Space Telescope Science Institute
COS Data Handbook 2.00
Table of Contents Previous Next Print

COS Data Handbook > Chapter 3: COS Calibration > 3.4 Descriptions of Spectroscopic Calibration Steps

This section provides a more detailed description of the calibration processing steps and algorithms applied by calcos, including the switches, reference file inputs, science file inputs, and the output products. Setting the calibration switch to PERFORM enables the execution of the corresponding pipeline calibration task.
Future modifications and updates to calcos will be announced in STScI Analysis Newsletters (STANs) and documented at the Web site:
The calibration steps for FUV time-tag data in the pipeline processing flow are described next. When present, each sub-section header begins with the switch that activates the module.
3.4.1 Initialization
When the pipeline is initiated, it first opens an association file to determine which files should be processed together. For time-tag data, it also creates a _corrtag file before anything else is done. The initial contents of this file are simply a copy of the _rawtag file, except that new columns have been added to the _corrtag file. It is then updated throughout the running of the pipeline.
3.4.2 BRSTCORR: Search for and Flag Bursts
This module flags “event bursts” in the FUV time-tag data for removal.
Header keywords updated: TBRST_A, TBRST_B (time affected by bursts in segments A and B), NBRST_A, NBRST_B (number of events flagged as bursts in segments A and B), EXPTIME,EXPTIMEA, EXPTIMEB.
Updates _corrtag file for time-tag data.
The COS FUV detectors are similar to the FUSE detectors, which experienced sudden, short-duration increases in counts while collecting data. These events, called bursts, led to very large count rates and occurred over the entire detector. Thus far, no bursts have been recorded on-orbit, and the default setting for BRSTCORR is OMIT. Nevertheless, since it is uncertain whether COS will suffer bursts at some point, the BRSTCORR module remains in the pipeline and is available to identify bursts and flag their time intervals should they occur. This module can only be applied to FUV TIME-TAG data.
The first step in the screening process is to determine the count rate over the whole detector, including stim pulses, source, background, and bursts. This rate determines which time interval from the BRSTTAB table to use for screening.
Screening for bursts is then done in two steps. The first step identifies large count rate bursts by calculating the median of the counts in the background regions, defined in the XTRACTAB table, over certain time intervals (DELTA_T or DELTA_T_HIGH for high overall count rate data). Events with count rates larger than MEDIAN_N times the median are flagged as large bursts.
The search for small count rate bursts is done iteratively, up to MAX_ITER. This step uses a boxcar smoothing of the background counts (taking the median within the box) and calculates the difference between the background counts and the running median. The boxcar smoothing is done over a time interval MEDIAN_DT or MEDIAN_DT_HIGH. Elements that have already been flagged as bursts are not included when computing the median. For an event to be flagged as affected by a small burst the difference between the background counts and the running median has to be larger than the following quantities:
The source counts value in 3) is the number of events in the source region defined in the XTRACTAB table minus the expected number of background counts within that region.
All events that have been identified as bursts are flagged in the data quality column (DQ in the corrtag table) with data quality bit = 64. In addition calcos updates the following header keywords to take into account time and events lost to burst screening: TBRST_A and TBRST_B (time lost to bursts in segments A and B); NBRST_A, NBRST_B (number of events lost to bursts in segments A and B), EXPTIME, EXPTIMEA and EXPTIMEB. Keywords EXPTIMEA and EXPTIMEB are the sums of the values in the input _x1d files. For _x1dsum files the value of the EXPTIME keyword is that corresponding to the larger value of EXPTIMEA and EXPTIMEB. The EXPTIME for each segment is contained in a table column in the first extension of the x1dsum file.
When running calcos a user can specify that the information about bursts be saved into a file. This output file contains four columns, each with one row per time interval (DELTA_T or DELTA_T_HIGH). Column 1 contains the time (seconds) at the middle of the time interval, column 2 contains the background counts for that time interval, column 3 contains a 1 for time intervals with large bursts and is 0 elsewhere, and column 4 contains a 1 for time intervals with small bursts and is 0 elsewhere.
3.4.3 BADTCORR: Bad Time Intervals
This module flags time intervals in TIME-TAG data that have been identified as bad for some reason.
Header keywords updated: EXPTIME, Exptimea and exptimeb (for FUV data), NBADT, or Nbadt_A and nbaDt_B (number of events flagged for NUV or FUVA and B, respectively) and TBadt or Tbadt_a and tbadt_b (time lost to bad events in NUV or FUVA and FUVB, respectively).
Updates corrtag file for TIME-TAG data.
The BADTTAB table lists zero or more intervals of time which will be excluded from the final spectrum for various reasons. This file is currently empty, but it could be populated by the COS team if events occur on orbit which they feel render data collected during specific time intervals not scientifically useful. It is also available for the convenience of the user. For example, the user may wish to eliminate observations obtained in the daytime portion of the orbit to minimize airglow contamination, or they may want to isolate a certain portion of an exposure. In these cases, modifying BADTTAB may be the most convenient means to accomplish this. Events in the rawtag file having times within any bad time interval in BADTTAB are flagged in the DQ column of the corrtag table with data quality = 2048. The exposure time is updated to reflect the sum of the good time intervals. This step applies only to TIME-TAG data.
3.4.4 PHACORR: Pulse Height Filter
This module operates on FUV data and flags events whose pulse heights are outside of nominal ranges.
Reference file: PHATAB, PHAFILE
Input files: FUV rawtag, FUV rawaccum
This module works differently for FUV TIME-TAG and ACCUM data. It is not used for NUV data.
