GALEX DATA GLOSSARY

 

 

ACS

Attitude Control System

AIS

All-sky Imaging Survey [EXPTIME=100 s]

All-sky survey tile

A tile composed of multiple, separate dithered pointings, called legs, occurring within a single eclipse.

ATS

Absolute Time Sequence, a list of Spacecraft and Instrument commands accompanied by absolute command execution times.

BFA

Back Focal Assembly

CAG

Calibration Grism, a calibration program designation

CAI

Calibration Imaging, a calibration program designation

CAO

Calibration Opaque, a calibration program designation

CRP

Count Rate Protection

cps

counts per second

DIB

Data Interface Box (part of FEE)

DIS

Deep Imaging Survey [EXPTIME=30,000 s]

Dither

A controlled motion of the satellite to move the telescope boresight in a tight, slow spiral pattern that moves outward to 1.5 diameter across the sky.

Dose

Number of detected photon counts at a each location on a detector

DPU

(Instrument) Data Processing Unit, which contains most of the instrument electronics and commands or controls the rest. The electronics not in the DPU are the FEE, and thermal sensors, heaters, and mechanisms and their sensors.

DSS

Deep Spectroscopic Survey [EXPTIME=1,500,000 s]

Eclipse

Time interval during which the Sun is occulted by the Earth during a particular orbit of the spacecraft, typically 1800-2000 sec in length. All GALEX observations occur while GALEX is in eclipse. One target is observed per eclipse. The data acquired during an eclipse is sometimes referred to as an eclipse.

EE

encircled energy

EXPTIME

Nominal exposure time in seconds at each location in a GALEX survey.

FEE

Front End Electronics for Detector operation

FSW

Flight software

FUV

Far Ultraviolet detector (1344 1786 )

GALEX

Galaxy Evolution Explorer

Grism

A ruled prism which disperses the light into a spectrum. The GALEX grism is slitless so it acts on the entire field of view. (See the Mission & Instrument Overview in GR1 documentation.)

GROW

Grism rotator and optical wheel mechanism. The optical wheel places one of three optics in the optical beam between the telescope and the detector. These are and imaging window, a grism

HK

House Keeping (data)

IOC

In-Orbit Checkout

Leg

A single, dithered pointing within an all-sky survey tile.

MCP

Microchannel Plate, the high-voltage electron-multiplier array that is the basis for the GALEX detectors.

MIS

Medium-Imaging Survey [EXPTIME=1500 s]

MOC

Mission Operations Center

MPS

Mission Planning System, semi-automatic ground software at the SOC and MOC used to plan observations and create the weekly ATS.

MSS

Medium Spectroscopic Survey [EXPTIME=100 ksec]

NGS

Nearby Galaxy Survey [EXPTIME=1500 s]

NUV

Near Ultraviolet detector (1771 2831 )

Observation

The acquisition of photon data from a region of the sky during a single orbital night (eclipse).

ObsFile

Observation commands generated by the SOC to be sent to the spacecraft 

OOL

Out Of Limit (a value for a housekeeping parameter that is outside a preferred range)

PA

Passive Analog (thermistor temperature read by the spacecraft)

PHA

Pulse Height Analyzer

PhotoObj

Extracted objects from direct images

Plan

The requirements, constraints, and priorities for a series of observations of a single target. Requirements might include minimum exposure time and predecessor targets. Constraints may include start and end dates and South Atlantic Anomaly avoidance flags. Priority is used to create a hierarchy of plans based on a simple rank order scheme. These are incorporated into the Mission Planning System.

Pointing

Generic term referring to the location on the sky of a single, dithered GALEX field of view. A single tile or a target may consist of one or more pointings.

Primary Tile

A tile whose center coincides with a non-AIS target center or whose legs coincide with all-sky survey leg centers. Thus a target can be roughly described as a primary tile plus an optical wheel setting.

Programs

A collection of plans designed to accomplish a particular scientific goal. This can include survey programs, calibration programs and targeted observation programs.

PSF

Point Spread Function, the distribution of light in an image of an unresolved object

PTAG

Photon Tag (to temporally delimit raw photon data in the telemetry stream)

QDE

Quantum Detector Efficiency

Region

A pre-defined area on the sky where one or more targets (see list below) are clustered. A region is used for planning purposes.

RTS

Relative Time Sequence, a list of Spacecraft and Instrument commands accompanied by relative command execution times.

SAA

South Atlantic (Magnetic) Anomaly, in which elevated radiation levels require reduced detector high voltage, and no data is taken

SC

Spacecraft

SDSS

Sloan Digital Sky Survey

Secondary Tile

A tile with no single target counterpart, i.e., a secondary tile's photon data will come from multiple targets. Note that no data from secondary tiles exist in the GR1 release.

SOC

Science Operations Center

SODA

Science Operations and Data Analysis

SpecExtract

 Data describing each spectral extraction  (extraction type may be optimal (default) or simple boxcar)

SpecFlux

 Spectrum consisting of wavelengths, fluxes, & error  tables

SpecStrip

 2-D spectrum

SSR

 

Solid State Recorder (non-volatile memory on spacecraft used to store science data and housekeeping telemetry)

Sub-visit

A sequential count of legs detected and processed by the pipeline (see leg and all-sky survey tile). If all data for an AIS tile reach ground and are processed normally, the sub-visit number will correspond to the leg number, but if there are data gaps or the processing is aberrant in some way, legs may be missing (or possibly split in two), but sub-visit numbering will always be sequential. (Note the rough correspondence between visit vs. eclipse, and sub-visit vs. leg.)

Survey

A type of observation designed to study the sky at given depth (exposure time), with a given sky coverage and location, and in Imaging, Grism, or Opaque GROW mode. (See Survey Summary in the GR1 documentation.)

Target

An area of sky observed by GALEX together with the spacecraft-motion and instrument commands used for that observation. Note that because the commanded spacecraft motion can be complex, a target can have a complex shape. In particular, All-sky Imaging Survey (AIS) targets are composed of several separate pointings, which may or may not overlap. Also note that the same geometric part of the sky (see Primary Tile) may be observed in both direct and grism modes, making two targets. A target can be roughly described as a primary tile plus an optical wheel setting. Targets may overlap one another. Most targets will consist of a single, roughly-circular, 1.25 degree diameter GALEX field of view, or, for AIS targets, a series of overlapping fields of view.

TBD

To Be Determined

TDC

Time to Digital Converter (part of FEE detector electronics)

Tile

An area of sky for which the pipeline has generated (or will generate) an image using GALEX photon data. A tile may have an arbitrary shape (see Target), but in general will comprise one or more images with north in the +Y direction. (See Primary Tile, and Secondary_Tile.) Most tiles will consist of a single, roughly-circular, 1.25 degree diameter GALEX field of view, or, for AIS tiles, a series of overlapping fields of view.

TLM

Telemetry

VC

 

Virtual (telemetry) Channel (VC18=spacecraft and VC32=instrument) defined in the protocol for transmission and processing of data from the satellite

Visit

The planned sequential repeat count of an observation from a single eclipse of a single target. For example, if 10 widely-separated eclipses are allocated for observing a particular target, the visits will numbered 1 through 10. Note that since not all planned observations are made successfully, and some data may not reach ground, some visits may be missing from a sequence. See also sub-visit.

VisitData

Processed data from a single visit

WCS

World Coordinate System (FITS CRVAL1,CDELT2,CROTA2,etc.)

WSS

Wide Spectroscopic Survey [EXPTIME=30 ksec]