The following terms and acronyms are used in this Handbook.

ACCUMOperating mode for COS in which only the locations of detected photons are recorded; no time information is recorded. ACCUM mode is designed for bright objects with high count rates. See also TIME-TAG.
Along Dispersion (AD)The dispersion direction, corresponding to the long/major axis on both the FUV and NUV detectors.
Aperture Mechanism (ApM)

The Aperture Mechanism is 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.

APTThe 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 estimate of the actual time needed to obtain an observation. For more information, go to
BOAThe Bright Object Aperture is 2.5 arcsec in diameter with a neutral-density filter that attenuates flux by a factor of about 200.
CalCOSThe COS calibration pipeline, a software package that performs image and spectroscopic data reduction to produce output files useful for scientific analysis.
central wavelengthFor the NUV gratings, the central wavelength is the approximate midpoint of the stripe B spectrum. For the FUV gratings, the central wavelength refers approximately to the shortest wavelength recorded on Segment A. This is frequently referred to as "cenwave" throughout this handbook.
channel (FUV or NUV)One of the two COS optical systems, FUV and NUV, including mirrors, gratings, and detectors.
COS 2025

A policy that took effect at the beginning of Cycle 25 with the goal of retaining full science capability of the COS/FUV channel until 2025. It places restrictions on the use of particular G130M cenwaves at Lifetime Position 4 in order to reduce gain sag due to geocoronal Lyman α.

CVZThe Continuous Viewing Zones are regions of the sky where HST can observe without interruptions caused by target occultation by the Earth. These zones are approximately 24 degrees in size centered on the orbital poles, which are 28.5 degrees from the celestial poles.
ETCExposure 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 required to acquire or observe an object. The ETC is used together with the APT to plan HST observations. For more information, go to
FCAFlat-field Calibration Aperture, the aperture through which the on-board deuterium lamps illuminate the COS optical system.
FGSFine Guidance Sensor. By tracking guide stars, the three FGSs can maintain the pointing stability of HST with a precision of 2 mas or less.
FP-POSA command used to move the spectrum on the detector (in the dispersion direction) to reduce the effects of fixed-pattern noise.
FUSEFar Ultraviolet Spectroscopic Explorer, a moderate-resolution (R ~ 15,000), far-UV spectrograph that used micro-channel plate detectors similar to those employed by the FUV channel of COS.
FUVThe far-ultraviolet channel of COS can observe wavelengths from less than 900 to 1800 Å.
gain sagA reduction in the efficiency of the COS FUV micro-channel plate detector at converting incoming photons into detectable events. It is a consequence of detector use.
GALEXGalaxy Evolution Explorer, a NASA mission observing the sky in two ultraviolet bandpasses. GALEX data are useful for determining the UV fluxes of COS targets. For more information, go to
GSC2/ICRSGuide Star Catalog II/International Celestial Reference System. The GSC2 is an all-sky optical catalog based on 1'' resolution scans of the photographic Sky Survey plates from the Polomar and UK Schmidt telescopes. The ICRS is the fundamental celestial reference system adopted by the International Astronomical Union for high-precision astrometry. Uncertainties in this system are dominated by the 0.3'' uncertainty of the GSC2.
GTOGuaranteed 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 positionThe default position for a mechanism. COS is reconfigured at the start of each visit, and mechanisms are returned to their home positions. For the ApM, the home is the PSA; for OSM1, home is G130M, CENWAVE=1222; and for OSM2, home is MIRRORA.
IDTInvestigation Definition Team, NASA's term for the group that proposed and built COS.
lifetime positionA region on which a spectrum illuminates the FUV detector. Due to the onset of gain sag, the lifetime position has been changed four times since the beginning of COS operations. Starting in Cycle 29, COS began using multiple lifetime positions at the same time. The most recently commissioned lifetime position was LP5, which took place on October 4, 2021. The next lifetime position will be LP6, which will be commissioned at the beginning of Cycle 30.
 LSFLine Spread Function, the shape of a spectral feature emitted by a monochromatic point source.
MAMAMulti-Anode Micro-channel Array, a photon-counting UV detector, used in the NUV channel.
MASTThe Mikulski Archive for Space Telescopes, which makes available data from a number of NASA missions, including HST. Go to
MCPMicro-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 electronic processing.
MIRRORA, MIRRORBMIRRORA and MIRRORB are internal flat mirrors 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 bright targets.
NUVThe near-ultraviolet channel of COS can observe wavelengths from ~1650 to 3200 Å.
OSM1, OSM2The Optics Select Mechanisms place gratings or mirrors in the optical path.
OTAOptical Telescope Assembly, HST's optical system of primary and secondary mirrors, plus the structure that holds them and maintains alignment.
pixelThe 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 to a pixel based on calculations, but there are no physical pixels as such.
PHDPulse-Height Distribution, a histogram of the charge cloud sizes collected in a particular exposure or portion thereof. The PHD is a useful measure of data quality and is recorded as a data product for FUV exposures. PHD data are not available for NUV exposures or in ACCUM mode.
POS TARGThe "POS TARG X, Y," special requirement is used to request a target offset in APT. POS TARG offsets are specified in the COS user coordinate system, which is used in all COS data products (Section 13.6). Note that the POS TARG coordinates represent motion of the target in the aperture; the telescope moves in the opposite direction.
PSAPrimary Science Aperture, a circular aperture 2.5 arcsec in diameter and completely open.
PSFPoint Spread Function, the two-dimensional distribution of light at the detector plane produced by the HST+COS optics.
reselResolution element of a spectrum or image. For spectra, a resel corresponds to the FWHM of a narrow emission line. Using pre-flight data, resels were determined to be roughly 6 pixels wide (dispersion direction) by 10 tall for the FUV channel and 3 × 3 pixels for the NUV. On-orbit data suggests that the FUV resel is somewhat larger than this, while the NUV resel is somewhat smaller. Note that spectra are recorded in pixel units and that any rebinning into resels is performed on the ground during data reduction.
segmentThe COS FUV detector consists of two independent segments. In all spectroscopic modes, the long-wavelength end of the spectrum falls on Segment A, and the short-wavelength end on Segment B.
SMOVServicing 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 performed in the ensuing cycle.
stim pulseArtificially induced events on each segment of the FUV detector. The stim pulses allow for the correction of thermal distortion and aid in determining the dead-time correction.
STMAGIn this system, the flux density is expressed per unit wavelength, and the reference spectrum is flat in Fλ. STMAG = 2.5 log Fλ 21.10.
stripeTo accommodate the NUV detector format, COS NUV spectra are split into three non-contiguous stripes, each of which covers a relatively small range in wavelength.
TAGFLASHUse of TIME-TAG mode with FLASH=YES selected. In this mode, wavelength-calibration spectra are obtained at periodic intervals during a PSA TIME-TAG observation so that any drifts of the spectrum due to residual motion of the optics can be removed.
TIME-TAGA COS observing mode in which the locations (pixels) and times (to the nearest 32 msec) are recorded for each detected photon. Doing this consumes memory but allows great flexibility in reducing and analyzing the data.
wavecalA wavelength calibration exposure; i.e., an exposure of the Pt-Ne wavelength calibration lamp through the WCA.
WCAWavelength Calibration Aperture, which is illuminated by a Pt-Ne wavelength calibration lamp.
XDCross-dispersion direction, corresponding to the Y axis on both the FUV and NUV detectors.
XDLCross Delay Line, the type of detector used in the FUV channel of COS.