For FUV TIME-TAG data, each photon event includes a 5 bit (0 - 31) Pulse Height Amplitude (PHA). The value of the pulse height is a measure of the charge produced by the microchannel plate stack, and can be used to identify events which are likely due to cosmic rays or detector background. The PHATAB reference file lists lower and upper pulse height thresholds expected for valid photon events for each detector segment. The PHACORR module compares each event’s pulse height to these thresholds, and if the pulse height is below the Lower Level Threshold (LLT) or above the Upper Level Threshold (ULT), the event is flagged in the DQ column of the corrtag table with a data quality bit of 512. The upper and lower thresholds are also written to the PHALOWRA (PHALOWRB) and PHAUPPRA (PHALOWRB) keywords in the output data files for segment A (B), while the number of events flagged is written to the NPHA_A and NPHA_B keywords.
Default values of the lower (LLT) and upper (ULT) thresholds have been chosen based on the properties of the detector and are implicit in data used when generating other reference files (e.g. FLUXTAB)
With continuing exposure to photons, pulses from the MCPs have smaller amplitudes, a phenomenon known as “gain sag”. As this occurs, the thresholds in the PHATAB will be updated to maximize the number of real events counted. Which PHATAB is used for data collected at a particular time will be handled by the USEAFTER date keyword in the calibration file header.
The PHAFILE reference file is an alternative to the PHATAB, and allows pulse-height limits to be specified on a per-pixel basis rather than a per-segment basis. The PHAFILE has a primary header and four data extensions, consisting of the FUVA PHA lower limits, FUVA PHA upper limits, FUVB PHA lower limits, and FUVB PHA upper limits respectively. The use of a PHAFILE instead of a PHATAB (if both are specified and PHACORR = ‘PERFORM’, the PHAFILE will take precedence) allows a number of adjustments, including (for example) the use of a lower PHA threshold in gain-sagged regions, thus allowing more background events to be filtered out while still continuing to detect photon events in gain-sagged regions. As of August 2012, no PHAFILE has been produced by the COS team, but in the future one or more such files may be produced for use with FUV TIME-TAG data. Note that the use of a PHAFILE requires calcos 2.14 or later.
For FUV ACCUM data, pulse height information is not available for individual events. However, a 7 bit (0 - 127) Pulse Height Distribution (PHD) array, containing a histogram of the number of occurrences of each pulse height value over the entire detector segment, is created onboard for each exposure. PHACORR compares the data in this pha file to the values in the PHATAB file. Warnings are issued if the peak of the distribution (modal gain) does not fall between the scaled values of LLT and ULT; or if the average of the distribution (mean gain) does not fall between the MIN_PEAK and MAX_PEAK values in PHATAB. The PHALOWRA and PHAUPPRA, or PHALOWRB and PHAUPPRB keywords are also populated in the output files with the LLT and ULT values from the PHATAB.
3.4.5 WALKCORR: Y-Walk Correction
This module corrects for the tendency of the FUV XDL detector to register events at lower Y-locations as gain sag accumulates.
Updates corrtag file for TIME-TAG data.
The WALKTAB table contains parameters for the polynomial used to correct Y-walk caused by gain sag. Y-walk causes events on the XDL detector to appear to have originated lower on the detector than their actual location. Currently, the Y-walk correction involves an equation of the form:
dx = x1 * (PHA - x0) + x2 * (PHA - x0)2
dy = y1 * (PHA - y0) + y2 * (PHA - y0)2
where xn and yn are coefficients which are, in turn, calculated from the event X and Y location, and a variable-length series of other coefficients stored in the reference file. For example, for the case with three X coefficients and three Y coefficients, the xn coefficients are calculated as follows:
x0 = a000 + a100*x + a010*y + a200*x2 + a110*x*y + a020*y2
x1 = a001 + a101*x + a011*y + a201*x2 + a111*x*y + a021*y2
x2 = a002 + a102*x + a012*y + a202*x2 + a112*x*y + a022*y2
where amno is the item at location m,n,o in the coefficient matrix.
The current Y-walk correction is considerably simpler than this, with many of the coefficients equal to zero:
FUVA: dx = 0; dy = (PHA - 12)*0.47
FUVB: dx = 0; dy = (PHA - 14)*0.47
However, it is possible that future revisions will result in a more complex correction, and the correction may certainly vary with time and lifetime position.
The WALKTAB table contains the following items for each FUV segment:
X0: subtracted from x before evaluating coefficient polynomials. Set to 8192.
Y0: as X0, but for y. Set to 512.
3.4.6 RANDCORR: Add Pseudo-Random Numbers to Pixel Coordinates
This module adds a random number between -0.5 and +0.5 to each x and y position of a photon detected by the FUV detectors.
Input files: FUV rawtag, FUV rawaccum
Updates corrtag file for TIME-TAG data and a virtual corrtag file for ACCUM data.
For FUV TIME-TAG data RANDCORR adds random numbers to the raw coordinates of each event, i.e.:
Where Δx and Δy are uniformly distributed, pseudo-random numbers in the interval -0.5 < Δx, Δy+0.5.
The result of this operation is to convert the raw, integer pixel values into floating point values so that the counts are smeared over each pixel’s area.
For FUV ACCUM data, a pseudo TIME-TAG list of x and y values is created with an entry for each recorded count. Next, a unique Δx and Δy are added to each entry.
If the RANDSEED keyword in the raw data file is set to its default value of -1, the system clock is used to create a seed for the random number generator. This seed value is then written to the RANDSEED keyword in the output files. Alternatively, an integer seed (other than -1) in the range -2147483648 to +2147483647 can be specified by modifying the RANDSEED keyword in the raw data file. Doing so will ensure that identical results will be obtained on multiple runs.
