Glossary

This glossary defines those terms (single words or phrases) concerning Software Engineering, Telescope Control and Interferometry, mentioned in this document, but firstly introduced and extensively described in other documents. The meaning of each term is carefully explained, focusing on its usage in the context of this specific document. Words, belonging to a term, which are enclosed in brackets, are implicit when no ambiguity can arise. Those words, contained in the definition of a term and included also in the Glossary, which are used with a specific technical meaning, are printed in italics. Entries are arranged alphabetically.

• active optics. A method of maintaining a highly accurate optical surface in a reflecting telescope by means of a computer-controlled feedback system that continually monitors the quality of the image and uses this information to adjust, at low spatial and temporal frequencies, a motorized support system under the primary telescope mirror and to perform corrections to the secondary mirror.
• actor. An actor is a role of an entity external to the system. Actors can be humans, machines, or devices. One physical object may play several roles and therefore be modeled by several actor. A primary actor is one having a goal requiring the assistance of the system. A secondary actor is one from which the system needs assistance to satisfy its goal. One of the actors is designated as the system under discussion.
• adaptive optics. A technique for improving the quality of the image produced by an astronomical telescope in which an optical system compensates for constantly varying distortions, generated by turbulence in the Earth's atmosphere (seeing), of the incoming wavefront. The distortions are corrected by the rapid control, at high spatial and temporal frequencies, of a small, deformable mirror.
• apparent motion. Motion of an astronomical object with respect to the observer.
• array configuration. An arrangement of a number of telescopes, delay lines and transfer optics, which cooperate in an interferometric observation.
• autoguiding. A system that automatically ensures the precise guiding of a telescope during an observation. It measures the position error of a star image and performs the required corrections driving the telescope's axes.
• auxiliary telescope. A 1.8 meter diameter telescope, which can be relocated among a set of predefined observing stations. It is used solely by the VLTI, either with other identical auxiliary telescopes or in combination with the unit telescopes.
• baseline. The modulus of the baseline vector.
• baseline vector. A tridimensional vector joining the intersection point of the altitude and azimuth axes of a pair of telescopes.
• beam combiner. An optical system which combines the light beams coming from different telescopes used for interferometry.
• blind tracking. The telescope is continuously pointing the object under observation using its theoretical trajectory, taking into account the sidereal motion, the complete standard coordinate conversion, atmospheric refraction as well as telescope-dependent errors included in the pointing model. It involves the motion of the telescope's axes only.
• calibration object. An astronomical object associated to a specific target object and used for calibration purposes on the acquired data.
• classical observing. An operating mode of the ESO VLT where the observer attends personally his observing program, supervising each step of the observation run.
• command. An expression that can be input to a computer system or a software module to initiate an action or affect the execution of a function.
• database attribute. A location, uniquely identified by a name, within the WS or the LCU real-time database, where a value is stored.
• data store flow. A flow through a store that holds information to be acted upon by one or more data processes without altering that information.
• event store flow. A flow through a store that records occurences of event flows in a First In/First Out (FIFO) order.
• dual feed. An optical system, at a telescope focus, capable to select two narrow field-of-view beams: a primary one on-axis and a secondary one off-axis within a certain maximum radius.
• exposure. The smallest task composing an observation, requiring three main actions: (1) to reset or clear the detector, (2) to accumulate photons and (3) to read-out the detector and store the acquired data.
• exposure time (see also integration time). The time during which the photons are accumulated in an exposure.
• external optical path difference (see also internal optical path difference). One of the terms contributing to the optical path difference, namely OPDext, which takes into account the geometric difference in the optical path length from the observed object to the telescopes. It depends on the object position in the sky and on the baseline vector, and varies in time as the Earth rotates (sidereal motion). It is computed by using the formula: OPDext = S·B, where S is the unit vector pointing to the object, and B the baseline vector.
• field of view. The angular extent of the image formed by an optical instrument, such as a telescope.
• field stabilization. A system which allows to correct automatically for image position errors at a higher frequency than the one provided by autoguiding. It allows to compensate for image jitter generated by wind and, to some extent, atmospheric turbulence. It measures the position error of a star image and performs the required corrections adjusting the tip/tilt angles of a telescope's mirror.
• function. A defined objective or characteristic action of a system or component.
• functional requirement . A requirement that specifies a function that a system or component must be able to perform.
• functional specification. A document that specifies the functions that a system or component must perform.
• guide star. A bright star in the same telescope field of view of the object under observation, which is used as a reference point by the autoguiding system of the telescope.
• integration time. The sum of many short exposure times, needed to acquired data on a specific astronomical object. The integration time is equal to a single long exposure time only when the VLTI is in phase tracking mode.
• interferometric arm. The optical path length of the beam from the astronomical object to the point of beam combination through a given telescope.
• internal optical path difference (see also external optical path difference). One of the two terms contributing to the optical path difference, namely OPDint, which takes into account the internal difference in the optical path length, due to the interferometer layout, from the telescopes to the detector. It is also called delay offset or delay constant.
• mode. A condition of existence that a system, subsystem, or component, may be in. Normally it comprises a set of possible states.
• module. See software module.
