Glossary

Terms used in Sidralune and this manual.

Term	Meaning
AAVSO	American Association of Variable Star Observers, the main archive for variable-star observations
ADES	Astrometry Data Exchange Standard, the Minor Planet Center's modern report format
ADQL	Astronomical Data Query Language, an SQL dialect for querying astronomical databases
Airmass	The amount of atmosphere along the line of sight relative to the zenith: 1 overhead, 2 at 30° altitude, about 38 at the horizon
Alignment point (AP)	In lucky imaging, a small box aligned separately in every frame to correct local seeing distortion
Alpaca	The network version of ASCOM, using HTTP
Altitude / azimuth	Angle above the horizon / direction along the horizon measured from north through east
Apparent coordinates	Coordinates of date, including precession, nutation and aberration; where a telescope must point
Arcminute (′), arcsecond (″)	1/60 and 1/3600 of a degree
ASCOM	The standard Windows interface for astronomy equipment drivers
Astronomical twilight	When the Sun is between 12° and 18° below the horizon; full darkness begins when it is 18° below
AU	Astronomical unit, the mean Earth-Sun distance (149.6 million km)
Barycentric correction	Correction of a time or velocity for the Earth's motion relative to the solar system's centre of mass
Bayer matrix	The pattern of red, green and blue filters over a colour sensor's pixels (RGGB, BGGR, GRBG, GBRG)
Bayer / Flamsteed designations	Star names like α Lyrae (Bayer) or 61 Cygni (Flamsteed)
Bias frame	A zero-length exposure recording the sensor's read-out offset
BJD / HJD	Barycentric and heliocentric Julian dates: times corrected to the solar system's barycentre or the Sun's centre
BLS	Box least squares, a period search for transits
Bortle scale	A 1–9 scale of sky darkness, 1 being the darkest
B−V	Colour index: blue minus visual magnitude. Negative for hot blue stars, around 0.65 for the Sun, above 1.5 for red stars
Calibration frames	Bias, darks, flats and dark flats, used to remove the camera's and telescope's own signal
Central meridian (CM)	The longitude on a planet facing the observer
Circumpolar	Never setting from your latitude
Comparison star	A constant star of known brightness used as a reference in photometry
Critical focus zone	The range of focuser positions within which the image is in focus
Dark frame	An exposure with the telescope covered, recording dark current and hot pixels
Dark flat	A dark frame with the same exposure as the flats
DE440	JPL Development Ephemeris 440, the current high-precision planetary ephemeris; DE440s is its 1849–2150 subset
Debayering	Interpolating a full-colour image from a Bayer-matrix sensor
Deconvolution	Mathematically reversing blurring caused by the atmosphere and optics
Detection margin	In Sidralune's visibility model, how many magnitudes an object is above the threshold of visibility
Dithering	Moving the telescope slightly between exposures so defects fall on different pixels
Drizzle	A stacking method that recovers resolution from undersampled, dithered frames
ΔT	The difference between Terrestrial Time and Universal Time (about 69 seconds in 2026)
Ecliptic	The Sun's apparent yearly path across the sky; the planets stay near it
Elongation	Angular distance of an object from the Sun
Ensemble photometry	Using several comparison stars together for a more stable reference
Ephemeris	A table of positions of a celestial body over time; also the underlying theory
Equivalent width	The width of a rectangle of continuum height with the same area as a spectral line; a measure of line strength
Exit pupil	The diameter of the light beam leaving the eyepiece: aperture ÷ magnification
Extinction	Dimming of starlight by the atmosphere, in magnitudes per airmass
Field rotation	Rotation of the field of view on an altitude-azimuth mount
FITS	Flexible Image Transport System, the standard astronomical image format
Flat frame	An exposure of an even light source recording vignetting and dust shadows
FWHM	Full width at half maximum: the diameter of a star image at half its peak brightness
Gaia DR3	The European Space Agency's third Gaia data release: positions, motions and brightnesses of 1.8 billion stars
GHS	Generalised hyperbolic stretch
HFR	Half-flux radius: the radius containing half of a star's light; used for focusing
Hour angle	Time since an object crossed the meridian
INDI	Instrument Neutral Distributed Interface, the open standard for astronomy equipment on Linux
J2000 / ICRS	The standard celestial reference frame, equivalent to the equator and equinox of 1 January 2000 for most purposes
Julian Date (JD)	A continuous count of days since 4713 BC, used for astronomical timing
Kappa-sigma clipping	Rejecting values more than κ standard deviations from the mean
Libration	The Moon's apparent wobble, which shows slightly more than half its surface over time
Light curve	A graph of brightness against time
Limiting magnitude	The faintest star visible or recorded
LSR	Local standard of rest, the reference frame moving with the average motion of stars near the Sun; used for radio velocities
Lucky imaging	Taking many short exposures and keeping only the sharpest moments
Magnitude	The astronomical brightness scale; larger numbers are fainter, 5 magnitudes is a factor of 100
MCMC	Markov-chain Monte Carlo, a method for estimating parameter uncertainties
Meridian	The line from north to south through the zenith; objects are highest when they cross it
Meridian flip	Moving a German equatorial mount to the other side of the pier after the target crosses the meridian
Mosaic	An image made of several overlapping fields
MPC	Minor Planet Center, the clearing house for asteroid and comet observations
Nautical twilight	Sun between 6° and 12° below the horizon
Nyquist sampling	Sampling with at least two pixels across the smallest detail
O−C	Observed minus calculated: the difference between observed and predicted times
Occultation	One body passing in front of another, hiding it
Optimal sub-exposure	The shortest exposure at which read noise becomes negligible compared with sky noise
Parallactic angle	The angle between the direction to the celestial pole and the direction to the zenith at an object; governs field rotation
PDM	Phase dispersion minimisation, a period search method
Phase angle	The angle Sun-object-observer
PixelMath	Applying a mathematical formula to every pixel of an image
Plate solving	Determining an image's exact sky position by matching its stars to a catalogue
PSF	Point spread function: the image of a point source (a star)
Radial velocity	Speed along the line of sight, from the Doppler shift
Read noise	Noise added by the camera when reading out the sensor, in electrons
Registration	Aligning images to each other
Rejection	Discarding outlying pixel values (satellites, cosmic rays) when stacking
SER	A video format for planetary imaging with full bit depth and time stamps
SGP4	The standard orbit model for satellite two-line elements
SIP	Simple Imaging Polynomial, a convention for describing optical distortion in a WCS
SNR	Signal-to-noise ratio
SQM	Sky Quality Meter reading, the sky's brightness in magnitudes per square arcsecond; larger is darker
Stacking	Combining many aligned frames to reduce noise
Starlet / à trous wavelets	A multiscale image decomposition used for sharpening and noise reduction
Stretching	Brightening the faint parts of linear image data to make them visible
TAP	Table Access Protocol, used to query astronomical databases
TLE	Two-line element set describing a satellite's orbit
Transit (meridian)	An object crossing the meridian, at its highest point
Transit (planet)	A planet or moon passing in front of its star or planet
Tsys	System temperature of a radio receiver, a measure of its noise
WCS	World coordinate system: the mapping between image pixels and sky coordinates
WinJUPOS file names	A widely used naming convention for planetary images that records the mid-time of the recording, YYYY-MM-DD-HHMM_T
XISF	Extensible Image Serialization Format, an astronomical image format with metadata and optional compression
ZHR	Zenithal hourly rate: the meteor rate one observer would see with the radiant overhead under a limiting magnitude 6.5 sky