A

Achromat
Sort of refractor, with primary lens corrected for 2 wavelengths. Most common refractor; suffers from color bleeding with very bright objects (due to dispersion).

Adhese, to
To join 2 pieces of glass by heat treatment without distortion.

Airy disk
Center spot of the diffraction pattern caused by a point-like source (star).

Alignment
Adjusting the optical axis such that it is coincident with the mechanical axis of the tube.

Aloxite
A mount on which one axis moves up and down (altitude) and the other axis moves around to the left and to the right (azimuth).

Aluminizing
Putting an aluminum coating on the optical surface of a mirror, so that it reflects more light.

Annealing
Thermal treatment of materials to alleviate or even totally eliminate internal stresses. The material is heated at a relatively high temperature at which material diffusion occurs at a reasonable speed. The material is held at that temperature for a certain time, and then is cooled very slowly, to avoid material stresses being reintroduced.

Aperture
Diameter of the light inlet of a telescope.

Appleply
Sort of plywood (strong and lightweight).

APO
Apochromat.

Apochromat
Sort of refractor, with primary lens corrected for 3 wavelengths. Very expensive, but marvelous images.

ATM
Amateur Telescope Making, Amateur Telescope Maker

B

Backlash
When you rotate an axis of a gear box, then you will often experiment that when you invert the rotation direction, the output axis does not change immediately its rotation direction. There is some play on the system, called backlash. Every mechanic system has backlash, but it only becomes a problem when it becomes noticeable.

Backpacking scope
A telescope that can be "carried on your back", thus a portable telescope.

Baffle
Sort of diaphragm in the light path, used for enhancing the image contrast by reducing the incidence of unwanted scattered light.

Barlow
Optical interface that artificially lengthens the focal length of the primary; by doing so the barlow interface increases the magnifying power of the eyepiece. Common barlow interfaces multiply the focal length of the primary by 2 or 3, resulting in 2 or 3 times more power with the same eyepiece.

Barn door mount
Sort of telescope mount, mostly used for photographic cameras.

Bartelize, to
This verb refers to Mel Bartels' motorized Dobson, and means motorize the alt-azimuth telescope with steppers and a laptop.

Bearing
Interface that allows much smoother rotation of an axis (e.g.: roller bearing)

Beral coating
Sort of high reflective, dielectric coating used on mirrors, with very low reflection losses (US brand name)

Beveled edge
Reason 1: To reduce the stress on the edge of a mirror, beveling of the edge is important. With a beveled edge you reduce the risk of breaking chips of glass, if the mirror touches/hits a harder material (e.g. the mirror cell) Reason 2: Sharp edges (which are normal in blank production) are dangerous for the hands of the ATM'er.

/^^--..__..--^^\ - Surface of Mirror
! !
\______________/ - Back of Mirror The slash and the backslash represent the "Beveled edge".

Blueing
Coating a lens with an anti-reflective coating. Since the reflective losses are minimized at about 550 nanometer (wavelength of green light to which the human eye is the most sensitive), the coated lens appears to be bluish, since the green part of the light is not reflected anymore and hence disappears, leaving only the complementary color(s).

C

CAT
1. Catadioptric. 2. Computer Aided Telescope.

Catadioptric
Telescope that combines reflection and light bending (refraction), like Schmidt-Cassegrains, Maksutovs.

Cell
System that supports a mirror or a lens.

Coating
Layer of metal or dielectric that is applied on a surface. Most important coatings are optical coatings. Lens coatings help minimizing the reflection losses at the surfaces, while mirror coatings have enhanced reflective properties and also protect the reflective film against corrosion (as a SiO2 coating atop of a reflective Al coating on mirrors).

Coil
Spring made of a wire put in a coil (like a wine-opener)

Collimation
Adjusting all optical elements such that they share a single optical axis.

Computer Aided Telescope
Telescope that is driven with the help of a computer. Examples are stepper motor driven Alt-Azimuth mounts.

