Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

A Glossary of Terms and Abbreviations

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ACCUM

Operating mode for COS in which only the locations of detected photons are recorded. No time information is recorded, and this makes it possible to deal with higher count rates and hence brighter objects. See also TIME-TAG.

ApM

Aperture Mechanism, used to place either the BOA or PSA into position as the science aperture. The ApM is also moved to place the FCA into position if a flat-field exposure is to be taken.

APT

The Astronomer's Proposal Tool, software provided by STScI for writing Phase I proposals and Phase II programs. The use of APT is encouraged in all cases, even for Phase I proposals, because it provides an accurate estimation of the actual time needed to get an observation. For more information, go to:

 http://www.stsci.edu/hst/proposing/apt

BOA

Bright Object Aperture. Like the PSA, the BOA is 2.5 arcsec in diameter, but it also includes a neutral density filter that attenuates by a factor of about 200. Because of a 15 arcmin wedge in this optical element, the BOA also degrades the spectral resolution when it is used. See Figure 3.4.

ETC

Exposure Time Calculator, software provided by STScI to estimate exposure times needed to achieve, say, a given signal-to-noise level on a source. Although information is provided in this Handbook on exposure estimation, the ETC provides the most accurate way to determine the exposure times involved in acquiring or observing an object. In addition to what is in the ETC, APT includes factors such as instrumental overheads. For more information, go to:

 http://etc.stsci.edu/webetc/index.jsp

FCA

Flat-field Calibration Aperture, the aperture through which the on-board deuterium lamps illuminate the COS optical system.

FEFU

"femto-erg flux unit." 1 FEFU = 10^-15 erg cm^-2 sec^-1 Å^-1

FP-POS

A command used to move the spectrum on the detector so as to use different portions, thereby reducing the effects of fixed-pattern noise.

FUV

Far ultraviolet, the channel of COS that is used from about 1150 to 1800 Å.

Galex

Galaxy Evolution Explorer, a NASA mission observing the sky in two ultraviolet bandpasses. Galex data is useful for determining the likely UV fluxes of COS targets. For more information, go to:

 http://www.galex.caltech.edu/

GTO

Guaranteed Time Observer, a member of the COS science team who has been granted a share of telescope time as part of their involvement in designing and building COS.

home position

The default position for a mechanism. COS is reconfigured at the start of each new visit and so the mechanisms will be found in their home positions at that time. For the ApM the home is the PSA. For OSM1, home is G130M, and for OSM2 home is G185M.

IDT

Instrument Development Team, NASA's term for the group that proposed and built COS.

LSF

Line Spread Function, the shape of a point source along the direction of dispersion.

MAMA

Multi-Anode Micro-channel Array, a photon-counting UV detector, used in the NUV channel.

MAST

The Multi-mission Archive at Space Telescope, which makes available data from a number of NASA missions (including HST) and other sources. Go to:

 http://archive.stsci.edu

MCP

Micro-Channel Plate, a resistive glass plate with 10-15 micron-sized holes used within both the XDL and MAMA detectors to amplify photo-electrons into charge pulses large enough for downstream electronic processing.

MIRRORA, MIRRORB

MIRRORA and MIRRORB are used for NUV imaging in COS. MIRRORA provides the highest throughput. MIRRORB uses a reflection off of the order-sorting filter of MIRRORA to get lower throughput, which can be helpful when observing brighter targets.

NUV

The near ultraviolet channel of COS.

OSM1, OSM2

The Optics Select Mechanisms on COS that place gratings or mirrors in the optical path.

OTA

Optical Telescope Assembly, HST's optical system of primary and secondary mirrors, plus the structure that holds them and maintains alignment.

pixel

The basic stored unit of data. In the NUV channel, MAMA pixels correspond to physical portions of the detector. In the FUV channel, the position of a detected event is assigned a pixel based on calculations, but there are no physical pixels as such.

PHD

Pulse-Height Distribution, a histogram of the charge cloud sizes collected in a particular exposure or portion thereof. The PHD is a useful diagnostic tool of data quality and is recorded as a data product for FUV exposures. No PHD data are available for NUV exposures. See Section 4.1.7.

PSA

Primary Science Aperture, which is 2.5 arcsec in diameter and is completely open.

PSF

Point Spread Function, the two-dimensional distribution of light produced by HST's optics.

resel

Resolution element, the basic unit of resolution in a spectrum. In the FUV channel, resels are 6 pixels wide (dispersion direction) by 10 tall. In the NUV channel, resels are 3 × 3 pixels. Note that spectra are recorded in pixel units and that any rebinning into resels is done later during data reduction. Also note that a resel corresponds approximately to the FWHM of narrow wavelength calibration lines, although the actual resolution is somewhat different from one grating to the next.

SMOV

Servicing Mission Observatory Verification, the period immediately following a servicing mission in which HST's instruments are activated, tested, and made ready for science observing. Only a minimal set of calibrations are done in SMOV to confirm instrument performance; more detailed calibrations are done in the ensuing cycle.

stim pulse

A virtual point source that is located in pixels at opposite corners of each segment of the FUV XDL detector system. These point sources allow for thermal distortion to be calibrated and aid in determining the dead-time correction. For more information, see Section 4.1.6.

TAGFLASH

Use of TIME-TAG mode with FLASH=YES selected. This adds wavelength calibration spectra at periodic intervals during a TIME-TAG observation so that any drifts of the spectrum due to residual motion of the optics can be removed.

TIME-TAG

A COS observing mode in which the locations (pixels) and times (to the nearest 32 msec) are recorded for each detected photon. Doing this can consume buffer capacity but allows great flexibility in reducing and analyzing the data later.

wavecal

A wavelength calibration exposure; i.e., an exposure of the Pt-Ne wavelength calibration lamp through the WCA.

WCA

Wavelength Calibration Aperture, which is illuminated by a Pt-Ne wavelength calibration lamp.

XDL

Cross Delay Line, the type of detector used in the FUV channel of COS.