Terminology

Following is information gleaned from the archived Cloudy Nights thread "Common CN Contractions & Acronyms". Many thanks to CN members for their work in collecting and archiving this list.

(This page is responsible for the hover-over tool tips on this website)

Hardware/Software/Imaging & Observing/General:
where the mirror is on top of the grinding tool.
 * Achro:Achromatic Refractor, or Achromat
 * achro:Achromatic Refractor, or Achromat
 * AFoV:Apparent Field of View. Refers to the visual size of the field seen through an eyepiece, not the actual angular sky coverage.
 * AG:Autoguider
 * Alt:Altitude
 * Alt/Az:Altazimuth mount
 * APO:Apochromatic Refractor, or Apochromat
 * ATM:Amateur Telescope Making (or Maker)
 * AU:Astronomical Unit. 1 AU is approximately the mean Sun-Earth distance.
 * Az:Azimuth
 * Cass:Cassegrain Reflector.
 * Cat:Catadioptric Telescope.
 * COL:Computerized Object Locator
 * CW:Counterweight
 * Dec:Declination
 * Dob:Dobsonian Telescope
 * DSC:Digital Setting Circles. Electronically display direction scope is pointed in RA/Dec coordinates.
 * DT:Dovetail plate
 * EP:Eyepiece
 * EQ:Equatorial Mount
 * FF:Field Flattener
 * FF/FR:Field Flattener/Focal length Reducer.
 * FL:Focal Length
 * FR:Focal Ratio. FR also Focal length Reducer.
 * GEM:German Equatorial Mount.
 * HC:Hand Controller
 * JD:John Dobson. Inventor of the Dobsonian.
 * LP:Light Pollution
 * Mak:Maksutov-Cassegrain Telescope
 * MN:Maksutov-Newtonian Telescope
 * MOT:Mirror On Top. ATM term for mirror grinding,
 * NELM:Naked Eye Limiting Magnitude
 * Newt:Newtonian Reflector
 * OAG:Off-axis Guider
 * OTA:Optical Tube Assembly
 * PE:Periodic Error (for Equatorial Drives). Tracking errors caused by the imperfections inherent in any worm gear system. These errors repeat every rotation of the worm, hence Periodic.
 * PEC:Periodic Error Control or Correction. Records corrections made during RA tracking, either by hand or autoguider; when activated, PEC replays the corrections, reducing periodic error (PE). Mount does not retain the recording after powering down.
 * Plate:Photographic plate, generally refers to an image made using a telescope. (In the context of astronomy)
 * PPEC:Permanent Periodic Error Correction. Mount retains corrections in permanent memory.
 * RA:Right Ascension
 * RACI:Right-Angle Correct-Image finder - flips image to correspond to what the eye sees unaided. Aids in locating objects when using charts (stars are oriented correctly - not flipped left-right and up-down as in regular finders).
 * RC:Ritchey-Chretien Telescope
 * RDF:Red Dot Finder [Form of Unity or Unit-power (1x magnification) finder]
 * Sb:Surface brightness
 * SBS:Side-by-side saddle plate
 * SCT:Schmidt-Cassegrain Telescope
 * SN:Schmidt-Newtonian Telescope
 * SNR:Signal to noise ratio of an image.
 * Stack:An image created from several frames (sub-exposures), which have been aligned with image processing software.
 * stack:An image created from several frames (sub-exposures), which have been aligned with image processing software.
 * Sub:Sub-exposure
 * sub:Sub-exposure
 * Subs:Sub-exposures
 * subs:Sub-exposures
 * TFoV:True Field of View. The actual angular measurement of sky seen through an eyepiece.
 * TOT:Tool On Top. ATM term for mirror grinding, where the grinding tool is on top of the mirror.
 * UTA:Upper Tube Assembly; seldom-used acronym for the secondary cage of a Dobsonian telescope.

Objects

 * DSO:Deep Sky Object (clusters, nebulae, galaxies...objects beyond the Solar System)
 * SSO:Shallow Sky Object (planets, asteroids...anything orbiting the Sun)
 * BN:Bright Nebula
 * DN:Dark Nebula
 * EN:Emission Nebula
 * GX:Galaxy
 * GC:Globular (Star) Cluster
 * OC:Open (Star) Cluster
 * PN:Planetary Nebula

Magnitude

 * Mag:Magnitude
 * VMag:Visual Magnitude
 * BMag:Blue Magnitude
 * Vtip:Magnitude of the brightest star in a globular cluster
 * VHB:Magnitude of the horizontal branch stars in a globular cluster
 * UBVRI:Ultraviolet, Blue, Visual, Red, Infrared (photometry)
 * SB:Surface Brightness

Galaxy Types
Note: These are often used in catalogs and on wikipedia as a brief description for galaxies, and the above is only a subset of the full list of galaxy type abbreviations.
 * E:Elliptical (E0...E6: degrees of ellipticity)
 * S:Spiral (Sa...Sd: range of spiral strcuture; from tight and well-defined to loose and ill-defined)
 * SA:Ordinary Spiral
 * SB:Barred Spiral (SBa...SBd: See "S" above)
 * SAB:Mixed Spiral
 * Irr: Irregular
 * Pec: Peculiar

Deep Sky Catalogs
Note: These letters usually precede a number representing an object from the catalog. For example, M42 is the 42nd entry in the Messier catalog. As with the preceding section, the above is only a subset of published catalogs.
 * M:Messier
 * H:William Herschel Catalog
 * h:John Herschel Catalog
 * GC:General Catalog
 * NGC:New General Catalog
 * IC:Index Catalog
 * CGCG:Catalogue of Galaxies and Clusters of Galaxies
 * MCG:Morphological Catalogue of Galaxies
 * PGC:Principal Galaxies Catalog
 * PNG:Strasbourg-ESO catalog of planetary nebulae
 * UGC:Uppsala General Catalog (Galaxies)
 * B:Barnard catalog of dark nebulae
 * Cr:Collinder catalog of open star clusters
 * Sh2:Sharpless catalog of bright nebulae
 * Tr:Trumpler catalog of open star clusters
 * RC3:Third Reference Catalog of Bright Galaxies. This catalog, written by de Vaucouleurs, de Vaucouleurs and Corwin, et al, is a primary source of data on galaxy positions, sizes, brightnesses and types.
 * PK:Perek-Kohoutek (catalog of planetary nebulae)

Stellar Catalogs
Note: As with the preceding section, these letters usually precede a number representing an object from the catalog and the above is only a subset of published stellar catalogs.
 * HD:Henry Draper catalog (stars)
 * SAO:Smithsonian Astrophysical Observatory (Harvard) catalog (stars)
 * GSC:(Hubble) Guide Star Catalog
 * T:Tycho catalog (stars)
 * ADS:Aitken Double Star (Catalogue)
 * WDS:Washington (Visual) Double Star (Catalog)