Glossary
This page lists some of the more common terms regarding telescopes, telescope building, and astrophotography. I have tried to give a brief definition of the terms within the context they are used in these web pages. There may be other or more correct definitions.KIS
Keep it Simple (KIS). The best all around design strategy. The simpler a thing is, the better it generally works. This is because there is just less to go wrong.Accessory Terms
Auto Focuser
A motorized focuser that adjusts the position of the eyepiece while reducing vibrations from manual focusing.Cheshire Eyepiece
A collimation tube with a 45 degree reflector (white or silvered) that reflects light onto the primary mirror to allow adjustment of the primary mirror orientation.Combination Tube
A sight tube with a cheshire type reflector in the peep hole end.Dew Heater
An electrical heater that prevents dewing of optical components. Dew heaters use resistance wire or resistors to heat mirrors, eyepieces, or other components.Sight Tube
A simple collimation tube with a peep hole in one end that is used to center the secondary mirror in the telescope draw tube.Astrophotography Terms
Autoguider
An automated system for tracking an object during astrophotography. A guidestar is selected and centered in a camera connected to a guide scope or off axis guider. A computer makes corrections to the telescope mount to keep the guide star at a constant position in the digital camera frame.ccd Camera
A ccd (charge couple device) camera uses a light sensitive chip to convert light into an electric signal; this technology is used in consumer digital cameras and camcorders.DSI Pro
The DSI (Deep Sky Imager) Pro is a monochrome ccd camera produced my Meade Instruments.Field Derotator
A motor that compensates for field rotation by rotating an astronomical camera. The field derotator rotates the camera at the same rate as a celestial object rotates, negating field rotation.Field Rotation
The rotation of celestial objects during tracking with an Alt-Az mounted telescope.Guide Scope
A small telescope mounted onto the main telescope. The guide scope is orientated to point an illuminated crosshair (reticle) at a predetermined star during imaging. The stars location in the guidescope is periodically checked and the main telescope is repositioned to keep the guidescope on the same star. This (guiding) corrects for small drive errors and keeps the image correctly centered in the ccd camera during imaging.Guiding
Making small adjustments to keep the telescope centered on a celestial object during astrophotography. Guiding corrects for the small errors that all telescope mounts and polar alignments contain.Illuminated Reticle Eyepiece
A eyepiece with an illuminated reticle (crosshair) that is used with a guide scope. These eyepieces are also used in aligning equatorial mounts to the celestial pole.RGB Filter Set
A set of red, green, and blue filters that are used with a monochrome ccd camera to produce a color image. Multiple pictures (monochrome + one picture through each color filter) are acquired and digitally merged to form a color photo. Since many monochrome ccd cameras are more sensitive than color ccd cameras, there are sensitivity advantages to this type system.Back To Top
Drive and Gear Terms
Backlash
A cumulative error in gear systems occurring when the gears change direction. Backlash occurs because when the gears change direction, the teeth must move a small distance before they engage one another. Over time, backlash error accumulates and can affect GoTo pointing accuracy.Band Clamp Gear
A worm wheel formed by wrapping a band clamp around a cylinder or disk.Friction Drive
A drive system that transfers rotational motion by frictional contact between multiple wheels.Gear Reduction
Using a series of different sized gears, shafts, wheels, or other components to reduce the rate of rotation between elements in a drive train.Hob
A rotating cutting tool used to cut the teeth in a worm wheel.Hobbing Gears
Using a rotating hob to cut a worm wheel from a rotating metal or plastic gear blank.Moulded Gear
A gear produced by pressing a threaded rod into a wheel with a center groove filled with resin or polymer. The resin or polymer hardens to produce molded gear teeth.Shaft Coupler
A device that couples two rotating shafts of equal or different diameter and transfers torque.Spur Gear
A gear with teeth around it's circumference that meshes with similar gearing on another device and transfers rotation.Threaded Rod Gear
A worm wheel formed by wrapping a threaded rod around the circumference of a circle.Worm
A gear formed in the shape of a screw that transfers rotation to a worm wheel.Worm Drive
A drive system consisting of a worm transferring rotation to a worm wheel, where both drive axis are at 90 degrees relative to each other.Worm Wheel
A gear with the same thread as a matching worm.Back To Top
GoTo System Terms
GoTo
A computerized system employing motors, drive gears, and computer programming where an object is selected from a database and the telescope will automatically GoTo the selected object.Meade 492 Motor Kit
A system manufactured by Mead Instruments that includes two drive motors, 60 tooth worm wheels, matching worms, mounting assemblies, computer interface board, and simple hand controller. This system, supplied with the simple hand controller, allows manual control of the telescope drive motors. A version 495 or 497 Autostar is required for full GoTo control.Meade Autostar
A system manufactured by Mead Instruments for automated telescope positioning and tracking of celestial objects. The Autostar hand controller contains necessary programming to construct a mathematical model relating telescope position to celestial coordinates. The latest version (497 Autostar controller) can be updated via the www.Back To Top
