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This glossary covers astronomical terms, or other LSST terms related to the astronomy domain. See the DMS Glossary for technical terms related to the Data Management System software. Many terms (and in fact, algorithms) are borrowed from the vocabulary of SDSS. Actually, some of the definitions below are taken almost verbatim from the SDSS Glossary for terms in common. We gratefully acknowledge the curators of this excellent resource, however quotations are omitted in the interest of clarity in this Glossary. 

Here are quick links to the alphabetized glossary terms: 

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | ST | U | V | W | X | Y | Z


Adaptive moments

The second moments of the source intensity distribution, which are used for measuring source shapes. This approach is close to optimal for measuring the shapes of faint galaxies. 


The pathlength of light from an astrophysical source through the Earth's atmosphere. It is given approximately by sec z, where z is the angular distance from the zenith (the point directly overhead, where airmass = 1.0) to the source. 


An electronic component of a CCD that is used to recover the signal during read-out. For LSST, multiple amplifiers on each CCD will enable simultaneous read-out of adjacent regions of each detector. Often this term is used, not quite correctly, as a synonym for a read-out channel.


Refers to the structured communication that is issued rapidly via the internet to the community during the Alert Production. It is intended to characterize the detection of one or more sources that are new, or have changed significantly in position or brightness, relative to the applicable image template.


The appearance of a known object after having been invisible (or, undetected). 

Aperture correction

A correction that is applied to fluxes of sources that were measured within a finite aperture, to account for the source flux that lies outside the aperture. This correction is usually based upon a model of the PSF as derived from bright, isolated stars. From the model one can derive the magnitude of the correction with aperture size and its variation with position in the image, which asymptotically approaches 1.0 at infinite aperture. Fluxes of sources in crowded fields are often measured with small apertures to avoid contamination, and then corrected with this approach. 


In astronomy, the sub-discipline of astrometry concerns precision measurement of positions (at a reference epoch), and real and apparent motions of astrophysical objects. Real motion means 3-D motions of the object with respect to an inertial reference frame; apparent motions are an artifact of the motion of the Earth. Astrometry per se is sometimes confused with the act of determining a World Coordinate System (WCS), which is a functional characterization of the mapping from pixels in an image or spectrum to world coordinate such as (RA, Dec) or wavelength. 



In an image, the background consists of contributions from the sky (e.g., clouds or scattered moonlight), and from the telescope and camera optics, which must be distinguished from the astrophysical background. The sky and instrumental backgrounds are characterized and removed by the processing software using a low-order spatial function whose coefficients are recorded in the image metadata. 



The sequence of pointings, visit exposures, and exposure durations performed over the course of a survey


A fully qualified, calibrated science image, consisting of science pixels, a quality mask, a PSF characterization, and a WCS mapping. 


An imaging device mounted at a telescope focal plane, composed of optics, a shutter, a set of filters, and one or more sensors arranged in a focal plane array. For details see Representation of a Camera in the LSST Stack


In the SDSS survey, a camera column is the range (in declination) covered by a single sensor in the camera


A Charged-Coupled Device is a particular kind of solid-state sensor for detecting optical-band photons. It is composed of a 2-D array of pixels, and one or more read-out amplifiers


A 5-passband legacy imaging survey conducted at the Canada-France-Hawaii Telescope from 2003-2008. 


The raw data from a read-out amplifier of a sensor. For LSST there are 16 amplifiers for each science sensor, resulting in 16 parallel data channels from each device. 

Chi-squared Co-Add (image)

An image that is the weighted sum of multiple input images, where for each input: 

coadd.image += image.image**2 / image.variance
coadd.mask |= image.weightMap += weight

For bad pixels, coadd and weightMap are not altered. Note that the inputs must be aligned to a common projection and pixel grid and corrected to the same photometric scale and zero-point.  

Co-add image

An image that is the combination of multiple input images. The inputs are aligned to a common projection and pixel grid, corrected to the same photometric scale and zero-point, with bad pixels and artifacts rejected. (Image PSFs may also be matched prior to co-addition.) Co-Add images have had non-astrophysical background removed. 



Deblending is the act of inferring the intensity profiles of two or more overlapping sources from a single footprint within an image. Source footprints may overlap in crowded fields, or where the astrophysical phenomena intrinsically overlap (e.g., a supernova embedded in an external galaxy), or by spatial co-incidence (e.g., an asteroid passing in front of a star). Deblending may make use of a priori information from images (e.g., deep Co-Adds or visit images obtained in good seeing), from catalogs, or from models. 


Often abbreviated Dec, it is a part of an equitorial coordinate pair that expresses the angular distance (usually expressed in degrees) from the Celestial Equator, measured along great circles that intersect the Equatorial poles. Positions south of the equator are given negative sign. 

deVaucouleurs profile

The radial distribution of flux of an astronomical source that is characterized as: 

{I(r) = I_0 * exp(-7.67(r/r_e)^{1/4})}

An elliptical version of this profile can be fit to every detected source, yielding the deVaucouleurs parameters. See the page on Measurement in the LSST Stack for details. 

