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A Back to top of glossary
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| Abrasive |
Grinding powder, grit, the stuff used to remove glass during the rough and fine grinding stage. See grit. |
| Achromat |
A refractor or lens element that is optimized to reduce chromatic aberration to a minimum. |
| Airy disk |
| Named for Sir George Airy, the airy disk is the central spot in the diffraction pattern created in any telescope. Due to the wave nature of light, the light of a star cannot be focused to a point. Instead, it forms a pattern as shown here, with a central disk surrounded by progressively dimmer rings. The diameter of the airy (central) disk is (1.22 * lambda * f)/D, where lamda is the wavelength of light, f is the focal length, and D is the aperture diameter. | 
Size and brightness
of rings exaggerated. |
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| Alt-az mount |
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 |
The process of applying a very thin (few atoms thick) layer of aluminum and often other substances to make a mirror reflective. |
| Annealing |
A process by which strain is removed from a glass blank. It involves a carefully controlled heating and cooling cycle (to within fractions of a degree over many hours). |
| Aperture |
The diameter of the primary objective of your telescope. |
| Aperture fever |
A seriously debilitating illness, sufferers endlessly crave larger and larger telescopes to the exclusion of all reason! |
| APO |
apochromatic, also slang for an apochromatic telescope, as in "I wish I owned an APO" |
| Apochromat |
A refractor or lens element that is optimized and uses special glasses to eliminate chromatic aberration. |
| Astigmatism |
An asymmetric optical defect, characterized by figure differences on two or more axes. Picture a saddle or a Pringles™ potato chip -- examples of severe astigmatism. |
| ATM |
Abbreviation for "amateur telescope maker." |
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B Back to top of glossary
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C Back to top of glossary
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| Cat |
Abbreviation for catadioptric scope. |
| Catadioptric scope |
A telescope that combines both reflecting and refracting elements. |
| Cerium Oxide |
A polishing agent (CeO), sometimes abbreviated to "cerium." Preferred by many over rouge because it doesn't stain like rouge. |
| Cervit™ |
A brand-named low expansion glass. |
| Chordal stroke |
A grinding stroke used during hogging to remove large quantities of glass quickly. In this stroke, you grind mirror on top with the center of the mirror over the edge of the tool. You grind first on one chord, rotate mirror and tool, then on another, and so forth. Picture a stop sign (an octogon) - that's the pattern the center of your mirror will make after you have done a few wets. |
| Chromatic aberration |
An optical defect in which not all colors come to focus at the same point, commonly associated with refractors. Typically seen in the eyepiece as a blue or purple fringe around bright objects. |
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D Back to top of glossary
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| Dobsonian mount |
A simple, low-cost alt-azimuth mount, popularized by John Dobson, for whom this mount style is named (rumor has it, much to his displeasure). |
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E Back to top of glossary
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| Equatorial mount |
A type of telescope mount in which one axis is parallel to the earth's spin axis. |
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F Back to top of glossary
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| Figuring |
The final stage of grinding a mirror. At this stage, your mirror is already polished. You use altered stroke patterns, altered laps, or other techniques to turn your sphere into a parabola (assuming you are making a Newtonian or similar design). After this, it's off to the coater. |
| 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. |
| Focal length |
The length at which parallel rays of light are brought into focus for a particular lens or mirror. |
| Focal ratio |
A ratio of the focal length and diameter of a mirror or lens. |
| Foucault test |
A test of optical surfaces, developed by Leon Foucault, a French optician. The test uses reflection and geometrical optical principles to amplify shadows of defects on the mirror so that they are easily visible. |
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G Back to top of glossary
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| Grit |
Grinding powders, silicon carbide (SiC) or aluminim oxide (Al2O3). Stephen Tonkin maintains information on abrasives (grits) at
http://www.aegis1.demon.co.uk/abrasiv.html. His information includes the particle size related to various classification schemes and also the elutriative time. The times given are the time to settle in a 1m column of water. |
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H Back to top of glossary
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| Hogging |
The initial rough grinding you do to put the initial curve in your mirror. You do not need to worry as much about obtaining the correct figure at this point, just getting your mirror close. Rough grinding will produce the spherical shape you are aiming for. |
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I Back to top of glossary
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J Back to top of glossary
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K Back to top of glossary
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L Back to top of glossary