RANDCORR is only applied to events in the active area of the detector, as defined in the BRFTAB. Stim pulses, for example, do not have this correction applied.
3.4.7 TEMPCORR: Temperature-Dependent Distortion Correction
This module corrects for linear distortions of the FUV detector coordinate system that are caused by changes in the temperature of the detector electronics.
Input files: FUV rawtag, FUV rawaccum
Updates corrtag file for TIME-TAG data and flt file for ACCUM.
The FUV XDL detector has virtual, not physical, detector elements that are defined by the digitization of an analog signal. The charge packet associated with a photon event is split and transported to opposite sides of the detector where the difference in travel time of the two packets determines the location of the photon event on the detector. Since the properties of both the delay line and the sensing electronics are subject to variations as a function of temperature, apparent shifts and stretches in the detector format can occur.
To measure the magnitude of this effect, electronic pulses (Figure 1.6) are recorded at two reference points in the image (“electronic stims”) at specified time intervals throughout each observation. TEMPCORR first determines the locations and separations of the recorded stim positions and then compares them to their expected locations in a standard reference frame (as defined in columns SX1, SY1, SX2, and SY2 of the BRFTAB file). The differences between the observed and reference stim positions are used to construct a linear transformation between the observed and reference frame locations for each event (or pseudo-event in the case of ACCUM data). TEMPCORR then applies this transformation to the observed events, placing them in the standard reference frame.
3.4.8 GEOCORR and IGEOCORR: Geometric Distortion Correction
This module corrects geometric distortions in the FUV detectors.
Input files: FUV rawtag, FUV rawaccum
Updates corrtag file for time-tag data and flt file for accum.
The GEOCORR module corrects for geometric distortions due to differences between the inferred and actual physical sizes of pixels in the FUV array (ground measurements indicated that geometric distortions in the NUV MAMA are negligible). It produces a geometrically corrected detector image with equal sized pixels. This is done by applying the displacements listed in the reference file, GEOFILE, which lists the corrections in x and y for each observed pixel location. The geometric distortion varies across the detector, and the GEOFILE gives the distortion only at the center of each pixel. If IGEOCORR is ‘PERFORM’ (the default), the displacements to correct the distortion at (XCORR, YCORR) will be interpolated to that location, which includes a fractional part (even before geometric correction) due to TEMPCORR and RANDCORR. If IGEOCORR is ‘OMIT’, the correction will be taken at the nearest pixel to (XCORR, YCORR).
GEOFILE was created by using a ray-trace analysis of the COS FUV optical system. A set of wavelength calibration exposures was taken while stepping the aperture mechanism in the cross-dispersion direction to create an image of dispersed line profiles. The ray trace and measured line positions were compared to determine the shift between the measured (uncorrected) and predicted (corrected) line positions (see Figure 3.6).
The distortion corrections are given as images in the GEOFILE in the following order:
Figure 3.6: A Map of the FUV Geometric Correction
A map of the FUV geometric correction, scaled by a factor of 10 in the x-direction for illustration purposes, for detector segment A (left) and segment B (right) in user coordinates. The arrow points from the observed to the corrected position.
3.4.9 DOPPCORR: Correct for Doppler Shift
This module corrects for the effect that the orbital motion of HST has on the arrival location of a photon in the dispersion direction.
Reference files: DISPTAB, xtractab
Input files: FUV/NUV rawtag, FUV/NUV rawaccum
Updates corrtag file for TIME-TAG data
During a given exposure the photons arriving on the FUV and NUV detectors are Doppler shifted due to the orbital motion of HST. The orbital velocity of HST is 7.5 km/s, so spectral lines in objects located close to the orbital plane of HST can be broadened up to 15 km/s, which can be more than a resolution element.
DOPPCORR is the calcos routine which corrects for the orbital motion of HST. It operates differently on TIME-TAG and ACCUM files:
For TIME-TAG files the raw events table contains the actual detector coordinates of each photon detected, i.e., the photon positions will include the smearing from the orbital motion. In this case DOPPCORR will add an offset to the pixel coordinates (the XCORR column) in the events table to undo the Doppler broadening. The corrected coordinates are written to the column XDOPP in the corrtag file for both FUV and NUV data.
For ACCUM files Doppler correction is applied onboard and is not performed by calcos. This means, however, that the pixel coordinates of a spectral feature can differ from where the photon actually hit the detector - a factor which affects the data quality initialization and flat-field correction. Therefore for ACCUM images DOPPCORR shifts the positions of pixels in the bad pixel table to determine the maximum bounds that could be affected. It is also used to convolve the flat-field image by an amount corresponding to the Doppler shift which was computed on orbit. The information for these calculations are contained in the following header keywords:
The “T” suffix at the end of each of these keywords indicates that they were derived from the onboard telemetry, whereas the other keywords described below were computed on the ground from the orbital elements of HST. The two sets of keywords can differ by a small amount, but they should be nearly identical.
DOPPCORR assumes that the Doppler shifts vary sinusoidally with time according to the orbital movement of HST. The following keywords are used to perform the correction and are obtained from the first extension (EVENTS) in the _rawtag:
EXPSTART - start time of the exposure (MJD)
DOPPZERO - the time (MJD) when the Doppler shift was zero and increasing (i.e., when HST was closest to the target)
DOPPMAG - The number of pixels corresponding to the Doppler shift (used only for shifting the data quality flag arrays and flat fields)
ORBITPER - the orbital period of HST in seconds
The data columns used in the correction are TIME (elapsed seconds since EXPSTART) and RAWX (position of photon along dispersion direction). The Doppler correction to be applied is then
SHIFT = -(DOPPMAGV/(c*d))*λ(XCORR)*sin(2*pi*t/ORBITPER)
where c is the speed of light (km/s), d is the dispersion of the grating used in the observation (?/pixel), λ(XCORR) is the wavelength at the XCORR position being corrected (obtained from the dispersion solution for that grating and aperture in the DISPTAB reference file) and t is defined as
where the factor of 86400 converts from days to seconds.