Observatory Terms
Arch Support
Small wood blocks that connect the steel drywall frame arches to the dome ring.Base Ring
A three layer plywood ring that sits on top of the base structure. The dome ring sits on top of bearings attached to the observatory base structure or base ring (depending upon the design).Base Structure
The observatory structure that supports the dome at the same height as the telescope mount altitude bearings. The dome ring sits on top of bearings attached to the observatory base structure or base ring (depending upon the design).Birdcage Dome
An observatory dome frame consisting of two central arches and usually 10 peripheral arches. The arches are usually constructed from 2 or 3 layers of thick plywood. This type of dome frame resembles a birdcage.Bitumen
The left over tar fraction from distillation of crude oil. Bitumen is used for paving roads and in waterproofing materials.Bituwell Plates
Bitumen impregnated sheets of corrugated inorganic-organic fibers, mainly used in Europe for carport and shed roofs. Bituwell plates are very strong yet lightweight (5.4 kg per 0.93 m x 2 m x 3 mm sheet).Central Arch
Two birdcage dome structural elements that form the observatory dome slit. The two central arches are chords across the base ring, and are less than the base ring diameter.Dome Centering Bearings
Bearings contacting the inside and top surfaces of the dome ring, preventing the dome from moving side-to-side or lifting off of the base structure as it's rotated.Dome Bearing
Bearings mounted on top of the base structure or base ring. The dome ring rotates on top of the dome bearings.Domed Observatory
An observatory consisting of a rotating dome placed on top of a base structure. These observatories are more difficult to build than roll off observatories, but give better wind protection.Dome Ring
The mulitlayered plywood ring that supports the dome. The central and peripheral arches attach to the top of the dome ring. The dome ring underside rotates on bearings attached to the observatory base structure or base ring (depending upon the design).Dome Slit Cover
A removable door that covers the dome slit openingDome Slit Opening
The opening between the central arches allowing light into the telescope.Foam Geodesic Dome
An observatory dome design consisting of a geodesic structure of styrofoam triangles (or other shapes), covered with fiberglass or some other structural, weather resistant material.Peripheral Arch
Birdcage dome structural elements that start on the dome ring, extend toward the dome ring center, and connect with the central arches. There are usually 5 peripheral arches equally spaced along each central arch.Pier
The structure that supports the telescope mount. The pier extends down through the observatory floor without making contact with the floor or observatory structure; this reduces vibrations to the telescope from dome rotation.Rain Skirts
Exterior sheets or strips that cover the gap between the observatory dome ring and the base structure, reducing drafts and makes the observatory weathertight.Roll Off Observatory
An observatory consisting of a simple building with a roof that rolls off on tracks. These observatories are generally easier to build than domed observatories, but give less wind protection than domes.Steel Drywall Supports
3 sided bars of 1 mm sheet steel, used in the construction of non-load bearing interior walls. These supports often contain pressed channels and/or folded lips that can greatly increase their strength without adding significant weight.Telescope Terms
Achromatic
Lens
A lens consisting of two different lenses (crown glass and flint glass)
that converges red and blue light to the same focal point; other
wavelenghts
(green) will still give some degree of chromatic
aberration.
Aperture
The diameter of an astronomical mirror or lensApochromatic
Lens
A lens consisting of two or more different optical materials,
which converge red, blue, and green light to the same focal point.
C8
An 8 inch aperature ƒ10 Schmidt-Cassegrain telescope (SCT) manufactured by Celestron International. The C8 was introduced in 1970 and was the first high volume-low cost SCT.C90
A 90mm Maksutov-Cassegrain telescope manufactured by Celestron International.Catadioptic Telescope
A telescope type containing both lenses and mirrors. Catadioptic telescope designs offer advantages over reflector telescopes due to shorter tube length and reduced optical aberrations.Chromatic Aberration
Chromatic aberration is when different frequencies (colors) of light converge at different focal lengths and is caused by a len's refractive index deecreasing as the light wavelength increases.Collimation
Adjusting the orientation of the primary and secondary mirrors to give the best possible image.Collimating Screws
Screws on the secondary mirror holder that adjust the secondary mirror's orientation (alignment) with respect to the primary mirror.Comatic Aberration
Comatic aberration (coma) is a property of parabolic mirrors where point sources of light are focussed at different places depending on their distance from the mirror's central axis. This causes stars in the center of the mirror's field to be in focus where stars become more out of focus toward the edge of the mirror's field.Comatic Circle
A circle formed in the optical plane when an off axis point of light strikes a lens or mirror with comatic aberration. Overlapping comatic circles form a comet or cone like image.Concave
Curving inward.Convex
Curving outward.Corrector Plate
A lens placed in the entrance to a telescope with a spherical primary mirror to correct for comatic aberration.Diffraction Spikes
Spikes or points formed on star images, arising from diffraction of light around the secondary mirror spider vane assembly.Dobsonian Telescope
