Hubble Legacy Archive
Glossary

A

ACS

Advanced Camera for Surveys. The ACS contains three cameras; the wide field camera WFC, the high-resolution camera HRC, and the solar blind camera SBC.

ACS grism

ACS grism data are slitless spectra acquired with the Advanced Camera for Surveys grism mode. The Space Telescope European Coordinating Facility (ST-ECF) has created data products for the HLA by extracting and calibrating the spectra of individual objects in the ACS grism observations.

B

C

COS

Cosmic Origin Spectrograph. Installed in May 2009 during SM4, COS is best at observing points of light, like stars and quasars. COS focuses exclusively on ultraviolet light and has two channels, one to examine far-ultraviolet light, and one to examine near-ultraviolet light.

cosmic rays

Energetic particles traveling through space at high velocity. CRs create image artifacts (bright pixels) in exposures and need be removed in the image processing by combining multiple exposures or by filtering based on the shapes of the artifacts.

D

DADS

Data Archive and Distribution System. This is the "classic" archive retrieval system for Hubble data. Data that are not yet in the HLA will be usually be available through DADS. Observations taken within the past year are often proprietary, meaning that special archive account privileges are required for data retrieval.

DSS

Digitized Sky Survey. The DSS comprises a set of all-sky photographic surveys in E, V, J, R, and N bands conducted with the Palomar and UK Schmidt telescopes. The glass photographic plates were scanned and digitized by STScI for the Guide Star Catalog projects. The HLA uses color images created by combining red and blue DSS plates as the background image in the footprints view.

Dec

Declination. Dec is the celestial latitude measured in degrees north or south of the celestial equator.

DrizzlePac

DrizzlePac is a software package for aligning and combining HST images, which brings an array of new features and algorithmic improvements, including AstroDrizzle, which removes geometric distortion, corrects for sky background variations, flags cosmic-rays, and combines images with optional subsampling. It is the successor to the older, widely used MultiDrizzle software.

E

F

FIRST

Faint Images of the Radio Sky at Twenty-cm. FIRST is a project designed to produce the radio equivalent of the Palomar Observatory Sky Survey over 9,000 square degrees of the North and South Galactic Caps. Data from the NRAO Very Large Array (VLA) are reduced using an automated mapping pipeline to produce images with 1.8" pixels, a typical rms of 0.15 mJy, and a resolution of 5". At the 1 mJy source detection threshold, there are ~90 sources per square degree, ~35% of which have resolved structure on scales from 2-30". 30% of the sources have counterparts in the Sloan Digital Sky Survey.

FITS

Flexible Image Transport System. FITS is the standard astronomical data format endorsed by both NASA and the IAU. FITS is much more than an image format (such as JPEG or GIF) and is primarily designed to store scientific data sets consisting of multi-dimensional arrays (1-D spectra, 2-D images or 3-D data cubes) and 2-dimensional tables containing rows and columns of data.

FOS

Faint Object Spectrograph. The FOS was designed to make spectroscopic observations of astrophysical sources from the near ultraviolet to the near infrared (1150 - 8000 Angstroms). It was used from the Hubble launch in 1990 until its removal in 1997.

G

Gaia

The Gaia astrometry mission. Gaia is an ESA mission that is determining astrometry to accuracy of a milliarcsecond or better for stars brighter than approximately 20th magnitude. The HLA and HSC are using the Gaia DR1 catalog as an astrometric reference catalog. Note that Gaia DR1 does not include proper motions (except for stars that are too bright to measure accurately in Hubble images), so the astrometric accuracy degrades for observations that are not close to the Gaia reference epoch in 2015.

GALEX

The Galaxy Evolution Explorer. GALEX is a NASA mission led by the California Institute of Technology aimed at investigating how star formation in galaxies evolved from the early Universe up to the present. GALEX uses microchannel plate detectors to obtain direct images in the near-UV (NUV) and far-UV (FUV) and a grism to disperse light for spectroscopy. The GALEX data archive is located at MAST; GALEX data products include a series of all sky surveys and deep sky surveys in the imaging mode and partial surveys in the near and far UV spectroscopic modes.

