ACS - Advanced Camera for Surveys (See Instruments).
Arc minute (am) - See arcsecond.
Arcsecond (as) - For the measurement of small angles, degrees are too large. 1 Degree = 60 arc minutes ; 1 arc minute = 60 arc seconds. Further divisions are possible by using the SI prefixes e.g. mas = milli arc second or 1/1000th of 1 arc second. The "arc" in arc second assures that this is a measure of angle, not time.
Axial Instrument - Science Instruments for HST are classified according to their shape and how they are mounted in the telescope. Axial instruments are about the size and shape of a telephone booth. There are a total of four of these instruments in HST at any one time. Currently the four axial instruments are: STIS, NICMOS, FOC and COSTAR. Axial instruments are mounted in the rear of the telescope along the optical axis. Light from the telescope's mirrors enters these instruments directly.
Charge Coupled Device (CCD) - Charge coupled devices are the detectors used in modern video cameras and in digital cameras. Like all electronic photosensors, they turn light (photons) into electric charges (electrons). CCD's have many characteristics which are desirable for astronomy. A good CCD has a better dynamic range than film, enabling a CCD based camera to record many levels of light and dark. CCD's are linear over much of their range, meaning that the intensity of the image may be measured. They are fast, and respond to a wide range of wavelengths (see Electromagnetic Spectrum).
COS - Cosmic Origins Spectrograph (See Instruments).
COSTAR - Corrective Optics Space Telescope Axial Replacement (See Instruments).
Electromagnetic Spectrum - Light is an electromagnetic wave. Different colors of light correspond to different wavelengths. The Hubble Telescope has instruments which can see wavelengths from Near Ultra-violet (NUV) to the Near Infra-Red (NIR).
FGS - Fine Guidance Sensors (See Instruments).
Field of View (FOV) - The field of view of an astronomical instrument is the angular size of an image. Although the WFC3 instrument will have a large field of view, this is large compared to other instruments. The entire field of view of the Hubble telescope is comparable to the "O" in "One Dime" held at arm's length. Although this seems like a small area, images from astronomical targets are frequently smaller than this.
FOC - Faint Object Camera (See Instruments).
Focal Plane - The imaginary surface at which the light to all of the HST instrument is focused. Also the diagram which shows how each of the instruments see the sky.
FOS - Faint Object Spectrometer (See Instruments).
GHRS - Goddard High Resolution Spectrograph (See Instruments).
Instruments - The Hubble Space Telescope Program uses many instruments to probe the heavens. The past, present, and future instruments are:
Near Infrared (NIR) - See Electromagnetic Spectrum.
Near Ultra-Violet (NUV) - See Electromagnetic Spectrum.
Nanometer (nm) - Wavelengths of light are extremely small. Units like nanometers (billionths of meters) are frequently used. Visible light is 400-700 nm.
Radial Instrument - Science Instruments for HST are classified according to their shape and how they are mounted in the telescope. Radial instruments are mounted above the axial instruments and to the side. Radial instruments receive light by use of "pickoff mirrors" which redirect the light path into the instrument, rather than directly like axial instruments. There are a total of four radial instruments on HST. Three of the radial instruments are the FGS's and the fourth is WFPC2. WFC3 will replace WFPC2.
Selectable Optical Filter Assembly (SOFA) - This is the name of the carousel which was removed form WF/PC (1) and which held the filters used on orbit in this instrument. By re-using legacy equipment in this way, development costs are held down for WFC3.
Servicing Mission (SM) - A servicing mission is a Space Shuttle flight during which repairs and upgrades are performed on the Hubble Space Telescope. There have been two Servicing Missions to date, and three more are planned.
Spectrum - If white light is passed through a prism, it is broken into many colors, like a rainbow, called a spectrum. The spectrum of a star or luminous source can tell us many things about it, like temperature, composition, and velocity. See Electromagnetic Spectrum.
STScI - Space Telescope Science Institute - The STScI is located in Baltimore, Maryland at the John's Hopkins University. The STScI maintains all operations for the telescope including proposal selection, scheduling, data calibration, distribution, and archiving. Their website can be found here.
WF/PC (1) - Wide Field / Planetary Camera (See Instruments).
WFPC2 - Wide Field / Planetary Camera II (See Instruments).
WFC3 - Wide Field Camera 3 (See Instruments).