DIA Object

DIA object (often concatenated as DIAObject) is one that has been detected during difference image processing. It is distinguished from a regular object in that its brightness varies in time. 

DIA Source

A DIA source (often concatenated as DIASource) is one that has been detected during difference image processing. It is distinguished from a regular source by at least one of the following properties: its uniqueness (i.e., no association to known objects); its variability in brightness; or, if it is associated with a Solar System Object, a change in coordinates. 

Difference image

Refers to the result formed from the pixel-by-pixel difference of two images of the sky, after warping to the same pixel grid and scaling to the same photometric scale. The pixels in a difference thus formed should be zero (apart from noise) except for sources that are new, or have changed in brightness or position. In the LSST context, the difference is generally taken between a visit image and template

Difference image processing

A pipeline that constructs differences between two images (usually a recent visit exposure and a template image) in order to detect sources that are new or that have changed brightness. The input images are transformed to a common grid, scaled to the same photometric response, and matched to the same PSF. Sources are detected above a configurable threshold on the difference image. This pipeline feeds the Alert processing. 



An output product of PhoSim, an Eimage is a simulation of the response of a single sensor, where the outputs of the constituent amps have been integrated, and the effects of variations in pixel-to-pixel sensitivity and amplifier gains have been removed. 




An ephemeris (pl: ephemerides) gives the predicted positions of astronomical objects or artificial satellites in the sky with time. The ephemerides are computed from mathematical models of motion of the object and the Earth.  

Exponential profile

The radial distribution of flux of an astronomical source that is characterized: 

{I(r) = I_0 * exp(-1.68 (r/r_e))}

The normalization 1.68 is chosen so that the model radius is a half-light radius. An 2-dimensional elliptical version of this profile is fit to every detected source. 



filter in astronomy is an optical element used to restrict the passband of light reaching the focal plane–i.e., it transmits a selected range of wavelengths. Filters elements are often named after standard photometric passbands, such as those used in the SDSS survey: u, g, r, i, z


Flexible Image Transport System, an international standard in astronomy for storing images, tables, and metadata in disk files. See the IAU FITS Standard for details. 


Shorthand for the spatial extent of a source in an image, or of the (often complex) boundary of an image, a co-add image, or a survey. For a source, the footprint is the set of image pixels that that contain flux from that source


Shorthand for radiative flux, it is a measure of the transport of radiant energy per unit area per unit time. In astronomy this is usually expressed in cgs units: erg/cm2/s

Focal plane array

A focal plane array (FPA) is the arrangement of multiple sensors in the focal plane of a camera. For LSST, the FPA is divided into an array of continguous rafts, upon which 9 science sensors are mounted 3x3. Additional engineering sensors are mounted on rafts near the periphery to support wavefront sensing and telescope guiding. 

Forced photometry

A measurement of the photometric properties of a source, or expected source, with one or more parameters held fixed. Most often this means fixing the location of the center of the brightness profile (which may be known or predicted in advance), and measuring other properties such as total brightness, shape, and orientation. 



The Hierarchical Triangular Mesh (HTM) is a partitioning scheme to divide the surface of the unit sphere into spherical triangles. It is a hierarchical scheme and the subdivisions have roughly equal areas. HTM is used to index the coordinates in the object databases for faster querying speeds. 


Image template

A co-added, single-band image of the sky that is deep, and where all transients, SSObjects, and artifacts have been removed. Constituent images for Image Templates may be selected from a limited range of quality parameters, such as PSF size or airmass. Such images are used as templates to perform difference image processing in order to detect variable, transient, and Solar System astrophysical objects

Instance Catalog

A catalog of astronomical sources containing source type, coordinates, brightnesses, and SEDs for use in creating simulated LSST images with PhoSim. Synonym with trim file


Julian Date

The Julian Date (JD) of any instant is the Julian day number for the preceding noon (UTC), plus the fraction of the day elapsed since that instant. The Julian day number is a running sequence of integral days, starting at noon, since the beginning of the Julian Period; JD 0.0 corresponds to noon on 1 January 4713 BCE. Various Julian Date converters are available on the Web. For example, 18h 00m 00.0s UT on 2014-July-01 (near the start of LSST construction) corresponds to JD 2456840.25. 


Magnitude, Pogson

Usually simply magnitude, it is a logarithmic measure of integrated source brightness, usually within a standard photometric passband, such that: {M - M_0 = -2.5 log(F/F_0)}where the zero-point flux is defined by a photometric standard. 

Magnitude, Petrosian

A magnitude determined from a fit to a Petrosian brightness profile: 

{R_P(r) = stuff}

Appropriate for galaxies. 

Magnitude, PSF

For isolated stars that are well described by the PSF, the optimal measure of the total flux is determined by fitting a PSF model to the object. 


The Modified Julian Day is shorthand for the Julian Day: MJD = JD - 2400000.5, which makes MJD=0 correspond to UT midnight in 1858-Nov-17. The half-day offset from JD aligns the start of the day with modern civil timekeeping. 