A design attributed to John Dobson, which is a large Newtonian telescope on a very inexpensive but sturdy Alt-Az type mount.Draw Tube
A tube that holds the eyepiece and can be rasied or lowered relative to the secondary mirror. Most draw tubes are 0.96, 1.25, or 2 inch. inner diameter.Eyepiece
The lens that magnifies the image formed at the telescope focal point.Focal Length
The distance from the surface of a parabolic telescope mirror to where the image is formed. Note that for a thin lens the definition is slightly different.Focal Plane
An object located an infinite distance from an optical system forms an image on the rear focal plane. If the object is not at infinity, the image is formed at a different location.Focal Point
Convex lenses and concave mirrors focus light to converge at a focal point that lies at the focal length from the lens or mirror.Magnification
The eyepiece enlarges the image formed at telescope focal point. The magnification equals the telescope mirror or lens focal length divided by the eyepiece focal length.Maksutov Telescope
A telescope type invented in 1941 by Dimitri Maksutov. The Maksutov (MAK) telescope is a catadioptic telescope containing a spherical primary mirror, a spherical secondary mirror, and a spherical meniscus corrector plate.Meniscus Corrector Plate
A spherical meniscus shaped corrector plate, found on Maksutov-Cassegrain telescopes. The corrector plate is located on the entrance to a telescope with a spherical primary mirror to correct for comatic aberration.Mirror Box
The truss tube telescope sections containing the mirror cells and mirrors, connected together with truss tubes.Mirror Spot
A small secondary mirror attached directly to the back of the corrector plate in a Maksutov telescope.Newtonian Telescope
A reflector telescope invented by Isaac Newton that consists of a parabolic primary mirror and a flat secondary mirror.Objective Lens
The large front lens in a refractor telescope, that gathers light and focuses it to the focal point.OTA
Optical Tube Assembly. Just another term for a telescope.Parabolic Mirror
A mirror with a parabolic cross section that focuses light to a common focal point.Porro Prism
A device containing two right triangular glass blocks that uses total internal reflection to reinvert an upside down terrestrial image.Primary Mirror
The large mirror in a telescope that gathers light and focuses it to an image.Primary Mirror Cell
A device that holds the primary mirror and has adjustment screws to change the mirrors orientation (collimation).Reflector Telescope
A telescope which uses mirrors to collect and focus light.Refractor Telescope
A telescope which uses lenses to collect and focus light.Schmidt-Cassegrain
The Schmidt-Cassegrain telescope (SCT) consists of a spherical primary mirror, convex secondary mirror, and a Schmidt corrector plate (thicker in the middle and edge) to correct spherical aberration.Schmidt Corrector Plate
The Schmidt corrector plate is a lens that corrects incomming light so that light striking the outer portion of a spherical primary mirror focuses onto the same spot as light striking the inner portion. The Schmidt corrector plate is thicker at the edges and center., and flat on the side facing the primary spherical mirror.Secondary Mirror
The small mirror that reflects the light gathered by the primary mirror out of the optical tube assembly and into the eyepiece.Secondary Mirror Cell
A device that holds the secondary mirror and has adjustment screws to change the mirrors orientation (collimation).Sector Box
A box that connects the two sectors (altitude axis bearings) to the OTA.Spherical Aberration
Spherical aberration is a property of spherical mirrors where light striking the mirror's center is focused farther from the mirror than light striking near the edge.Spherical Mirror
A mirror with a circular cross section that focuses light to different focal points depending on where the light strikes the mirror. Light striking the mirror's center is focused farther from the mirror than light striking near the edge (spherical aberration).Spider
A device consisting of vanes or tensioned wires that holds the secondary cell centered in the OTA.Telrad Zero Pointing Device
A positioning or finder system that does not magnify or invert the image. The Telrad uses a system similar to a military heads up display to project illuminated circles onto a clear plate, through which the sky is viewed.Truss Tube
A series of tubes forming a cage assembly that holds the primary and secondary mirrors.Truss Tube Connector
A device that connects the truss tubes to the telescope mirror boxes.Back To Top
Telescope Mount Terms
Altitude
The rotational axis that elevates or depresses a telescope (up-and-down direction)Alt-Az
A telescope mount that allows the telescope to move up-and-down and side-to-side. The type of mounts typically found in toy telescopes.Azimuth
The rotational axis that moves the telescope parallel to the ground (side-to-side direction)Bearings
A coupling device allowing two components to rotate relative to each
other.Celestial
Equator
The intersection of the plane containing the Earth's equator with the
celestial sphere. Declination coordinates are measured in degrees above
or below the celestial equator.
Celestial Sphere
An imaginary non-rotating sphere, centered on the Earth and
its axis, that contains all celestial objects. As the
Earth rotates within the celestial sphere (W. to E.), an observer on
the Earth will see celestial objects move across the sky on the inside
of this sphere (E. to W.).
OTA Centering Bearings
A bearing placed on each side the telescope mount that keeps the telescope centered in the altitude axis. This prevents the telescope from moving side-to-side as it's elevated or depressed.
