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Calibration Reference Data System

COS Reference File Tables

Reference files used by the COS calibration pipeline (CALCOS)) are updated on a regular basis. Current reference files files can be retrieved from the HST archive, or downloaded directly from the lref directory. If you choose to access the files via this ftp server, keep in mind that there is a known bug in trying to access files via ftp server using Safari. The recommended browser is Firefox.

Flat Field Files (FLATFILE)

Accounts for pixel-to-pixel variations.

FLATFILE provides a flat field image which is used by the pipeline to remove the pixel-to-pixel variations in the detector. The FUV FLATFILE consists of a primary header and two IMAGE extensions, one for each segment. The NUV FLATFILE consists of a primary header and a 1024 x 1024 IMAGE extension.

Currently (08/2013), the FUV flat field corrects the grid wires for the G130M and G160M. The grid wires are flagged but not corrected in G140L. With the OPUS 2013.2 installation, expected in Sept 2013, there will be a single flat field applied to all FUV observations (including G140L) which corrects the grid wires as well as edge features on segment B caused by inaccuracies in the geometric distortion correction.

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.




Geometric Distortion Correction Images (GEOFILE)

Corrects for the intrinsic nonlinearity (INL) of the FUV detector.

This file is only used for FUV data. The GEOFILE is 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.




Delta Geometric Distortion Correction Image (DGEOFILE)

Improves the geometric correction for the FUV detector.

The delta geometric distortion reference file is used to improve the geometric correction for the FUV detector. It is defined and applied in the same way as the Geometric Correction, and is only applied to data that have been geometrically corrected. At a given (X,Y) location in the geometrically corrected COS data, the value at that location (corrected for binning and offset) in the delta geometric correction image gives the distortion to be subtracted from the X or Y coordinate.




X Walk Correction File (XWLKFILE)

Gives the correction for the pixel coordinates XCORR and YCORR of FUV TIME-TAG data.

The X walk correction file gives the correction for the pixel coordinates XCORR and YCORR of FUV TIME-TAG data, depending on the XCORR value and pulse height amplitude (PHA). This correction, along with the Y walk correction, supersedes the old WALK correction and became active after calcos version 3.2.1 in June 2017. The reference file is an image with each pixel (i, j) containing the walk correction for an event with XCORR value i and PHA value j, where i and j correspond to the indices of the pixel in the x (most rapidly varying) direction and y (least rapidly varying) direction. Bilinear interpolation is used to calculate the correction for non-integer values of XCORR and PHA, although the latter coordinate is currently always an integer.




Y Walk Correction File (YWLKFILE)

Gives the correction for the pixel coordinates YCORR of FUV TIME-TAG data.

The Y walk correction file gives the correction for the pixel coordinates YCORR of FUV TIME-TAG data, depending on the XCORR value and pulse height amplitude (PHA). This correction, along with the X walk correction, supersedes the old WALK correction and became active after calcos version 3.2.1 in June 2017. The reference file is an image with each pixel (i, j) containing the walk correction for an event with XCORR value i and PHA value j, where i and j correspond to the indices of the pixel in the x (most rapidly varying) direction and y (least rapidly varying) direction. Bilinear interpolation is used to calculate the correction for non-integer values of XCORR and PHA, although the latter coordinate is currently always an integer.




Bad Time Interval Table (BADTTAB)

Lists the start and end times of known bad time intervals.

The BADTTAB reference file lists the start and end times of known bad time intervals. It is used by the BADTCORR calibration module to flag events in TIME-TAG events 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. The segment column can be populated with either FUVA, FUVB or ANY; the start and end columns are in Modified Julian Date.




Baseline Reference Frame Table (BRFTAB)

Used when applying the thermal distortion correction.

The BRFTAB reference 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 BRFTAB file 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.




Burst Parameters Table (BRSTTAB)

Provides the parameters needed for deciding when the count rate in some time interval in FUV TIME-TAG data is too high to be regarded as normal.

The BRSTTAB file 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 of the COS Data Handbook.




1-D Extraction Parameters Table (XTRACTAB)

Gives the location of the spectrum to be extracted from a 2-D image.

This table provides the information needed to extract the spectrum from a geometrically corrected image of the detector for each optical element and central wavelength.




Bad Pixel Table (BPIXTAB)

Gives the locations of rectangular regions that cover portions of the detector that are known to be less than optimal.

The bad pixel table identifies rectangular regions on the detectors that are known to be less than 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).




Deadtime Reference Table (DEADTAB)

Used to obtain the true number of events received compared to the number of events counted by the detector electronics.