3.4.10 DEADCORR: Nonlinearity Correction
This module corrects for count rate dependent non-linearities in the COS detectors.
Input files: FUV/NUV rawtag, FUV/NUV rawaccum, NUV images
Updates corrtag file for TIME-TAG data and flt for ACCUM.
DEADCORR corrects for non-linear photon detection in the COS detector electronics. Both the FUV and NUV detector electronics have a small temporal overhead when counting events. This overhead becomes noticeable when the count rates become large.
The efficiency of the detector’s photon counting is calculated as the ratio of the true count rate and the observed count rate. This value is referred to as the deadtime. The deadtime for each detector is modeled and the reference file DEADTAB contains a lookup table of the correction for various count rates. Figure 3.7 shows how the measured count rates deviate from the actual count rates as a function of the actual count rate for the NUV detector, and segment B of the FUV detector (the FUV segment A curve is nearly identical).
Figure 3.7: FUV and NUV Deadtime
The solid curves are the observed count rates versus true count rates for the COS detectors and the dashed lines are for perfect detectors. TOP: The NUV MAMA. BOTTOM: Segment B of the FUV XDL detector (the curve for Segment A is nearly identical). Significant deviations from the true count rates occur at about 15,000 counts per second for the XDL detectors, and at roughly 10 times this rate for the MAMA.
For TIME-TAG data the deadtime correction is computed every 10 seconds. The observed count rate is the number of events within that time interval, and the deadtime factor is determined by interpolation within the values in DEADTAB. The values in the EPSILON column in the _corrtag file for events within that time interval will then be divided by the deadtime factor. For ACCUM data the observed average count rate is taken from a header keyword for the digital event counter. The deadtime factor is then found by interpolation within the DEADTAB, the same as for TIME-TAG data, and the science and error arrays will be divided by the deadtime factor.
3.4.11 FLATCORR: Flat-field Correction
This module corrects for pixel-to-pixel non-uniformities in the COS detectors.
Input files: FUV/NUV rawtag, FUV/NUV rawaccum, NUV images
Updates corrtag file for TIME-TAG data and flt for ACCUM.
The FLATCORR step corrects for high frequency, pixel-to-pixel sensitivity differences across the detector. It uses a flat-field image located in the file specified by the FLATFILE header keyword. Figure 3.8 shows an NUV flat. For spectroscopic data, any wavelength dependence of the detector response or remaining low frequency spatial variations are removed by the flux calibration step (FLUXCORR, Section 3.4.17). Flat fielding is performed in geometrically corrected space, and because the pixel-to-pixel variations should be largely wavelength independent, only one reference image is used per detector or detector segment (NUV, FUVA, and FUVB). The flat-field correction is applied differently for TIME-TAG and ACCUM mode data for both spectroscopic and imaging modes.
For spectroscopic TIME-TAG exposures, each photon in the events list is individually corrected. In the _corrtag file, the photon weight in the EPSILON column is divided by the flat-field value at the event’s detector location rounded to the nearest pixel (XCORR, YCORR for FUV; RAWX, RAWY for NUV).
For spectroscopic ACCUM mode data, photons are summed into an image onboard by the COS electronics. To compensate for the motion of HST during the observation, spectroscopic exposures are normally taken with Doppler compensation performed during the accumulation (science header keyword DOPPONT=TRUE). During Doppler compensation, photon locations are shifted as the data are received, and the underlying flat field at each imaged pixel is an average of the original pixel position sensitivities. FLATCORR replicates this averaging for the flat-field correction using the same control parameters as those onboard (DOPPMAGT, DOPZEROT, ORBTPERT) if DOPPCORR=PERFORM (Section 3.4.9). The convolved flat-field image is divided into the rawaccum data, which is then divided by the exposure time to create the flt image file.
NUV images using the mirrors are not Doppler corrected. In this case, DOPPCORR=OMIT, and the input data are divided by the flat field without convolution.
For both the flt and counts files, error arrays are created based on counting statistics (Section 2.7), but they are not used in further processing.
It was discovered on-orbit, that the NUV suffers some vignetting. This causes a structure in “pixel space”, affecting roughly the first 200 pixels of all three spectral stripes by as much as 20%. The NUV flat field was originally modified to correct for this effect, but variation in the vignetting caused sufficient errors that the vignetting is no longer included in the NUV flat field. Work continues on grating-specific vignetting corrections.
For the FUV channels, the ground flats proved inadequate. Consequently, the current FUV flats correct only for the effects of grid wires in the M-mode gratings. The project is currently investigating a variety of correction schemes to eliminate the flat-field effects in FUV spectra.
Figure 3.8: Flat-field Images of the NUV MAMA Detector.
The image at left shows the full detector, and the one on the right has been enlarged to illustrate structure in the flat-field images. The hex structure associated with the microchannel plate is visible in both FUV and NUV flat fields.
3.4.12 WAVECORR: Wavecal Correction
For spectroscopic data, this module determines location of the wavelength calibration spectrum on the detector relative to a template, and then applies zero point shifts to align the wavecal and the template.
Input files: FUV/NUV rawtag, FUV/NUV rawaccum
Header keywords updated: SHIFT1[A-C], SHIFT2[A-C], LMP_ONi, LMPOFFi, LMPDURi, LMPMEDi.
Updates corrtag file for TIME-TAG data.
