Glossary
A selection of common terms and acronyms
This is not an exhaustive glossary but a selection of terms that may not have had extensive explanation in the main text.
AAVSO: American association of variable star observers.
Achromat: A refractor made of objective lenses of different materials to bring two colors of light to almost the same focal point.
Adobe RGB (1998): A popular color space (profile) for photographers and image editing.
ADU: Refers to Analog to Digital Units, the digital pixel values from a CCD.
Afocal Projection: In astrophotographic terms a telescope, complete with eyepiece, is coupled to a camera (with its lens) to image onto the sensor. The camera lens is in effect replacing the human eye in a visual setup. The term “digiscoping” is a form of afocal photography.
Aggressiveness: In autoguiding, the aggressiveness setting sets the proportion of the tracking error that is removed by the next guiding command.
Apochromat: A refractor made of several objective lenses of different dispersion characteristics that minimizes spherical and chromatic aberration.
ASCOM: A non-profit initiative to create an open source standard for interfacing astronomy software, and hardware, on the Windows platform.
Astigmatism: This is an optical defect that renders stars as ovals. More common with eyes than optics!
Asterism: This describes a convenient pattern of stars, often as part of a constellation. An example is “The Plough”.
Astrometry: This is the measurement of a star’s position and motion in relation to catalog databases.
Bahtinov Mask: A focus aid that looks like a drain cover, which, when placed over the front of a telescope, creates 3 diffraction spikes that intersect when the system is in focus.
Bias Current: This is sensor noise that occurs with every exposure irrespective of temperature or duration. It also sets the dynamic range of the sensor and its effect can any be reduced by combining techniques.
Blooming: The unsightly effect of a CCD well becoming overexposed and the excess electrons leaking into adjacent photosites. Some CCDs have electronics to reduce this effect.
C-Mount: A thread standard often used on cine lenses but also used on small CCD cameras: 1-inch diameter, 32 threads per inch and with a flange distance of 17.5 mm.
Centroid: The position of a star’s center. Used during autoguiding and astrometry.
Chromatic Aberration: In glass optics, the optical elements refract light to different degrees, depending on its wavelength. The aberration arises as the different color components of white light do not focus at the same point.
Clipping Mask: In Photoshop, a clipping mask associates an adjustment layer to the layer below.
Collimation: This describes the alignment of optical elements, often in context to mirror systems, which are sensitive to mechanical stresses.
Convolution: In an astronomy sense, the smearing effect of the optical system on the signal.
Cosmic Rays: These are random high energy particles from space. They trigger electrons in a CCD detector and leave small white streaks. They are normally processed out during image calibration.
Dark Current: This is the ongoing thermally induced accumulation of non-image electrons, the number of which increase with exposure time and temperature. There is a mean and a random value; the latter can only be reduced by averaging many images.
Deconvolution: A process that models and corrects for the smearing effect of an optical system on a perfect point light source.
Diagonal: A mirror or prism that deflects the light path to enable more convenient viewing. Often fitted into the back of a focuser on a refractor or SCT.
Dither: A deliberate random image shift, executed between exposures, typically up to 1 pixel. Some references assume several pixels in cases where hot pixels are not removed by calibration frames.
Drizzle: A technique for statistically combining multiple images, typically under-sampled, to increase resolution. It requires several images that are deliberately misaligned by sub-pixel amounts. (See Dither above.)
Dovetail: A metal rail with angled edges that clamps onto a telescope mount. Popular standards are Vixen (~43 mm flange) and Losmandy (~75 mm flange).
ED (Extra-low Dispersion): Refers to glass in an optical system with little false color.
Half Flux Density (HFD): Often used by autofocus algorithms. The pixel diameter of a star within which half the energy or flux occurs. Similar to Full Width Half Max (FWHM) measurement but more robust in poor seeing conditions.
Field Rotation: If a mount is not accurately polar aligned, during a long exposure, stars will appear to rotate around the guide star.
G2V: Refers to a star of a particular spectral type and used for color calibration. Our Sun is a G2V star.
Gamma: Is a non-linear transform applied in imaging systems using a simple power-law expression. Some color spaces, such as sRGB and Adobe RGB(1998) are based on gamma 2.2. A linear image has a gamma of 1.0.
German Equatorial Mount (GEM): Most commonly used for imaging, especially with Newtonian and refractor designs.
GSC Catalog: The guide star used for control and alignment of the Hubble Space Telescope and our own more humble mounts on terra firma.