The DEADTAB reference 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 DEADTAB reference 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 rate

and can be used to calculate the true count rate.




Dispersion Coefficients Table (DISPTAB)

Gives a set of polynomial coefficients for computing wavelength from pixel number.

This table provides the dispersion relations for each segment, aperture, optical element and central wavelength.

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 + DELTA




Calibration Lamp Spectra Table (LAMPTAB)

Used to determine the pixel offset of the observed data.

The LAMPTAB file consists 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.




Pulse Height Parameters Table (PHATAB)

Lists the lower and upper thresholds for valid individual pulse heights in TIME-TAG mode as well as the minimum and maximum values for the location of the mean value of the pulse height distribution used in ACCUM mode.

The PHATAB reference file is only valid for FUV data, and is applied during the PHACORR step of calcos to filter non-photon events. The table lists the lower and upper thresholds for valid individual pulse heights in TIME-TAG mode. In TIME-TAG mode, 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 in ACCUM mode. In ACCUM mode, 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 in ACCUM mode the pulse height is a 7 bit number with values ranging from 0 to 127.




Photometric Sensitivity Reference Table (FLUXTAB)

Provides the information needed to convert from corrected detector counts to flux units.

This table provides 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.

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.




Spectroscopic WCS Parameters Table (SPWCSTAB)

The spectroscopic WCS parameters table gives values to be used for populating the world coordinate keywords.

Files are selected on DETECTOR and OBSTYPE and only for SPECTROSCOPIC data




Time Dependent Sensitivity Table (TDSTAB)

Gives information about the relative sensitivity as a function of time and wavelength.

There are two such files, one for the FUV and one for the NUV and 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.




Wavecal Parameters Table (WCPTAB)

Gives information about the relative sensitivity as a function of time and wavelength.

The WCPTAB file contains information relevant for the wavecal pipeline processing. A fixed RESWIDTH (the number of pixels per resolution element in the dispersion direction) 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 (one step of the OSM is this many pixels) is measured by calculating the displacement in pixels from a PtNe spectrum obtained at a position of FPOFFSET=0 to the position FPOFFSET=-2 for 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 of FPOFFSET=0 to the position FPOFFSET=-2 for stripe B of the WCA (and dividing by 2). The XC_RANGE (the maximum lag (amplitude) for cross correlation) was estimated as 110% of the STEPSIZE for both FUV and NUV.




High Voltage Reference Table (HVTAB)

Gives the times when the FUV high voltage was changed, and the value (raw counts) that was used in the command to set the high voltage.

Calcos will use the values in this table to populate extension header keywords HVLEVELA and HVLEVELB.

This file contains two extensions. Each extension is a binary table giving the date/time and commanded high voltage value for one FUV segment. Keyword EXTNAME is used for selecting the appropriate extension.




Walk Correction Table (WALKTAB)

Gives parameters for correcting the pixel coordinates XCORR and YCORR of FUV TIME-TAG data, depending on the pulse height amplitude (PHA).

The parameters are used for computing the coefficients of polynomials in PHA (one polynomial for XCORR, another for YCORR) that give the corrections to be subtracted from XCORR and YCORR. The coefficients of PHA are themselves computed by evaluating polynomial functions of XCORR and YCORR. The changes to be subtracted from XCORR and YCORR are:


dx = x_c1* (pha - x_c0) + x_c2*(pha - x_c0)^2+...+x_cN*(pha - x_c0)^N
dy = y_c1*(pha - y_c0) _ y_c2*(pha - y_c0)^2+...+y_cN*(pha - y_c0)^N


Where N is the value in the N_PHA_COEFF column, x_c0 is the value in the x0 column, y_c0 is the value in the y0 column, and x_ci and y_ci for i > 0 are the values in the XCOEFF and YCOEFF columns, respectively.




Gain Sag Reference Table (GSAGTAB)

Gives the locations of rectangular regions for portions of the FUV detector that have very low pulse height amplitude (PHA).

For each such region, a data quality flag value is given, although the value is the same in every row.

This file contains multiple extensions. Each extension is a binary table giving regions to be flagged as low gain regions. The appropriate extension to use is selected by SEGMENT and FUV high voltage (given by keyword HVLEVELA or HVLEVELB in the first extension header of the raw file); see also the section on extension header keywords below. Each row of the table gives the location and data quality value for one rectangular region. The format of a table is similar to the bad pixel reference table (_bpix.fits), except that the gain sag table includes a DATE column and does not include either a SEGMENT or TYPE column. The DATE column is used to select rows. A row will be used to flag a gain sagged region if the value in the DATE column is less than or equal to the exposure start time.