Creates lampflash file for TAGFLASH data.
The wavecal step of calcos determines the shift of the 2-D image on the detector along each axis resulting from thermal motions and drifts within an OSM encoder position. This step applies only to spectroscopic data, TIME-TAG and ACCUM, for both the FUV and NUV detectors. The shifts are determined from one or more contemporaneous wavelength calibration observations of a spectral line lamp (wavecal) which must be obtained without moving the OSM between the science and wavecal exposures.
There are two types of wavecals. For ACCUM data the spectrum of the calibration lamp is contained in an exposure that is separate from that of the science (AUTO or GO wavecals). For TIME-TAG data the wavecals can also be separate exposures, but the default when observing with the PSA aperture is tagflash mode. In the tagflash mode the line lamp is turned on and off (flashed) one or more times during a single science exposure, producing a wavecal spectrum that is offset in the cross-dispersion direction from the science spectrum (See Figure 1.7, and Figure 1.9). The algorithm used to determine the shifts is the same in either case, but the way that the shift is determined at the time of the observation differs. Thus, we begin by describing how the offsets are found.
Determining the offsets:
For each wavecal, the location of the spectrum in the cross-dispersion direction is determined by collapsing the spectrum along the dispersion direction using the extraction slope defined in the XTRACTAB table (SLOPE). The location of the brightest pixel, after boxcar smoothing, is taken as the spectrum location and that location is compared to the nominal position defined in the XTRACTAB table. The offsets from nominal positions for segments A and B (FUV) or stripes A, B, and C (NUV) are recorded in the lampflash file (which is created at this stage) in the SHIFT_XDISP field. The two FUV segments are processed independently. Cross-dispersion shifts are determined for each NUV stripe and then the average is computed and applied to all three stripes. The sign of the SHIFT_XDISP entry is positive if the spectrum was found at a larger pixel number than the nominal location.
To determine the offsets in the dispersion direction, the wavecal spectrum is collapsed along the cross-dispersion direction and compared to the template wavecal (LAMPTAB) taken with the same grating and central wavelength. For the NUV wavecal spectra for each stripe are determined independently. The positions are determined from a least squares fit to a shifted and scaled version of the template spectrum. The maximum range for shifting the wavecal and template wavecal spectra is defined by the value of XC_RANGE in the WCPTAB table. Calcos takes into account the FPPOS of the wavecal spectrum by shifting it by FP_PIXEL_SHIFT (from the column in the lamptab) before fitting it to the template wavecal. The final shift is stored as SHIFT_DISP in the lampflash file and the minimum value of chi squared is stored in the CHI_SQUARE array.
Applying the offsets:
The way the offsets are applied to the spectral data depends on whether the data were obtained with AUTO or GO wavecals or with TAGFLASH wavecals. For AUTO or GO wavecals, the wavecals are obtained at different times than the spectral data and temporal interpolation is done to determine the appropriate shifts. For TAGFLASH data, the wavecals are interspersed with the spectral data, allowing more precise and, consequently, more intricate corrections to be made. In either case, the result is the X[Y]FULL entries in the corrtag file. Because, as we shall see, the corrections can be time dependent, the differences between X[Y]CORR and X[Y]FULL can also be time dependent. This step of the calibration amounts to a time dependent translation of the detector coordinate system to a coordinate system relative to the wavecal spectrum, which is more appropriate for wavelength calibration.
AUTO or GO wavecals
For ACCUM science exposures which are bracketed by AUTO or GO wavecal observations, the shifts determined from the bracketing wavecal exposures are interpolated (linearly) to the middle time of the science observation, and the interpolated values are assigned to the SHIFT1[A-C] and SHIFT2[A-C] keywords in the science data header. If there is just one wavecal observation in a dataset, or if there are more than one but they don’t bracket the science observation, the SHIFT1[A-C] and SHIFT2[A-C] keywords are just copied from the nearest wavecal to the science data header.
For non-TAGFLASH TIME-TAG science exposures bracketed by AUTO or GO wavecal observations, the shifts determined from the wavecals are interpolated (linearly) so that each event in the corrtag file is shifted according to its arrival time. The SHIFT1[A-C] and SHIFT2[A-C] keywords recorded in the science data header are in this case the averages of the values applied. As in the ACCUM case, if there is only one wavecal observation in a dataset, or if there are more than one but they do not bracket the science observation, the SHIFT1[A-C] and SHIFT2[A-C] keywords are just copied from the nearest wavecal to the science data header.
A tagflash wavecal is a lamp exposure that is taken concurrently with a TIME-TAG science exposure, and the photon events for both the wavecal lamp and the science target are mixed together in the same events table. In many respects, TAGFLASH wavecals are handled differently from conventional wavecals.
The nominal start and stop times for each lamp flash are read from keywords in the corrtag table. The actual start and stop times can differ from the nominal times, so calcos determines the actual times (restricted to being within the nominal start-to-stop intervals) by examining the number of photon events within each 0.2-second interval in the wavecal region defined in the XTRACTAB table. A histogram of the count rate is constructed. The histogram is expected to have one peak near zero, corresponding to dark counts, and another at high count rate, due to the lamp illumination. The average count rate when the lamp is on is taken to be the count rate for the second peak of the histogram. The lamp turn-on and turn-off times are taken to be the times when the count rate rises above or sinks below half the lamp-on count rate.
Calcos uses the time of the median photon event within a lamp turn-on and turn-off interval as the time of the flash. The keywords LMP_ONi and LMP_OFFi (i is the flash number) are updated with the actual turn-on and turn-off times, in seconds, since the beginning of the science exposure. The keywords LMPDURi and LMPMEDi are updated with the actual duration and median time of the flash.