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M Back to top of glossary
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| MA |
When referring to eyepieces, means "modified achromat." |
| ma |
Refers to the "minor axis" (smaller axis) of a diagonal mirror - the
most common manner of specifying diagonal size. |
| Mak or Maksutov |
A catadioptric scope that uses all spherical components: a spherical mirror combined with a meniscus corrector (a deeply spherically curved lens). Renowned as good performers. |
| Millies-Lacroix |
Adrien Millies-LaCroix. Usually refers to a a graphical method of interpreting Foucault test data. Download an Excel spreadsheet (ml.xls) that will help you plot such a graph. |
| MOT |
Abbreviation for "mirror on top." |
| 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. |
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N Back to top of glossary
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| Newtonian scope |
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. |
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O Back to top of glossary
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| OTA |
Optical Tube Assembly (the tube part of the telescope, not including the mount) |
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P Back to top of glossary
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| Parabolic |
A 2-dimensional parabola-shaped curve. See paraboloid. |
| Paraboloid |
Take a parabola and rotate it around its origin (the "bottom" of the curve) and you get a paraboloid. In other words, a paraboloid is a 3-dimensional parabola. A Newtonian style telescope mirror has a paraboloidal (not a parabolic) shape. |
| Pitch |
A substance derived from tree sap used as a base for polishing mirrors. |
| Pitch lap |
A surface made of pitch against which you rub the mirror with specific grits to polish. |
| Plate |
A type of glass (window glass) commonly used for mirrors. Plate glass typically has a green color. |
| Polishing |
The stage of mirror making where you |
| Primary |
Shortened form of primary mirror. The main mirror in a telescope. |
| Pyrex® |
A type of glass with a lower coefficient of expansion (also harder) than plate glass. Generally, Pyrex is colorless, though it can range from gray to straw-colored. |
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Q Back to top of glossary
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R Back to top of glossary
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| Reflector |
A telescope that operates by reflecting light into focus: a mirror type telescope. |
| Refractor |
A telescope that operates by bending (refracting) light into focus: a lens type telescope. |
| ROC |
Abbreviation for "radius of curvature." |
| Rouge |
A polishing agent, stains everything red, too. Fe2O3 |
| Rough grinding |
The stage of grinding after hogging in which you produce a good, spherical shape. Generally, you use the coarse grits like 120 for this stage. |
| RTMC |
Riverside Telescope Maker's Conference, an annual ATM event held over Memorial Day weekend. |
| RTV |
Room Temperature Vulcanizing. A silicone-based adhesive, the clear version of which is used to glue a mirror to its cell. Cures with exposure to moisture. |
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S Back to top of glossary
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| 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-Cassegranian Telescope. A catadioptric telescope. |
| Secondary |
Shortened form of secondary mirror. A smaller mirror which redirects the reflected light created by the primary. |
| Sheifspiegler |
A tilted-component telescope. When built with three elements, it is referred to as a Tri-Sheifspiegler. On the list, these are often humorously called a "sheep-sprinkler." (I'm not positive I have the spelling of "sheifspiegler" correct.) |
| 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. Sleeks also occur when the polishing compound dries out during a polishing session. |
| Stellafane |
From Latin, "shrine to the stars." Stellafane is an ATMing convention held each summer in Springfield, Vermont. More info at http://www.stellafane.com |
| Strehl ratio |
A measurement of optical precision that expresses the fraction of light that the mirror concentrates into the airy disk. A perfect mirror, therefore, has a Strehl ratio of 1--it reflects all of the light into the airy disk. |
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T Back to top of glossary
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| TCT |
tilted-component telescope, one in which the optical elements are tilted with respect to their rotational axes for the purpose of reducing or eliminating obstructions. |
| Tex |
Either refers to Jean Texereau or a program written to reduce figuring data, called TEX. |
| TN |
Telescope Nut, termed used frequently in the ATM 1, 2, and 3 books |
| TOT |
Abbreviation for "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 Cassegranian. |
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U Back to top of glossary
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V Back to top of glossary
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W Back to top of glossary
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| wet |
One grinding or polishing spell. One wet = time from applying grit and water + grinding/polishing + cleaning up. |
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X Back to top of glossary
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Y Back to top of glossary
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| Yolo |
An unobstructed telescope design. |
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Z Back to top of glossary
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| Zerodur® |
A brand of low-expansion glass. |
