| HST Data Handbook for COS | ||||
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3.7 Reference Files
This section contains a description of the COS reference files. See Figure 3.1 - Figure 3.5 for which modules use these files and Section 3.4 for explanations of how their contents are applied by those modules.
3.7.1 BRSTTAB: Burst Parameters Table
The
BRSTTABfile provides the parameters needed to identify bursts. It consists of a primary header extension and a binary table extension with the columns listed in Table 3.5. Details of the burst rejection routine are given in Section 3.4.2.
Table 3.5:BRSTTABtable contents
3.7.2 BADTTAB: Bad Time Interval Table
The
BADTTABreference file lists the start and end times of known bad time intervals. It is used by the BADTCORR calibration module to flag events inTIME-TAGevents lists which occur during a bad time interval. In later processing the flagged events will be removed from the final calibrated data, and the exposure time header keyword,EXPTIME, updated. The bad time interval table consists of segment, start, and end columns (see, Table 3.6). The segments columns can be populated with either FUVA, FUVB or ANY. The start and end columns are in Modified Julian Date.
Table 3.6:BADTTABtable content
Column Name Data Type Description SEGMENT String Detector segment, FUVA, FUVB or ANY START Double Bad time interval start time in MJD END Double Bad time interval end time in MJD
3.7.3 PHATAB: Pulse Height Discrimination Table
The
PHATABreference file is only valid for FUV data, and is applied during the PHACORR step of calcos to filter non-photon events. The file consists of two extensions, the first being the primary header, and the second a binary table (see Table 3.7). The table lists the lower and upper thresholds for valid individual pulse heights inTIME-TAGmode. InTime-Tagmode, each detector event has an associated pulse-height of 5 bits with values ranging from 0 to 31, The table also gives the minimum and maximum values for the location of the mean value of the pulse height distribution used inACCUMmode. InACCUMmode, a pulse height distribution histogram is generated for the whole exposure and downloaded as part of the science data file. The histogram includes all the digitized events for each segment independently of the currently defined subarrays. Note inACCUMmode the pulse height is a 7 bit number with values ranging from 0 to 127.
Table 3.7:PHATABtable contents
3.7.4 BRFTAB: Baseline Reference Frame Table
The
BRFTABreference file is only applicable to FUV data and is used during pipeline processing in the TEMPCORR module to apply the thermal distortion correction. The FUV detector does not have physical pixels like a CCD. Instead, the x and y positions of detected photon events are obtained from analog electronics, which are susceptible to thermal changes. Electronic stim pulses are normally commanded during integration and are used as physical position reference points. To return the FUV data to a known physical space, the BRFTAB defines the stim positions.The
BRFTABfile consists of a primary header extension and a binary table extension. The table lists the stim locations, stim search regions, and the active detector areas (Table 3.8).
Table 3.8:BRFTABtable contents
Column Name Data Type Description SEGMENT String Segment name, FUVA or FUVB SX1 Double X pixel coordinate (zero indexed) of stim11 SY2 Double Y pixel coordinate (zero indexed) of stim1 SX2 Double X pixel coordinate (zero indexed) of stim22 SY2 Double Y pixel coordinate (zero indexed) of stim2 XWidth Long Half width of search region for stims YWidth Long Half height of search region for stims A_Left Long X pixel of left side of active region A_Right Long X pixel of right side of active region A_Low Long Y pixel of lower side of active region A_High Long Y pixel of upper side of active region
1Stim 1 is located in the upper left corner
2Stim 2 is located in the lower right corner
3.7.5 GEOFILE: Geometric Correction File
This file is only used for FUV data. The
GEOFILEis used by the GEOCORR calibration module to perform the geometric correction. From the nature and construction of the XDL detectors, the physical size of the pixels vary across the detector. The geometric distortion maps are used to correct for this variation and to transform the data into a constant physical pixel size early in the data reduction calibration process. After the thermal correction has been applied and the detector digital span and position are adjusted to their reference values, as defined in the reference table, the geometric correction can be applied. This implies that all the files used to determine the geometric correction were initially thermally-corrected.Each geometric correction reference file contains four IMAGE extensions. There are two for each segment, and for each segment, there is one for each axis. At a given (X,Y) location in the uncorrected COS data, the value at that location (corrected for binning and offset) in the geometric correction image gives the distortion to be subtracted from the X or Y coordinates.
3.7.6 DEADTAB: Deadtime Table
The
deadtABreference file is used in the DEADCORR module, to obtain the true number of events received compared to the number of events counted by the detector electronics.There is one
DEADTABreference file for the NUV and FUV detectors. They consist of a primary header extension and a binary table extension which contains the livetime values for a given observed count rate and segment. The livetime is defined as:livetime = observed rate / true rateand can be used to calculate the true count rate.
3.7.7 FLATFILE: Flat Field File
FLATFILEprovides a flat field image which is used by the pipeline to remove the pixel-to-pixel variations in the detector. The FUVFLATFILEconsists of a primary header and two 14000 x 400 IMAGE extensions, one for each segment. The NUVFLATFILEconsists of a primary header and a 1024 x 1024 IMAGE extension.Currently, there is no usable FUV flat field reference file from pre-flight testing, and a dummy file of all ones is being used. A plan is in place to obtain FUV flat-field data in orbit from standard stars for every grating and possibly every central wavelength of the FUV detector. Until these data are available, the FUV flat field processing will use a file consisting of ones, which leaves the data unchanged.