Cradle
The part of a mount, special on GEM's, to rest the tube.

Crayford focuser
Sort of focuser which is based on pure friction. Very often this focuser design has very little to no backlash.

CT
Cassegrain Telescope (without corrector plate). A reflector.

D

D'Autumne platform
Sort of platform on which you set an alt-az mounted scope, so that it becomes equatorial. Due to constructional constraints however, the platform can only track for a certain time (up to 2-3 hours max).

Derotator
Engine that compensates for field rotation.

Diagonal
Elliptical secondary mirror tilted at 45 degrees, between the eyepiece and the primary in e.g. Newtonians.

Diagonal holder
Support to keep the diagonal in place. It is often attached to a spider.

Dillpickel telescope
An all spherical catadoptric newtonian.

Dispersion
The fact that light with different wavelengths is refracted (bent) differently. Examples where dispersion is shown: light decomposition in a prism.

Dob
Contraction of "Dobsonian".

Dobsonian
Term that denotes both the Dobsonian mount (see Dobson, John) and the whole telescope assembly.

Dobsonian mount
Very stable alt-azimuth mount with huge bearings.

Dobson, John
Creator of the popular Dobsonian telescope: a very stable alt-azimuth mount with huge bearings. John Dobson's intention was to build a telescope with "off the shelf" parts at low expenses.

E

EP
Abbreviation meaning "Eyepiece".

Entrance pupil
Optical aperture of an optical system (telescope, eyepiece, binoculars, . . .), at which the light enters. Equatorial mount
Most common type of telescope mount (until the Dobson revolution): since one axis is parallel to the rotational axis of the Earth (this axis points to Polaris (Northern hemisphere) or sigma Octantis (Southern hemisphere)), you can follow an object by just counteracting the rotation of the earth.

Erfle
A wide-field eyepiece.

Exit pupil
Optical aperture of an optical system (telescope, eyepiece, binoculars, . . .), at which the light exits.

Eyepiece
Set of lenses the eye will look through.

Eye
Distance between the eye and the eyepiece.

F

Feathers
Feather shaped defects within transparent materials. The term is applied also to collections of bubbles or flecks.

Field rotation
Rotation of the field of view in your eyepiece (or on your CCD or camera), due to misalignment of the polar axis in an equatorial mount, or in general due to the fact that your mount is NOT equatorial. It is easy to visualize when you imagine that Saturn rises, culminates and afterwards sets, but during that motion the orientation of the rings of Saturn change (they follow the ecliptic): tilted to the left at rise time, then horizontal at culmination, then tilted to the right at set time.

Finder, Finder scope
Small telescope on the side of a bigger one, with a wider field of view, used to help find the object one wants to observe.

Fine grinding
The stage of grinding after rough grinding in which you improve your sphere, remove pits, and prepare for polishing. You start with fairly coarse grits like 220 and proceed one by one to the finest grits (5 micron aluminum oxide or so) during this stage.

First Light
Emotional moment, when the brand new telescope will be seeing the first star light.

Focal length
The length at which parallel rays of light are brought into focus for a particular lens or mirror.

Focal ratio, f-ratio, f-number
The ratio F/D of the focal length F and diameter D of a mirror or lens.

Focus
Point at which a parallel incoming beam is converged to after contact with an optical system.

Focuser
Part that allows the image to be seen sharply.

G

GEM
Abbreviation of "German Equatorial Mount".

Glare shield
Sort of light shield that is often seen in nose cones of (larger) dobsonians, opposite to the focuser. This shield blocks eventual light rays that are not originated from the primary via the diagonal; by doing so the contrast is enhanced.

H

H
Acronym for "Huygens".

Helical Focuser
Focuser that focuses by rotating the eyepiece holder (on which there is a thread).

HM
Acronym for "Huygens-Mittenzwey".