GHRS

Goddard High Resolution Spectrograph. The GHRS was a high-resolution spectrograph operating in the ultraviolet. It was used from the Hubble launch in 1990 until its removal in 1997.

GSC2

Guide Star Catalog 2. The Guide Star Catalog 2 is an all-sky optical catalog based on the 1" resolution scans of the photographic Sky Survey plates (the DSS), at two epochs and three bandpasses, from the Palomar and UK Schmidt telescopes. It includes positions, magnitudes and classifications for approximately 1 billion (10⁹) objects.

grism

A combination of a prism and grating arranged to keep light at a chosen central wavelength undeviated as it passes through the grism. The resolution of a grism is proportional to the tangent of the wedge angle of the prism in much the same way as the resolution of gratings are proportional to the angle between the input and the normal to the grating. The grism then creates a dispersed spectrum centered on the location of the object in the camera field of view.

H

HRC

High Resolution Camera. This is one of the 3 imaging channels on the ACS. The HRC was a high-resolution CCD camera with 0.025 arcsecond pixels (half the size of the WFC pixels). With a field of view only 22 arcsec wide, it was used to make the highest resolution images of small sky areas from the near infrared to the near ultraviolet. It also included a coronagraphic mode to suppress the light from bright astronomical sources, allowing for much fainter objects to be detected nearby. The HRC is inoperable due to the power supply failure in January 2007; it was not able to be repaired in the servicing mission in May 2009.

J

L

M

MAST

Multi-mission Archive at the Space Telescope. The Multimission Archive at STScI is a NASA funded project to support and provide to the astronomical community a variety of astronomical data archives, with the primary focus on scientifically related data sets in the optical, ultraviolet, and near-infrared parts of the spectrum. The HLA project is part of MAST, as are many other archival products and services used by the HLA including DADS, DSS, GSC2, GALEX, FIRST, Kepler, etc.

Moving Target

In relation to HST, a moving target is an object that is in motion in our solar system, such as planets or asteriods.

MultiDrizzle

MultiDrizzle combines dithered observations (different overlapping pointings that are shifted or rotated) to create combined images. It can correct for geometrical distortion in the camera, can orient the combined image to have north up (which is not the case for raw Hubble images), and automates and simplifies the detection of cosmic-rays. It is implemented using the Python scripting language and PyRAF, the Python-based interface to IRAF.

N

NICMOS

The Near Infrared Camera and Multi-Object Spectrometer. The NICMOS provides imaging capabilities in broad, medium, and narrow band filters, broad-band imaging polarimetry, coronographic imaging, and slitless grism spectroscopy, in the wavelength range 0.8-2.5 microns. NICMOS has three adjacent but not contiguous cameras, designed to operate independently, each with a dedicated array at a different magnification scale.

NICGrism

NICMOS grism data are slitless spectra acquired with the NICMOS grism mode. The Space Telescope European Coordinating Facility (ST-ECF) has created data products for the HLA by extracting and calibrating the spectra of individual objects in the NICMOS grism observations.

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P

photometry

Photometry is the term used for the study of light emitted by a star or other astronomical object. This brightness is generally defined in absolute terms.

PS1

PanSTARRS DR1 Catalog. PanSTARRS surveyed the sky north of declination -30 degrees and generated a 5-color (grizy) catalog that is somewhat deeper and higher-resolution than SDSS over a larger sky area.

Q

R

RA

Right Ascension. The RA is the value of a longitudinal angle as it occurs in the equitorial coordinate system. It is measured in hours, minutes, and seconds, or it may also be measured in decimal degrees (with 360 degrees = 24 hours).

