|COS Instrument Handbook for Cycle 25|
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 TIME-TAG.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 toThe 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.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 toFlat-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.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 toGuide 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=1309; and for OSM2, home is G185M, CENWAVE=1850.The Mikulski Archive for Space Telescopes, which makes available data from a number of NASA missions, including HST. Go toMicro-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 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.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.The “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.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.Use 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.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.