As before, the cross dispersion location of each wavecal spectrum is determined by collapsing it along the dispersion direction and comparing it with the template in the XTRACTAB table to produce the SHIFT_XDISP entries in the lampflash file. The wavecal spectrum is then collapsed along the cross-dispersion direction to produce a 1-D spectrum that is fit to the template spectrum to obtain the SHIFT_DISP entries. Typically there will be more than one wavecal flash during a science exposure; so the shifts will be piece-wise linearly interpolated between flashes. The SHIFT1[A-C] and SHIFT2[A-C] values that are recorded in the science data header are the average of the shift values found from the different flashes.
Additional Functions: WAVECORR also corrects the _flt and _counts files which result from both ACCUM and TIME-TAG science data for the offsets in the dispersion and cross-dispersion directions. However, since these images are in pixel space they can only be corrected by an integer number of pixels. The _flt and _counts images are corrected by the nearest integer to SHIFT1[A-C] and SHIFT2[A-C]. DPIXEL1[A-C] is the average of the difference between XFULL and the nearest integer to XFULL, where XFULL is the column by that name in the _corrtag table. This is the average binning error in the dispersion direction when the _flt and _counts images are created from the _corrtag table. DPIXEL1[A-C] is zero for ACCUM data. This shift is used when computing wavelengths during the X1DCORR step.
3.4.13 DQICORR: Initialize Data Quality File
This module identifies pixels which are suspect in some respect and creates the DQ extension for the flt and counts images.
Reference file: FUV/NUV BPIXTAB, FUV gsagtab
Input files: FUV/NUV rawtag, FUV/NUV rawaccum, NUV images
Updates the DQ column of the corrtag files and the DQ image array.
The DQICORR step maps data quality flags for suspect detector pixels listed in the BPIXTAB table to the science data. The COS data quality flags are discussed in Section 2.7.2 and are listed in Table 2.19. Figure 3.9 shows examples of the types of regions isolated by the DQ flags and the effect that they can have on an extracted spectrum. DQICORR proceeds differently for TIME-TAG and ACCUM mode exposures, but the flags in the flt and counts images are created similarly in preparation for spectral extraction. Consequently, we describe each mode separately.
TIME-TAG: DQICORR compares the XCORR, YCORR pixel location of each event in the corrtag file to the rectangular regions flagged in the BPIXTAB table. The value in the DQ column for that event is then updated with the flags of all the regions (if any) that contain that pixel location.When the flt and counts images are generated from the corrtag file, photons which arrived during bad times or bursts are omitted from the image and ERR array. For FUV data, events whose PHAs were flagged as out of bounds are omitted as well. However, data with spatial DQ flags are retained at this stage. Several of these blemishes are illustrated in Figure 3.9
The third FITS extension of the flt and counts files is an array of data quality values generated directly from the BPIXTAB table. If DOPPCORR=PERFORM, the BPIXTAB locations are Doppler-smeared and the flags from all neighboring pixels that contribute to the flt and counts image pixels are combined.
ACCUM: For ACCUM exposures, the rawaccum image file will already have a third FITS extension of data quality values if any pixel had been flagged when constructing the raw image. The extension will be a null image if all initial data quality flags are zero. This is usually the case for NUV data, but not FUV. For FUV ACCUM exposures, photons are collected for only part of the detector segment and an initial data quality array is created to mark the pixels outside those subimage boundaries (flag=128, outside active area). When calcos creates the flt and counts images, it first converts the rawaccum image to a pseudo-time-tag table. In this table, the DQ column is updated with the DQ flags from BPIXTAB just as for the TIME-TAG data. In addition, the third extension of the flt and counts files contains a Doppler-smeared version of the BPIXTAB table, but it also includes the initial flag assignments in the rawaccum DQ extension.
Figure 3.9: The FUV Flat Field.
An FUV flat field obtained during ground testing illustrates the different kinds of blemishes and regions of lower sensitivity that occur. These regions are flagged in BPIXTAB according to the feature type, e.g., a ‘wrinkle” is a kind of detector flaw and grid wire is an example of a detector shadow.
3.4.14 STATFLAG: Report Simple Statistics
This module computes some statistical measures that provide general information about COS science observations.
Reference file: XTRACTAB, BRFTAB
Input files: flt, counts, x1d, lamptab
Header keywords updated: NGOODPIX, GOODMEAN, GOODMAX
STATFLAG enables the reporting of statistics for COS observations. STATFLAG is enabled by default for all science observations and operates on x1d, counts, and flt data products. STATFLAG is intended to provide a very basic statistical characterization of the events and locations on the detectors that are known to be good. The SDQFLAGS header keyword (Serious Data Quality FLAGS), indicates which DQ values (see Table 2.19 for definitions) should be excluded from the statistical calculations. Numerically, the default value of SDQFLAGS for NUV is 184. For FUV, SDQFLAG varies with grating. For FUV/G130M and G160M the default value is 8376, and 8380 for FUV/G140L.
STATFLAG reports the following statistics:
NGOODPIX: The number of good pixels or collapsed spectral columns. For the counts and flt images, this is the number of pixels in the spectral extraction or imaging region. For the x1d file, each ’Y’ column in the spectral extraction region of the flt file is combined to produce the one-dimensional spectrum. The DQ of each column is the logical OR of the DQ flags of the individual pixels. Only collapsed spectral columns that pass the DQ conditions indicated by SDQFLAGS are considered good for purposes of calculating statistics.
GOODMEAN: The mean of the good bins in counts per bin. For the counts and flt files, a bin is an individual pixel, while for x1d files, a bin is a collapsed spectral column.