Huygens
Brand of eyepieces. The first ones ever made, today only used for solar observations in one single light frequency (thus in combination with solar filters placed in front of the telescope (not in the eyepiece).

Huygens-Mittenzwey
Brand of eyepieces. An enhanced version of the Huygens eyepieces. Often a very cheap eyepiece found in some cheap telescopes.

I

J

K

L

M

MA

Acronym for "Modified Achromat", a brand of eyepieces of Meade.

Minor axis
The minor axis is the smallest diameter of an ellipse. With diagonals in Newtonians, the m.a. is also the diameter of the obstruction caused by the diagonal (circular appearance of the diagonal when tilted at 45°).

Microfacet
Small "facets" added to a pitch lap by pressing a netting (plastic window screening, vegetable-bag netting, bath-type scrubby, etc.) material into the lap.

Mirror box
Box-like part of atruss tube Dobsonian, in which the mirror is mounted. It rests in the rocker box.

Monocentric eyepiece
Sort of Eyepiece, in witch all radii of the lenses fall on the same point, like the "Steinheil"-Type

MOT
Mirror On Top.

Mount
Construction that supports the telescope.

N

Nagler
Brand of eyepieces with a very wide apparent field of view (82°). Brand name from TeleVue. Excellent eyepiece.

Newt
Contraction of "Newtonian telescope".

Newtonian Telescope
A reflecting telescope that uses a large mirror (the primary) which reflects light to a smaller mirror (the secondary) which reflects light out the side of the tube to the eyepiece.

Nose cone
Part of a truss tube dobsonian where you find the focuser and spider (plus diagonal).

O

Objective
Lens (or mirror) which receives the light from the object at which we point the telescope.

Orthoscopic
Brand of eyepieces corrected for 1st order aberrations. Also called "Abbe eyepiece" or "aplanar eyepiece", due to the design. Good eyepiece.

OTA
Optical Tube Assembly (the tube part of the telescope, without mount).

P

PanOptic
Brand of eyepieces with a large field of view (68°). Brand of TeleVue. Almost comparable to the Nagler eyepieces.

PEC
Acronym for Periodic Error Correction.

Periodic Error Correction
Gears are repetitive systems (one tooth of a gear returns to its initial state after every revolution). Since most gears are not perfect, they induce errors, that come back with a repetitive pattern (repeats after every revolution). So does a system built with gears, like a gear box. Then you will have to take into account all gears.

Plossl
Brand of eyepieces. Consists of two doublets, modified Plossls also incorporate an intermediate lens in between the doublets. Very good eyepiece; it seems that this design will become the all-round eyepiece.

Polishing
Last action in making a mirror. The polishing consists in the smoothening of the figure of the mirror by means of a very fine grit.

Poncet platform
Sort of platform used to guide an alt-az mounted scope (mostly a Dob) for a certain amount of time.

Primary
Contraction of "primary mirror" - Largest mirror, used as a converging lens in reflectors. Syn.: objective.

Contraction of "Peak-to-Valley" error of an optical surface. This is the maximal error.

Pyrex
A type of glass with a lower coefficient of expansion (also harder) than plate glass. Generally colorless.

Q

R

Rack and Pinion Focuser
Focuser with a sliding eyepiece holder on which a pinion is mounted. Focusing is achieved by turning on a knob.

Reflector
Telescope with a primary that reflects (acts like a mirror) the incoming light, like Newtonians, Cassegrains.

Refractor
telescope with a primary that bends (refracts, like a lens) the incoming light, thus where the light passes through the optical system (glass, ...). Due to dispersion, a refractor needs to be corrected for color shift: see achromats and apochromats.

RFT
Acronym for "Rich Field Telescope". This is a kind of telescope used with low power in order to obtain a large field of view, often exceeding 1°.

Rifle Scope
Small finder which is often mounted on a rifle for aiming. Has a very big eye relief and oversized exit pupil.

ROC
Radius of curvature (of the mirror). The ROC is defined as the radius of the spherical surface of the mirror. It is about twice the focal length.

