The following terms and acronyms are used in this Handbook.
|Operating 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
|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.
|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 estimate of the actual time needed to obtain an observation. For more information, go to http://apt.stsci.edu.
|The Bright Object Aperture is 2.5 arcsec in diameter with a neutral-density filter that attenuates flux by a factor of about 200.
|The COS calibration pipeline, a software package that performs image and spectroscopic data reduction to produce output files useful for scientific analysis.
|For 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.
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.
|The 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.
|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 required to acquire or observe an object. The ETC is used together with the APT to plan HST observations. For more information, go to http://www.stsci.edu/hst/cos/software/planning/etc.
|Flat-field Calibration Aperture, the aperture through which the on-board deuterium lamps illuminate the COS optical system.
|Fine Guidance Sensor. By tracking guide stars, the three FGSs can maintain the pointing stability of HST with a precision of 2 mas or less.
|A command used to move the spectrum on the detector (in the dispersion direction) to reduce the effects of fixed-pattern noise.
|Far 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.
|The far-ultraviolet channel of COS can observe wavelengths from less than 900 to 1800 Å.
|A 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.
|Galaxy 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 http://www.galex.caltech.edu.
|Guide 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.
|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.
|The 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.
|Investigation Definition Team, NASA's term for the group that proposed and built COS.
|A 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.
|Line Spread Function, the shape of a spectral feature emitted by a monochromatic point source.
|Multi-Anode Micro-channel Array, a photon-counting UV detector, used in the NUV channel.
|The Mikulski Archive for Space Telescopes, which makes available data from a number of NASA missions, including HST. Go to
|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 electronic processing.
|MIRRORA 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.
|The near-ultraviolet channel of COS can observe wavelengths from ~1650 to 3200 Å.
|The Optics Select Mechanisms place gratings or mirrors in the optical path.
|Optical Telescope Assembly, HST's optical system of primary and secondary mirrors, plus the structure that holds them and maintains alignment.
|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 to a pixel based on calculations, but there are no physical pixels as such.
|Pulse-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
Y," special requirement is used to request a target offset in APT.
TARG offsets are specified in the COS user coordinate system, which is used in all COS data products (Section 13.6). Note that the
TARG coordinates represent motion of the target in the aperture; the telescope moves in the opposite direction.
|Primary Science Aperture, a circular aperture 2.5 arcsec in diameter and completely open.
|Point Spread Function, the two-dimensional distribution of light at the detector plane produced by the HST+COS optics.
|Resolution 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.
|The 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.
|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 performed in the ensuing cycle.
|Artificially 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.
|In 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.
|To 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.
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.
|A 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.
|A wavelength calibration exposure; i.e., an exposure of the Pt-Ne wavelength calibration lamp through the WCA.
|Wavelength Calibration Aperture, which is illuminated by a Pt-Ne wavelength calibration lamp.
|Cross-dispersion direction, corresponding to the Y axis on both the FUV and NUV detectors.
|Cross Delay Line, the type of detector used in the FUV channel of COS.
COS Instrument Handbook
- Chapter 1: An Introduction to COS
- Chapter 2: Proposal and Program Considerations
- Chapter 3: Description and Performance of the COS Optics
- Chapter 4: Description and Performance of the COS Detectors
Chapter 5: Spectroscopy with COS
- 5.1 The Capabilities of COS
- • 5.2 TIME-TAG vs. ACCUM Mode
- • 5.3 Valid Exposure Times
- • 5.4 Estimating the BUFFER-TIME in TIME-TAG Mode
- • 5.5 Spanning the Gap with Multiple CENWAVE Settings
- • 5.6 FUV Single-Segment Observations
- • 5.7 Internal Wavelength Calibration Exposures
- • 5.8 Fixed-Pattern Noise
- • 5.9 COS Spectroscopy of Extended Sources
- • 5.10 Wavelength Settings and Ranges
- • 5.11 Spectroscopy with Available-but-Unsupported Settings
- • 5.12 FUV Detector Lifetime Positions
- • 5.13 Spectroscopic Use of the Bright Object Aperture
- Chapter 6: Imaging with COS
- Chapter 7: Exposure-Time Calculator - ETC
Chapter 8: Target Acquisitions
- • 8.1 Introduction
- • 8.2 Target Acquisition Overview
- • 8.3 ACQ SEARCH Acquisition Mode
- • 8.4 ACQ IMAGE Acquisition Mode
- • 8.5 ACQ PEAKXD Acquisition Mode
- • 8.6 ACQ PEAKD Acquisition Mode
- • 8.7 Exposure Times
- • 8.8 Centering Accuracy and Data Quality
- • 8.9 Recommended Parameters for all COS TA Modes
- • 8.10 Special Cases
- Chapter 9: Scheduling Observations
Chapter 10: Bright-Object Protection
- • 10.1 Introduction
- • 10.2 Screening Limits
- • 10.3 Source V Magnitude Limits
- • 10.4 Tools for Bright-Object Screening
- • 10.5 Policies and Procedures
- • 10.6 On-Orbit Protection Procedures
- • 10.7 Bright Object Protection for Solar System Observations
- • 10.8 SNAP, TOO, and Unpredictable Sources Observations with COS
- Chapter 11: Data Products and Data Reduction
Chapter 12: The COS Calibration Program
- • 12.1 Introduction
- • 12.2 Ground Testing and Calibration
- • 12.3 SMOV4 Testing and Calibration
- • 12.4 COS Monitoring Programs
- • 12.5 Cycle 17 Calibration Program
- • 12.6 Cycle 18 Calibration Program
- • 12.7 Cycle 19 Calibration Program
- • 12.8 Cycle 20 Calibration Program
- • 12.9 Cycle 21 Calibration Program
- • 12.10 Cycle 22 Calibration Program
- • 12.11 Cycle 23 Calibration Program
- • 12.12 Cycle 24 Calibration Program
- • 12.13 Cycle 25 Calibration Program
- • 12.14 Cycle 26 Calibration Program
- • 12.15 Cycle 27 Calibration Program
- • 12.16 Cycle 28 Calibration Program
- • 12.17 Cycle 29 Calibration Program
- • 12.18 Cycle 30 Calibration Program
- • 12.19 Cycle 31 Calibration Program
Chapter 13: COS Reference Material
- • 13.1 Introduction
- • 13.2 Using the Information in this Chapter
- 13.3 Gratings
- • 13.4 Spectrograph Design Parameters
- • 13.5 The Location of COS in the HST Focal Plane
- • 13.6 The COS User Coordinate System
- • Glossary