Refers to an astronomical object, such as a star, galaxy, asteroid, or other physical entity. Objects can be static, or change brightness or position with time. Generally an object will be associated with more than one instance of a source detection. 


Refers to the portion of the channel read-out of either a) non photo-active pixels, or b) additional read-out of the serial register after all science pixels have been accumulated (sometimes called virtual overscan). The overscan is often appended to the science pixels in the assembled amplifier image as a separate region. This region is useful to science processing software for estimating the stability of the DC offset in the read-out electronics. 



The window of wavelength or the energy range admitted by an optical system; specifically the transmission as a function of wavelength or energy. Typically the passband is limited by a filter. The width of the passband may be characterized in a variety of ways, including the width of the half-power points of the transmission curve, or by the equivalent width of a filter with 100% transmission within the passband, and zero elsewhere. 


Redshift (of a galaxy) determined from multi-band photometry. Generally determined from a fit of source colors to grid of model SEDs with redshift. 


In astronomy, this is an exercise to find detections of a newly discovered object in prior-epoch images. For example, finding the progenitor of a supernova, or prior apparitions of a solar system object at predicted locations in archived images, based on orbit parameters. 


The Point-spread function is the distribution of intensity on a sensor (or image) originating from an unresolved point-source (i.e., a star). Often the PSF is not the same Airy shape as would be expected from a finite-aperture optical system, owing primarily to atmospheric effects and imperfections in the optical system and the detector. 



The sensors in the LSST camera are packaged into replaceable electronic assemblies, called rafts, consisting of 9 sensors in a 3x3 mosaic. 

Raw Exposure

A raw exposure is the output from a camera, consisting of a set of image sections from each amplifier on each sensor on the focal plane array, including overscan

Right Ascension

Often abbreviated RA, it is a part of an equitorial coordinate pair that expresses the angular distance along the Celestial Equator. It is analogous to terrestrial longitude. RA increases to the east along the projection of the Earth's equator, from the origin (i.e., the Vernal Equinox). Positions are customarily expressed in degrees (0 < RA < 360), or hours (0 < RA < 24, usually in sexagesimal format). 


The sensors in the LSST camera are packaged into replaceable electronic assemblies, called rafts, consisting of 9 sensors in a 3x3 mosaic. 



Sloan Digital Sky Survey is a digital survey of roughly 10,000 square degrees of sky around the north Galactic pole, plus a ~300 square degree stripe along the celestial equator. 


Spectral Energy Distribution, the radiated energy of an astrophysical object as a function of energy (or wavelength) across the entire spectrum of light. 


An astronomical term for characterizing the stability of the atmosphere, as measured by the width of the point-spread function on images. The PSF width is also affected by a number of other factors, including the airmass, passband, and the telescope and camera optics. 


A sensor is a generic term for a light-sensitive detector, such as a CCD. For LSST, sensors consist of a 2-D array of roughly 4K x 4K pixels, which are mounted on a raft


The shape of an object is a functional characterization of its spatial intensity distribution. The integral of the shape is the flux. A functional characterization has the advantages of offering an analytical form that can be convolved with another function, distinguished from overlapping source shapes, and integrated to large radius to obtain the source flux. 

Sky map

An all-sky tesselization for LSST. The Stack includes softare to define a geometric mapping from the representation of World Coordinates in input images to that used for LSST. 

Sky patch

An quadrilateral sub-region of a sky tract, with a size in pixels chosen to fit easily into memory on desktop computers. 

Sky tract

A portion of sky, a spherical convex polygon, within the LSST all-sky tessellation. The implementation for SDSS Stripe 82 divided this equatorial band into quadrangular tracts, bounded by constant RA and Dec. Each tract was subdivided into sky patches


In normal LSST operations, a standard visit consists of two snaps, or two consecutive exposures with the same exposure time, telescope alignment, and filter. 

Solar system object

A solar system object is an astrophysical object that is identified as part of the Solar System: planets and their satellites, asteroids, comets, etc. This class of object had historically been referred to within the LSST Project (rather imprecisely) as Moving Objects. (Everything moves, if you wait long enough.) 


A single detection of an astrophysical object in an image, the characteristics for which are stored in the Source Catalog of the science database. The Data Management System attempts to associate multiple source detections to single objects, which may vary in brightness or position over time. 

Source association

The process of associating source detections on multiple images taken at different epochs, or in multiple passbands, with a single astronomical object

Stripe 82

A 2.5° wide equitorial band of sky covering roughly 300 square degrees that was observed repeatedly in 5 passbands during the course of the SDSS, In part for calibration purposes. 



A transient source is one that has been detected on a difference image, but has not been associated with either an astronomical object or a solar system body



A sequence of one or more consecutive exposures at a given position, orientation, and filter within the LSST cadence. The baseline visit for the main survey consists of two snaps of 15 s duration. 



A World Coordinate System is a mapping from image pixel coordinates to physical coordinates; in the case of images the mapping is to sky coordinates, generally in an equatorial (RA, Dec) system. The WCS is expressed in FITS file extensions as a collection of header keyword=value pairs (basically, the values of parameters for a selected functional representation of the mapping) that are specified in the FITS Standard


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