirror) is carried by the spider in the tube and can be adjusted to precisely line up the optical centre of the telescope's optics.

Flexy Knobs: These are used to precisely operate the RA and Dec slow motions on a mounting to control precise RA and DEC movements.

They are flexy to cut down on vibrations and movement of the telescope when being used. They turn worm wheels or screws to minutely alter the viewing direction of the telescope allowing the observer to follow the object and offset the rotation of the Earth.

Focuser:  The device by which the draw tube is moved to allow the focusing of the telescope. Usually by turning small wheels.

Fork Mounting: A type of equatorial mounting which needs no counterweight. The scope is suspended between two arms.

GoTo Mounting:  A type of mounting (AZ or EQ) which uses motors to align the telescope on objects picked from a list on a computer or via an app on your mobile phone.

The example shown is the SkyWatcher 130AZ-GTI  It has its own WiFi and connects to the observer's mobile phone to control the telescope. This can be done by selecting objects off a list, or manually with direction buttons. Initial set up of the scope is also performed with the mobile app.

Guide Scope: Larger than a finder, this is an instrument designed to allow close following of the field stars in photography.

The telescope in the example was a 120mm OMNI Refractor from Celestron with a 70mm guide scope.  If you are tracking stars when taking a long exposure photograph, tou need to see faint objects clearly to keep them positioned acurately in the reticule.

Latitude Scale: Found on EQ mounts, this allows you to set your latitude of the EQ to align with the Earth's axis. To set up a German type EQ, set your latitude scale pointer to your location's latitude and sight polaris up the RA axis.

Maksutov or Maksutov-Cassegrain: A telescope type using lenses and mirrors to produce the image. They are expensive but very effective when made well. They are recommended for planetary and Lunar observation particularly. These are expensive telescopes with good capabilities, usually with high focal ratios.

Maksutov-Newtonian: A design of telescope using a corrector plate and spherical primary. These are very good quality, expensive telescopes.

Not to be confused with telescopes marketed as 'Catadipotric-Newtonians', which are generally of cheap and nasty, 'Bird-Jones' design.

Meniscus Lens: In a Maksutov or Maksutov/Cassegrain telescope the light first encounters a glass lens at the front of the tube. THis is a single piece of optical glass with two different radius curvs on the surfaces. The surface nearest the eyepiece is less curved than the front surface making the lens thicker at the rim than the center. As light passes through this meniscus lens it slightly modifies the direction of the light so that the spherical primary will focus it correctly.  The secondary mirror of a Maksutov is a silvered 'spot' on the back surface of the meniscus.  A very clever design and an effective planetary telescope.

Mounting: This is the means by which the telescope is supported and directed to view objects. There are many kinds. The most commonly found are: the German Equatorial, The simple AZ, the Dobsonian AZ and the Fork.

Motor Drive: A device that moves the RA axis to follow the sky.  More complicated ones have variable drive speeds.

Newtonian Telescope: Using a parabolic primary and flat mirror to direct the light out of the side of the tube to the eyepiece. These are usually low focal ratio telescopes, ideal for DSOs. The Dobsonian Telescope employs the Newtonian optical layout.

Object Glass, OG: This is the large lens at the front of a refractor. It collects and focuses the light that is examined by the eyepiece. In achromatic telescopes it comprises a doublet lens made of crown glass and flint glass. ED scopes have doublets made of Extra-Low Dispersion glass. Apochromatic telescopes have a three element triplet lens, for superb colour correction. Beware that some telescope sellers refer to all ED telescopes as Apochromatic (Strictly speaking, they are not).

Optical Finder: A small, wide-field telescope attached to the main tube to allow easy location of astronomical objects.

For best results, and to be able to line up on objects too faint for the eye to see, an achromatic magnifying optical finder is best. In this example a straight through 9x50mm finder is seen in its mounting bracket. To be most effective, especially for DSO observations, your finder should have a diameter of at least 20% of the main scope. (Also see 'RDF')

NB: Telescopes up to 150mm can use 30mm finder. Over 150mm should use a 50mm.

Pillar: A stout tube (sometimes on tripod legs and sometimes filled with concrete set into the floor) upon which the mounting head is mounted. More solid, and sturdy than a tripod, permanent pillars are sunk into the ground and remains separate from the floor of an observatory to avoid vibrations.

PolarScope:  This is a small telescope with a special reticule that is used to align the mounting precisely, using the pole star in the northern hemisphere, and stars of Octans, in the southern.

You use a polar scope by sighting Polaris or the Octans stars through it. There is a ring upon which you position the star at the correct angle for the time of year and night. For Polaris, the ring is 3/4 of a degree away from the actual pole, and so the EQ head is correctly offset, when used correctly.

Primary Mirror: The mirror of a reflector. This must be parabolic in a Newtonian for properly focused images.

In the example is a 114mm parabolic mirror of f4.4 and is fitted into its cell.  SCTs and the various Maksutov designs have spherical mirrors and corrector plates. The primary mirror  focuses the light that is examined by the eyepiece.