The NUV flat field is a combination of internal and external deuterium flat field lamp exposures from thermal-vacuum testing which illuminate the portion of the detector that will receive all of the incoming external light on orbit. The data cover the following pixel region of the detector: x (dispersion): 0 to 1023, and y (cross-dispersion): 495 to 964. The rest of the detector, where flat field data are not available, has a value of 1.0. The bottom four and top three rows of the detector do not fit well with the rest of the detector and they are flagged in the data quality table.
3.7.8 BPIXTAB: Bad Pixel Table
The data quality initialization table identifies rectangular regions on the detectors that are known to be less then optimal. The feature type describes the type of detector blemish enclosed within the bounding box and q is the quality value assigned to all events detected within the box. The regions were identified by visual inspection of the combined flat field data for each detector (and segment). The
BPIXTABfiles consist of a primary header and a binary table extension which consists of the columns listed in Table 3.9.
Table 3.9: BPIXTABtable content
In the
BPIXTABtable, the DQ field may have several different values, each associated with a unique issue as shown in Table 3.10.
Table 3.10: Data Quality Flag Values
3.7.9 LAMPTAB: Template Calibration Lamp Spectra Table
The
LAMPTABfiles consist of a primary header extension and a binary table extension which contains an extracted 1-D spectrum from the internal PtNe calibration lamp through the WCA aperture, for each grating and central wavelength setting. It is used in the calcos pipeline to determine the pixel offset of the observed data. The structure of the template calibration lamp spectra table is shown in Table 3.11.
Table 3.11: LAMPTABtable contents
3.7.10 WCPTAB: Wavecal Parameter Table
The
WCPTABfile contains information relevant for the wavecal pipeline processing. It consists of primary header extension and a binary table extension which is described in Table 3.12. A fixed RESWIDTH value of 6.0 pixels (per resolution element) is used for the FUV detector and a fixed RESWIDTH value of 3.0 pixels (per resolution element) is used for the NUV detector. The FUV STEPSIZE is measured by calculating the displacement in pixels from a PtNe spectrum obtained at a position ofFPOFFSET=0to the positionFPOFFSET=-2for segment A from the WCA (and dividing by 2). The NUV STEPSIZE is measured by calculating the displacement in pixels from a PtNe spectrum obtained at a position ofFPOFFSET=0to the positionFPOFFSET=-2for stripe B of the WCA (and dividing by 2). The XC_RANGE was estimated as 110% of the STEPSIZE for both FUV and NUV.
Table 3.12:WCPTABtable contents
3.7.11 DISPTAB: Dispersion Coefficient Table
There are two
DISPTABfiles with similar formats, one for the NUV, and one for the FUV. They consist of a main header and a binary table in the second HDU. These tables provide the dispersion relations for each segment, aperture, optical element and central wavelength. Each file has the format given in Table 3.13.
Table 3.13:DISPTABtable format
For Px = the Doppler corrected pixel value in the dispersion direction, the associated wavelength for a specific segment, optical element, aperture, and central wavelength is given by
lambda(Px) = COEFF[0] + COEFF[1]*Px + COEFF[2]*Px2 + COEFF[3]*Px3 + DELTA3.7.12 XTRACTAB: 1-D Spectral Extraction Table
There are two
XTRACTABfiles with similar formats, one for the NUV and one for the FUV. They consist of a main header and a binary table in the second HDU. These tables provide the information needed to extract the spectrum from a geometrically corrected image of the detector for each optical element and central wavelength. Each file has the format given in Table 3.14.
Table 3.14:XTRACTABtable format
The spectral extraction of a source is performed by collapsing the data within a parallelogram of height HEIGHT that is centered on a line whose slope and intercept are given by SLOPE and B_SPEC. Similarly, two background spectra are determined by collapsing the data within a parallelogram of height BHEIGHT centered on the lines defined by SLOPE and B_BKG1 and SLOPE and BKG2. The background spectra are then smoothed by a boxcar of width BWIDTH. These are then scaled and subtracted from the source spectrum.
3.7.13 PHOTTAB: Photometric Throughput Table
There are two
PHOTTABfiles with similar formats, one for the NUV, and one for the FUV. They consist of a main header and a binary table in the second HDU. These tables provide the information needed to convert from corrected detector counts to flux units of erg s-1cm-2A-1 for each segment, optical element, aperture and central wavelength. Each file has the format given in Table 3.15.
Table 3.15:PHOTTABTable Format
The units of the Sensitivity array are (count s-1 pixel-1)/(erg s-1 cm-2 Angstrom-1). For each segment, optical element, central wavelength setting, and aperture, these files contain arrays of wavelengths and sensitivities which can be interpolated onto the observed wavelength grid. The net counts can then be divided by the sensitivity curves to produce flux calibrated spectra.
3.7.14 TDSTAB: Time Dependent Sensitivity Table
There are two such files, one for the FUV and one for the NUV. They are only used for spectroscopic data. The files contain the information necessary to determine the relative sensitivity curve at any given time by interpolating between relative sensitivity curves given at fiducial times which bracket the observation, or else extrapolate the results from the last curve if the observation date is more recent than the last fiducial date. Interpolation data are provided for each segment, optical element, and aperture (see Table 3.16).
Table 3.16:TDSTABTable Format
For an observation obtained at time T, which lies between TIME[j] and
TIME[j+1], the sensitivity curve used to calibrate the spectrum will
be corrected by the following factor:
(T - REF_TIME) SLOPE[i,j]/(365.25*100) + INTERCEPT[i,j].where REF_TIME is a general reference time given in the header of the FITS extension.
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