• narrow angle astrometry. An observing mode of the VLTI, which allows the measurement of the relative separation of two astronomical objects at the level of few tens of marcsec. It requires the measurement of the differential optical path difference between the on-axis object (primary beam) and the off-axis one (secondary beam). This mode needs the dual feed and a high precision differential metrology.
• observation. The whole process needed to acquire astronomical data from a specified astronomical object. It implies setting up all the required equipment (instrument, telescope(s), adapters, delay lines, sensors), executing exposures, and perform on-line data analysis and display. Sometimes this term is used in a very generic way to mean the act of observing; in such a context a specific observation on a defined object shall be better qualified using the phrase observation run, or, to highlight the software representation of that, observation block.
• observation block. A logical unit of exposures needed to obtain a coherent set of data. Encompasses all relevant information for a successful data acquisition on a target. It consists of target information, a set of operation templates (or sequences), parameter files for the templates, conditions, requirements, and comments concerning the specified observation. It represents the entity the short-term scheduler deals with. Constructing Observation Blocks is part of the Phase II Proposal Preparation Process.
• observation run. The process of actually executing an observation.
• observation sequence. A series of single observations, that is of observing blocks, within an observing program.
• observing program. A set of planned observations, described in a formalized proposal, which provides essentially the list of the astronomical objects and the main technical requirements about the selected instrument. It may be carried out in one or more observing nights.
• observing station. A place on the ESO VLT Observatory site, on the Paranal mountain top, where an auxiliary telescope can be located to perform the actual observation. For convenience the observing station of a unit telescope is obviously defined as its own fixed location.
• operational mode. The mode of a system, subsystem, or component that is installed in its intended environment.
• operational requirement. A requirement that is applicable to a system or component that is installed in its intended environment.
• operational state. The state of a system, subsystem, or component that is installed in its intended environment.
• package. See software package.
• process. See software process.
• projected baseline. A baseline vector as seen from the observed object, that is the geometrical projection of the baseline vector on a plane perpendicular to the direction of propagation of the source radiation.
• reference object. An astronomical object observed for tracking purposes simultaneously with a target object, by using a dual feed.
• reference phase tracking. A sub-mode of phase tracking with a dual feed, where the optical path difference signal is obtained from the interference pattern created by a reference object.
• regression test. A test performed on a modified program to instill confidence that changes are correct and have not adversely affected unchanged portions of the program.
• seeing. The effect of random turbulent variation of the index of refraction in the atmosphere on the quality of the image of an astronomical object.
• service observing (see also classical observing). An operating mode of the ESO VLT where the observer specifies the observing program and the Observatory performs the observations execution on behalf of the applicant. The time of the observation run is not exactly predefined and can be intermixed with programs from other observers.
• setup file. An ASCII file in a special format, describing setup parameters for an equipment (telescope, delay line array, instrument, detector).
• sidereal motion. Apparent motion of a stellar astronomical object.
• software device driver. A collection of subroutines and data that constitutes the software interface to an I/O device.
• software life cycle. The period of time that begins when a software product is conceived and ends when the software is no longer available for use. The software life cycle typically includes a concept phase, requirements phase, design phase, implementation phase, test phase, installation and check-out phase, operation and maintenance phase, and, sometimes, retirement phase. These phases may overlap or be performed iteratively.
• (software) module. A relatively large subdivision of the implementation items in a software package. In most cases a software package correspond to one single software module, but complex software packages can be split across more software modules. A software module is handled as a single configuration control unit and follows a standardized directory structure. Software modules correspond to components in UML terminology.
• (software) package. A major subdivision of a software project that collects a set of correlated functions that are designed, developed and tested all together and independently from other packages. Software packages can be recursively defined as containing other software packages.
• (software) process. A program in execution. It consists of the executable program, the program's data and stack, the support data stored in the database, its program counter, stack pointer and other registers, and all the other information needed to run the program.
• state. The value assumed at a given instant by the variable used by the control software to represent the condition of a system, subsystem, or component. Normally it is a finer specification within a given mode.
• station. See observing station.
• status. The set of values of all the parameters (state, numeric read-outs, flags,...) that define the condition of a system, subsystem, or component.
• stereotype. UML term for "a new kind of model element defined within the model based on an existing kind of model element. Stereotypes may extend the semantics but not the structure of pre-existing metamodel classes."
• subsystem. A secondary or subordinate system within a larger system. It usually refers to a device equipped with the control electronics and low level software.
• system. A collection of components organized to accomplish a specific function or a set of functions. When no further characterized, it is generally used to refer to the whole of a complex equipment made up by heterogeneous parts.
• target object. The astronomical object the observer is interested in.
• tracking. The action of following the apparent motion of an astronomical object in the sky.
• unit telescope. A 8 meter diameter main telescope of the ESO VLT Observatory, which can be used for both stand-alone and interferometric observation.
• use case. "A specific way of using the system by performing some part of the functionality. Each Use Case constitutes a complete course of action initiated by an actor, and it specifies the interaction that takes place between an actor and the system.... The collected use cases specify all the existing ways of using the system"  [RD05].
• uv plane. The space of the projected baseline, measured in wavelengths.
• uv plane coverage. The portion of the UV plane that is spanned by the projected baselines during the observation run because of the Earth's rotation and telescope relocation.