S

SBC

Solar Blind Camera. This is one of the 3 imaging channels on the ACS. It uses an ultraviolet-sensitive photon-counting detector to observe small faint regions at wavelengths inaccessible to CCD detectors. It is deliberately designed not to have any sensitivity to photons at the longer visible wavelengths of light in order to allow accurate measurements of the faint ultraviolet brightness of objects that are very bright at redder wavelengths. The detector is the same model as is used in the STIS UV detectors.

SDSS

Sloan Digital Sky Survey. The SDSS used a dedicated 2.5-meter telescope at Apache Point Observatory, New Mexico to obtain deep, multi-color images covering more than a quarter of the sky and acquired spectra to measure redshifts for more than 930,000 galaxies and 120,000 quasars.

spectral element

Spectral element refers to the filter or grating used for an exposure.

spectroscopy

Spectroscopy is the use of light, sound or particle emission to study matter. The emissions are, in many cases, able to provide information about the properties of the matter under investigation. The device often used for such analysis is a spectrometer, which records the spectrum of light emitted (or absorbed) by a given material, especially in analytical chemistry and physical chemistry fields, where the light can be used to determine the chemical composition of a substance because of signature spectral lines emitted by known elements.

STIS

Space Telescope Imaging Spectrograph. The instrument provides spatially resolved spectroscopy from 1150 to 10,300 Å at low to medium spectral resolution, high spatial resolution echelle spectroscopy in the ultraviolet, solar-blind imaging in the ultraviolet, time tagging of photons in the ultraviolet for high time resolution, and direct and coronagraphic imaging in the optical.

T

2MASS

Two Micron All Sky Survey. 2MASS has uniformly scanned the entire sky in three near-infrared bands to detect and characterize point sources brighter than about 1 mJy in each band, with signal-to-noise ratio (SNR) greater than 10, using a pixel size of 2.0". 2MASS used two highly-automated 1.3-m telescopes, one at Mt. Hopkins, AZ, and one at CTIO, Chile. Each telescope was equipped with a three-channel camera, each channel consisting of a 256×256 array of HgCdTe detectors, capable of observing the sky simultaneously at J (1.25 microns), H (1.65 microns), and Ks (2.17 microns).

U

V

W

WCS

World Coordinate System. The WCS convention defines keywords and usage that provide for the description of astronomical coordinate systems in a FITS image header.

WFC

Wide Field Camera. This is one of the 3 imaging channels on the ACS. The WFC is the most utilized channel of ACS that conducts broad surveys of the universe. Its detector consists of two butted 2048x4096, 15 µm/pixel charge-coupled devices (CCDs) for a total of 16 megapixels. The WFC plate scale is 0.05″ per pixel and it has an effective field-of-view of 202″×202″. The spectral range of the WFC detector is 350-1100 nm.

WFC3

Wide Field Camera 3. Installed in May 2009 during SM4, WFC3 observes near-infrared light, visible light, and near-ultraviolet radiation. WFC3 has two "channels." Each channel detects and processes different wavelengths. The ultraviolet-visible channel can be used to study nearby galaxies and galaxies undergoing bursts of star formation. The near-infrared channel can be used to study the light from distant galaxies.

WFPC2

Wide Field Planetary Camera-2. The WFPC2 was used to obtain high resolution images of astronomical objects over a relatively wide field of view and a broad range of wavelengths (1150 to 11,000 Å). WFPC2 includes 4 800x800 CCDs, three with 0.1" pixels (the WF channels) and one with 0.05" pixels (the PC). Most observations include data from all 4 CCDs. The HLA includes two different data products for most WFPC2 observations. The WFPC2 images have all 4 CCDs combined onto a 0.1" pixel scale that matches the WF CCDs; the WFPC2-PC images include only the PC chip and have a 0.05" pixel scale. These images can be selected separately using the WFPC2 and WFPC2-PC instrument choices in the HLA advanced search options. The WFPC2 operated from 1994 until its replacement in the May 2009 servicing mission.

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