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APPENDIX A: Glossary of Terms and Abbreviations

GHRS Instrument Handbook


Blaze Function

The efficiency of an echelle grating drops sharply as one moves away from blaze center. The shape of the response function is virtually the same for the different orders and this function is known as the Ripple Function (see Section on page 112).

Corner diodes

The detector area of the Digicons is laid out into specific diodes, each of which acts as an independent detector. There are 500 science diodes, each of which is skinny but tall, four focus diodes (see below), and four corner diodes. The corner diodes are large rectangles (0.1 x 1 mm) of detector area above and below the science diodes and are used for measuring background.

Cycles

Proposals to use HST are solicited and reviewed on roughly an annual basis. However, because HST's properties changed fundamentally when COSTAR and WFPC2 were installed, Cycle 3 was defined to end at the time of the Servicing Mission. Cycle 4 began at the end of SMOV. Cycle 6 will end at the time of the 1997 Servicing Mission.

CVZ

Continuous Viewing Zones. The inclined orbit of HST allows for uninterrupted observations of objects in some declination ranges at certain times. See the Call for Proposals for further information.

DEFCAL

Short for Deflection Calibration. All GHRS acquisitions begin with a DEFCAL, which measures the instantaneous location of the images on the onboard spectrum lamps and then compares that location to the nominal coordinates stored in the onboard database. The differences can range over several deflection steps in response to thermal and magnetic drifts. The offsets are applied to the database coordinates of the science apertures to provide an updated estimate of their location.

Focus diodes

See Chapter 7 to see how the diodes in the GHRS Digicons are configured. At both ends of the array of 500 science diodes are two focus diodes. The focus diodes are smaller than the science diodes and are square. The image of the LSA is deflected to the focus diodes to generate MAPs and IMAGEs. The focus diodes are 25 microns square.

GIMP

Geomagnetically-induced image motion problem. This problem underlies our recommendation to have no single exposure be longer than about 5 minutes in length.

GSC

Guide Star Catalog, the list used to find stars upon which the Fine Guidance Sensors can lock to control the pointing of HST.

LSA

Large Science Aperture. This is a square opening at the front of the GHRS that is used to acquire stars and for some science observations. Its dimensions were 2.00 arcsec square before COSTAR is installed and 1.74 arcsec square afterwards. The name used for the LSA will continue to be "2.0".

OPUS

The "OSS and PODPS Unified System." The Observation Support System was the facility located at STScI for real-time interaction between the ground and the HST spacecraft, and PODPS was the pipeline data reduction system. They have now been combined into ne operation called OPUS.

Phase I

A Phase I proposal for HST includes just the information need by the Telescope Allocation Committee (TAC) and STScI to judge scientific merit and technical feasibility. In addition to the scientific justification, you are asked to provide a list of the targets that you wish to observe and a brief description of the observations themselves. We recommend adding comments to provide a clearer explanation of what you intend, even if they are not required.

Phase II

The Phase II proposal is written once the Phase I proposal has been accepted for the HST science program. The Phase II proposal includes all the detailed specifications that are needed to turn your science program into the commands that the spacecraft will execute. As with Phase I, we recommend the liberal use of comments to help ensure that your goals will be achieved.

Ripple Function

See Blaze Function

RPS2

RPSS was the Remote Proposal Submission System. RPS2 is a second-generation version of RPSS that allows the user to plan their observations in detail so as to make efficient use of on-orbit time. To learn more about RPS2, consult the Presto Web page:

http://www.stsci.edu

SAA

South Atlantic Anomaly. A region lying over southeastern South America where the earth's radiation belts dip low, leading to high particle background rates for satellites in Low Earth Orbit. GHRS observations are suspended during passage through the SAA.

Science diodes

These are the primary diodes used for data acquisition. There are 500 of them, and they are 25 microns wide by 400 microns tall.

Side 1, Side 2

GHRS is split into two "sides," one for the short-wavelength detector (D1) and one for the long-wavelength detector (D2). The sides operate independently but depend on each other for communication with the spacecraft. The installation of the GHRS Repair Kit during the HST Servicing Mission has meant that all GHRS communications are now through Side 2. Moreover, Side 2 now solely controls the grating carrousel and LSA shutter.

SPYBAL

SPectrum Y BALance. A SPYBAL consists of a quick observation of the spectrum calibration lamp SC2 at a standard wavelength setting to ensure that the spectrum is properly centered on the diodes in the cross-dispersion direction. The y position at this standard wavelength is compared to a stored value and the difference is applied to the observations made with the proposal configuration until another SPYBAL is done. A SPYBAL is normally done before each new use of a different spectrum element, such as a grating. The resultant spectrum is provided to the observer and can be used to improve the default wavelength calibration.

SSA

Small Science Aperture. The nominal (pre-COSTAR) size was 0.25 arcsec square, but after COSTAR it is 0.22 arcsec square. The name for this aperture will continue to be "0.25".

STEIS

Space Telescope Electronic Information Service. This service provides on-line news, information, and documents via anonymous ftp. To use it, ftp to stsci.edu (Internet node 130.167.1.2) and login with username anonymous, using your last name as password. Use get to transfer the README file in the entry directory; this will provide a general explanation of how to access STEIS information. STEIS is now obsolete and the World Wide Web should be used instead. The URL for GHRS information is:

http://www.stsci.edu/hst/ghrs

STEP-PATT

STEP-PATT is the pattern of operations undertaken in an ACCUM. A typical STEP-PATT defines the relative proportions of time spent accumulating on the science diodes versus time with the background diodes. See Section on page 112.

STEP-TIME

STEP-TIME is the exposure time for the smallest unit of an exposure. For example, during an acquisition, STEP-TIME is the amount of time spent at each dwell point while executing a spiral search pattern. During an ACCUM, the detector integrates for a STEP-TIME before reading the diodes and adding their contents to the memory. A unit of STEP-TIME is spent executing each portion of a STEP-PATT, for example.

Blaze Function
Corner diodes
Cycles
CVZ
DEFCAL
Focus diodes
GIMP
GSC
LSA
OPUS
Phase I
Phase II
Ripple Function
RPS2
SAA
Science diodes
Side 1, Side 2
SPYBAL
SSA
STEIS
STEP-PATT
STEP-TIME