3.4.15 X1DCORR: Locate and Extract 1-D Spectrum
This module extracts a one dimensional spectrum from the image of the spectrum on the detector.
Reference files: XTRACTAB, DISPTAB
Input files: flt, counts
Header keywords updated: sp_loc_[abc], sp_off_[abc], sp_nom_[abc], sp_slp_[abc], sp_hgt_[abc]
Creates x1d files
A 1-D spectrum and its error array are extracted from the flt and counts images by summing the counts in the cross-dispersion direction within a band centered on the spectrum. The data are not resampled in the dispersion direction. Wavelengths are assigned by evaluating a polynomial function (dispersion relation) of the pixel coordinates. Background is subtracted (see BACKCORR) to get the net count rate, and the absolute flux is computed from the net count rate (see FLUXCORR).
This section provides the details of the spectral extraction process and the construction of the arrays which populate the x1d files. Table 3.1 lists these arrays along with others that are used to calculate them. The following discusses how each array is calculated in detail. The summed _x1dsum[n] files are described in Section 3.4.19.
Table 3.1: Variables used in 1-D Spectral Extraction
Table 3.1 defines the variables used in the 1-D spectral extraction. Variables beginning with a capital letter are saved in the output x1d file. An ‘“[i]” represents array element i in the dispersion direction.
SEGMENT: A string array listing the segments/stripes, contained in the file.
NELEM: An array listing the number of elements in the extracted arrays.
EXPTIME: An array of the exposure times used for each segment (these can differ for FUV data).
GROSS: The GROSS count rate spectrum is obtained from the counts file. The extraction is performed over a parallelogram, whose shorter edges are parallel to the image boundaries and longer (slightly sloping) edges are parallel to the spectrum (see, Figure 3.10). The columns within the parallelogram are summed in the cross-dispersion direction to obtain each element of the GROSS spectrum. Note that in some cases the spectral lines are obviously tilted, i.e., not aligned with the columns, but the sum ignores any such tilt. The location of the parallelogram in the cross-dispersion direction is taken from column B_SPEC in the XTRACTAB.
Extraction routines of this sort are referred to as “boxcar” extractions because they do not weight the elements of the spectrum in the cross-dispersion direction.
GCOUNTS: This is simply the number of gross counts, or GROSS times EXPTIME.
BACKGROUND: Two background regions are sampled on the counts array to obtain a mean background count rate spectrum. For the FUV data, these are above and below the spectrum, and for the NUV data they are above stripe C and below stripe A (see, Figure 3.10 and Figure 3.11). The background regions are extracted in the same way as the spectrum. The values in the two background regions are added, boxcar smoothed in the dispersion direction, and scaled by the sizes of their extraction regions before being subtracted from the science spectrum. Details of the background extractions are given in Section 3.4.16.
NET: The NET spectrum is the difference between the GROSS spectrum and a properly scaled BACKROUND spectrum multiplied by an array which accounts for flat-field and dead-time effects. This array is eps[i] = e[i]/GROSS[i], where e[i] is an element in an array extracted from the _flt file in exactly the same way as the GROSS spectrum is extracted from the counts file. Consequently, this factor corrects the NET spectrum for flat-field and dead-time effects.
DQ: The DQ array in the x1d file is the bitwise OR of the members of the DQ array, contained in the third FITS extension of the counts file, for all of the points in the counts image that contribute to an element of the GROSS spectrum. Consequently, if anything is flagged within the extraction region, it is reflected in the x1d DQ array.
DQ_WGT: The DQ_WGT array has one point for each extracted point in the spectrum. It is 0 or 1 depending on whether the DQ for a given point is allowed according to the header keyword, SDQFLAGS. The SDQFLAG value depends on the configuration of the instrument. For FUV/G130M and G160M the SDQFLAG default value is 8376. For FUV/G140L the value is 8380, and for all NUV modes, 184. These SDQFLAG values set the DQ_WGTS to 0 for events that are near the edge of the detector, dead spots, hot spots or outside of the subarray (see, Table 2.19). Otherwise, DQ_WGTS = 1. The DQ_WGTS array is used to construct the x1dsum file discussed in Section 3.4.19.
ERROR: The ERROR array is calculated from a combination of variables needed to track the detected counts and the different scale factors which multiply them. The raw ERROR array involves elements from both the _flt and the _counts files. It is calculated as follows:
term1[i] = (NET[i]*exptime/(extr_height*snr_ff))2
term2[i] = eps[i]2 exptime*(GROSS[i] + BACKGROUND[i](bkg_norm/bkg_smooth))
ERROR[i] = (1 + sqrt(term1[i] + term2[i] + 0.75))/exptime
The ERROR array contained in the _x1d file differs from this one only in the sense that it has the absolute flux calibration applied (see Section 3.4.17).
WAVELENGTH: As part of the spectral extraction, calcos assigns wavelengths to pixels in the extracted spectra using dispersion coefficients from the reference table DISPTAB. For each grating, central wavelength, and aperture, the DISPTAB table contains the dispersion solution with respect to the template spectral lamp table that was used in the WAVECORR step. The dispersion solution has the following form:
WAVELENGTH[i] = A0 + A1 x[i] + A2 x[i]2 + A3 x[i]3
where WAVELENGTH[i] is the wavelength in Angstroms, x[i] is the pixel coordinate in the dispersion direction, and Ai are the dispersion coefficients. Offsets due to an OSM shift determined from WAVECORR (Section 3.4.12) are corrected for by applying a linear offset after all other corrections have been made. For all modes, small residual offsets occur because of thermal drifts and drifts in the positioning of the OSM. The precision of the OSM positioning and the impact of small offsets in the spectra are discussed further in Chapter 4:COS Error Sources
FLUX: The FLUX array in the x1d file is the NET spectrum corrected by the appropriate, time dependent sensitivity curve. The details of this process are discussed in Section 3.4.17.