Rocker box
Box-like part of a Dobsonian in which rests the mirror box. The rocker box rests on the ground board.

RTMC
Riverside Telescope Maker's Conference, an annual event held over Memorial Day weekend. (USA)

S

Sagitta
The vertical depth of the curve of a spherical mirror. Lay your mirror on its back. Carefully lay a ruler across its diameter. The distance, at the center of the mirror, from the ruler to the mirror surface is the sagitta.

SCT
Schmidt-Cassegrain Telescope. A catadioptric telescope.

Secondary
Contraction of "secondary mirror". A second mirror surface in a reflector or in a catadiopric telescope. In Newtonians, the synonym is "diagonal".

Secondary holder
Support to keep the secondary in place. It is often attached to a spider. (see also: diagonal holder).

Seeds
Opacke specks of unmelted material in glass.

Serrurier Truss
Sort of truss design consisting of 8 trusses arranged so that they form triangles (better rigidity).

Setting circle
Circle mounted on a telescope axis or bearing, on which you can read the angular position of the telescope.

Sleek
A very fine scratch visible at only certain angles. Sleeks are found during polishing, usually being caused by impurities in the polishing compound or other contaminates.

Smoothing
The step between Routh grinding and polishing using grits like 400 or higher [Detlev Markfort]

Spider
Construction that holds the diagonal holder in place.

Spider vane
Arm-like construction, that keeps the secondary holder in place in the spider. Vanes are made of any rigid material: steel, aluminum, or even carbon fiber, ...). These vanes result in the oh-s-famous "spikes" seen in many astrophotos and through the scope.

Spike
Straight protrusion that starts from any bright object seen in a telescope with a spider, and that follows the direction of any vane in the spider. Spider configurations with 3 or 6 vanes both give 6 spikes (look at the directions of the vanes), spiders with 4 arms give 4 perpendicular spikes.

Spring
Part of metal that has improved elasticity.

Squaring the focuser
Adjusting the focuser and diagonal so that the optical axis of the primary is bent at EXACTLY 90 degrees.

Star test
Final test of the telescope, which shows the degree of collimation and the quality of the optics (when the collimation is OK).

Strain
Tension in a material.

Stray light
Light, not coming from your object of interest which irritates your vision or interference with your sight, reducing contrast. Typically it comes from streetlights, headlights, yard lights, etc.

Strehl ratio
A method of expressing optical performance of telescopes.

T

TCT
Tilted Component Telescope, like schiefspiegler (Kutter telescope). These scopes have components that are not perpendicular to the optical axis (besides a diagonal), or that have no rotational symmetry anymore. Most of this designs were created to avoid obstruction by the secondary.

TDE
Acronym for "Turned Down Edge".

Threaded
On which a "thread" is put, this is a spiral-like groove on a cylinder. As with a threaded rod or a bolt.

Threaded rod
Rod on which a thread is put. Looks like a bolt without head.

TM
Telescope Maker.

TN
Contraction of "Telescope Nut", a term often used when referring to a person obsessed with telescope making. See for example the ATM 1, 2 and 3 trilogy.

TOT
Tool On Top Trepanning
The process of boring/drilling a hole in the center of a mirror for the purpose of making a pierced-mirror design, like a Cassegrainian.

Truss tube
Telescope tube made of trusses, making it lighter and transportable when you disassemble it.

U

UWA
Acronym for "Ultra-Wide Angle". Brand of eyepieces with an apparent field of view of 80° and even more.

V

Vane
See: Spider vane.

W

WA
Acronym for "Wide Angle". Brand of eyepieces with a wide apparent field of view (more than 50°).

Washer
round, flat (metal) ring with a hole in it.

Wingnut
Sort of nut with two wing-like extensions, that make the screwing and unscrewing easy by hand (without tools). Very popular for collimation purposes.

Wire spider
Sort of spider made of metal wire.

X

Y

Z