R.A. Axis: This is the axis that aligns with the pole to allow the telescope to follow the astronomical objects east to west along the lines of Right Ascention.

Point the RA axis directly at Polaris to set up your EQ, then use the other movements to follow your object.  (Also See: Dec axis).

Refractor: A telescope which uses a large achromatic Object Glass to collect and focus light which is examined by an eyepiece.

RDF (Red Dot Finder): Used to sight bright objects to align a GoTo mount.

Unfortunately these are increasingly used as a main telescope finder as a cheap option on many telescopes!  They can't show faint objects at all. Optical finders are recommended.

Note to manufacturers: If you supplied proper achromatic optical finders, you may have to charge a little more, but your customers would respect the fact that you haven't given them a cheap ans ineffective alternative just to get their money!

Right Angle Finder: An optical finder with a built-in 90 degree mirror.

Particularly useful on Newtonians and Maksutov telescopes to get the eyepiece of the finder more easily accessible to the observer.  On the Newtonian, for example, you can arrange the eyepieces of both to be viewable from the observing stool, so there is no getting up and moving about in the dark required!

Rotation of Field: This is seen as the objects are viewed as they cross the sky in an altazimuth mounted telescope.  Your head, in simple terms, is Alt-Az mounted. When you look at the Moon as it rises it is one way up, and as it sets it's another.  In the equatorial telescope the astronomical object is seen with North at the bottom (usually, but can be South), no matter where is is in the sky.

If you use a diagonal the view can change as you move the diagonal angle in the drawtube, but for the purposes of photography you wouldn't move the diagonal and the field would not rotate as the object moves in the sky.

SCT / Schmidt-Cassegrain: A type of catadioptric telescope which has a complicated 'corrector plate' at the front and a spherically figured primary. 

The convex secondary mirror is in a cell in the center of the corrctor plate. These are expensive telescopes with good capabilities, usually with high focal ratios between f11 and f15.  The example is a Celestron C8 and has a focal ratio of f11.

The design gets its name from combining the optical corrector plate and spherical main mirror of the Schmidt camera, with the optical layout of the Cassegrain telescope. The telescope therefore requires a spherical main mirror, which is much cheaper, and easier to make accurately, instead of a much more complicated figure.

Secondary Mirror: This is a flat mirror that directs the cone of light out through the side of a Newtonian. (see: Flat)

In an SCT it is a convex mirror in a cell that extends the focal length of the primary. A similar arrangement is found in the Maksutov telescope, except that it is a silvered part of the meniscus lens and has no cell of its own.

Slow Motions:  The mechanism by which the telescope can pe precisely moved.

Using gears, the worm wheel or screw connected to the various axes of the mounting head is turned by a flexy knob, or motor, to move the telescope to follow an object by turning the mounting head around the RA axis at siderial rate. Other rates are used for objects other than stars. There is also a slow motion control on the Dec' axis for centering of the object and adjustment of the view.

Spider: This device holds the secondary mirror in the Newtonian tube and has adjustments to allow precise alignment.  There are several different designs featuring three or four vanes, single pillar type support, or interlocking rings. All designs hold the secondary mirror in the correct position and allow for adjustment.

Stirrup Mounting:  Best avoided. Not a very good design which can be troublesome - Also, always AZ in operation.

Stop: The stop is a circle of metal with a circular hole cut in it. It 'stops' internal reflections from the tube in a refractor telescope. 

Actually, these are more correctly called baffles as a stop is properly used to reduce a lens' full diameter being used. In a quality refractor there are at least two 'stops', with precisely sized holes spaced within the tube. The example in this illustration is a SkyWatcher 120 and has two stops. 

NB: In a cheap finder, you may find that they have used a stop of about 50% just behind the 24mm lens (Making it about 15mm) to reduce chromatic abberation by increasing the focal ratio. It also reduces light gathering and usefulness of the finder! It is always best to use an achromatic finder.

Telrad: this is an unaided eye estimation device, similar to an RDF. With all the same drawbacks, but costing even more money to buy!

Tripod:  A three legged device which supports the head and telescope during use. It may be height adjustible and made of wood or metal.

We use three legs as it is the most stable arrangement of suppport legs. By adjustiing the length of the legs it is always possible to arrange the telescope mounting base to be properly horizontal and the head to function correctly.

Tube:  The structure that holds the optics in the correct relative positions. Any shape!

It is called a 'tube', even if it is not solid or tubular! Telescope 'tubes' can be tubular, square section, hexagonal, lattice, skeleton or 'flextube' using rods. They all do the same job of holding the optical elements in the correct relative positions.

Tube Rings: Metal rings that attach to the mounting. These allow the telescope to be rotated during use for eyepiece position.

Wedge: A device designed to attach a fork mounting to tripod at the correct angle to allow EQ observations. Properly termed 'equatorial wedge'.