Hartmann Mask: A focus aid, comprising a mask with 2 or 3 circular apertures, placed over the front of the telescope. These align at focus.
Liveview: A mode on (typically) digital cameras that streams a live image from the sensor, facilitating focus and framing.
Meridian Transit: When an object crosses the meridian at its highest point.
Mirror Flop: Some SCTs have a moving mirror. The heavy mirror will tilt within the mechanism in different orientations.
NOVAS: Naval Observatory Vector Astrometry Subroutines. A software library of astrometry related computations.
Nyquist Sampling Theorem: In an astronomy sense it is applied to spatial resolution and that the resolution of the sensor should be at least twice the resolution of the optics.
Over-sampled: When the sampling frequency or sensor resolution exceeds that to detect the signal frequency or resolution.
Parfocal: Refers to different optical elements have the same effect on focus position. Applies to filters and eyepieces.
Periodic Error Correction (PEC): Software based system that measures and corrects for worm-gear tolerance issues, in real time, using a look up table (LUT). The LUT may reside in the mount or computer software.
Peltier: A semiconductor that exhibits a thermoelectric effect. A heat difference between surfaces generates a voltage and likewise an applied voltage generates a heat difference. When sandwiched between a sensor and a heatsink, the practical upshot is that it transfers thermal energy from the sensor to the heat-sink.
Petzval Field Curvature: Describes the optical aberration where a flat object is imaged onto a curved plane. The term Petzval lens design is also sometimes associated with telescope field-flattener designs to describe their correcting effect.
Photometry: The measurement of the apparent magnitudes of an object, in this case, mostly stars.
Pixel: An ambiguous term that refers to the sensor’s light sensitive cells (photosites) as well as to composite RGB picture elements in an image or the elements of an image.
Plate-solve: The process of calculating an image’s position by matching the star pattern with a catalog database.
Prime Focus: This is the typical system used in astrophotography. The eyepiece is removed from a telescope and the main objective focuses directly onto a sensor in a bare camera body.
Point Spreading Function (PSF): Describes the effect of an imaging system on a point light source. Used in the devolution process to model the opposing function.
Pulseguide: An autoguiding system that uses software rather than hardware to control the mount. Often combined intelligently with PEC. Software Bisque have something similar called Directguide for Paramounts.
Pulse Width Modulation (PWM): Power is applied on and off to regulate the average amount, typically for a dew heater system. The ratio determines the power. The frequency is usually low at 1 Hz but if using motor control modules, it may be 10 KHz.
Off Axis Guider (OAG): A small mirror, normally before the filter wheel, deflects peripheral light to a guide camera, set at 90 degrees to the optical path.
One-Shot Color (OSC): A term used for conventional digital cameras or CCDs fitted with a Bayer color array and produce a color image with a single exposure.
OTA: Optical Tube Assembly. Some telescopes are sold as systems with mounts and tripods. An OTA is just the optical telescope component.
Quantum Efficiency (QE): An expression of the efficiency of incident photon conversion into electrons in a CCD sensor.
Residuals: Refers to the error between the observed and predicted position of a body. Often used to indicate quality of a plate-solve calculation.
RJ45: 8-way connector system used for LAN / Ethernet communications. Simple robust locking connector system also used in 6-way (RJ12) and 4-way (RJ10) for serial communications, autoguider ST4 and focuser systems.
sRGB: A color space (profile) that is used extensively for consumer imaging devices and Internet use.
ST4: The name given to an early SBIG autoguiding system and now adopted to mean the “standard” interface for autoguiding inputs into a mount, based on opto-isolated switch closures.
Strehl ratio: A measure of the optical perfection of a system. A ratio of 0.85 is 85% as good as a perfect system.
T-Mount: Sometimes also called T2 thread, this a M42×0.75 metric thread for optical systems, designed for a 55-mm flange spacing (thread to sensor or film). T-thread adapters for various cameras, are deliberately sized to maintain this 55-mm flange spacing to the camera’s sensor.
Transparency: Not to be confused with atmospheric turbulence or seeing, this is the clarity of the air and the absence of mist, dust and pollution.
TSX: Shorthand for Software Bisque’s TheSkyX planetarium and astrophotography program.
Under-sampled: A sample frequency or spatial resolution that is insufficient to detect the full details in the system signal or image.
USNO: US Naval Observatory. Also a resource for astrometry data and star catalogs, used in addition to the GSC catalog for plate solving.