The SP_ KEYWORDS: This set of keywords provides useful information on the location of the spectrum in the cross dispersion direction and the location where the spectrum is extracted. The actual location of the spectrum is found from the _flt file through a two step process. First, the image of the active area is collapsed along the extraction window to produce a mean cross-dispersion profile. Second, a quadratic is fit to full-width-half-maximum pixels (but minimum of 5) point region centered on the maximum of the profile. The difference between this value and the expected location, SP_NOM_A[B], is given as SP_OFF_A[B].
The actual location where the spectrum is extracted is given by SP_LOC_A[B]. For normal pipeline extractions, SP_LOC_A[B] = SP_NOM_A[B], and SP_OFF_A[B] is listed for informational purposes only. However, it is possible to override these values and extract a spectrum at the SP_OFF location or any other by using the stand alone version of x1dcorr discussed in Section 5.1.1
Figure 3.10: FUV Extraction Regions
Portions of the undistorted images of the FUV detector segments (compare to Figure 2.2) illustrating the regions used to extract the spectrum and define the background. The dashed lines indicate the spectral extraction window, and the dotted lines define the background extraction region.
Figure 3.11: NUV Extraction Regions
Portion of the NUV detector showing spectral extraction regions used for the three non-contiguous PSA spectra (dashed lines) and for the background (dotted lines).
3.4.16 BACKCORR: 1D Spectral Background Subtraction
This module determines the background contribution to the extracted spectrum and subtracts it.
Input files: flt, counts
Updates the x1d file
The BACKCORR module computes the number of counts in the background regions, scales them by the ratio of sizes of the spectral extraction region to background regions, and then subtracts that value from the extracted spectrum at each wavelength. There are two background regions defined. For FUV data, there is one above and one below the object spectrum (see Figure 3.10). For the NUV spectra, the two regions are above and below the three stripes (see Figure 3.11). Each background region is a parallelogram with the same slope used to define the object extraction region, but with different y-intercepts. The parameters of the background extraction region in the FUV are:
The centers of background regions 1 and 2 in the cross-dispersion (Y) direction follow a linear function in the dispersion (X) direction according to the function
Y = mX +b
where m is the slope of the background (keyword SLOPE), and b is the Y-intercept of the background region (B_BKG1 or B_BKG2). At the i-th pixel along the dispersion direction (X) the background is computed by first summing all of the counts in pixels in the cross-dispersion within +/- (BHEIGHT/2) of the central Y pixel of the background box. All DQ spatially related DQ flags are ignored (note that counts which occur during bad time intervals or which have out of bounds PHAs, never make it to the counts file). Once this is done for all X pixels, the result is averaged over +/- BWIDTH/2 pixels along the dispersion direction. This gives a local average background (with known anomalous pixels such as dead spots or strong hot spots excluded). The background regions below and above the object spectrum are both computed in this way, and then they are summed and divided by 2 to yield an average background rate. This average is then scaled to the number of pixels in the object extraction box by multiplying it by the factor “HEIGHT / BHEIGHT”. The result is the background count rate BK[i] in Table 3.1, which is written to the BACKGROUND column in the x1d file. The background-subtracted count rate (corrected for flat field and dead time) is written to the NET column in the x1d table.
3.4.17 FLUXCORR/TDSCORR: Conversion to Flux
This module converts extracted spectrum into physical units, and allows for time dependencies in the conversion.
Reference files: fluxTAB, TDSTAB
If FLUXCORR=PERFORM, FLUXCORR divides the NET and ERROR columns by the appropriate sensitivity curve read from the fluxTAB reference table, which converts them to flux units (erg cm-2 s-1 -1). The NET divided by the sensitivity is written to the FLUX column of the x1d file, while the ERROR column is modified in-place.
The sensitivity curves read from the reference files are interpolated to the observed wavelengths before division. The flux calibration is only appropriate for point sources and has an option to accommodate time-dependent sensitivity corrections.
If TDSCORR=PERFORM, then the module TDSCORR will correct for temporal changes in the instrumental sensitivity relative to the reference time given by REF_TIME keyword in the FITS header of TDSTAB. TDSTAB provides the slopes and intercepts needed to construct a relative sensitivity curve. The curve for the epoch of the observation is determined by piecewise linear interpolation in time using the slopes and intercepts closest to the time of the observation. The sensitivity may be discontinuous at an endpoint of a time interval. Different piecewise linear functions may be specified for each of the wavelengths listed in the table. This process results in a relative sensitivity at the epoch of the observation, at the wavelengths given in the reference table. Interpolation between these wavelengths to the observed wavelength array is also accomplished by piecewise linear interpolation.
3.4.18 HELCORR: Correction to Heliocentric Reference Frame
This module converts the observed wavelengths to Heliocentric wavelengths.
Input files: rawtag, x1d
In addition to the Doppler smearing from HST orbital motion, the photons acquired during an observation are also Doppler shifted due to the orbital motion of the Earth around the Sun (V ~ 29.8 km/s). The sign and magnitude of the Doppler shift depend on the time of the observation as well as the coordinates of the target (i.e., the position of the target relative to the orbital plane of the Earth).
The HELCORR module in calcos transforms wavelengths of a spectrum to the heliocentric reference frame. It is applied to the extracted 1D spectrum during the operation of X1DCORR, by utilizing the keyword V_HELIO, which is the contribution of the Earth’s velocity around the Sun to the radial velocity of the target (increasing distance is positive), in km/s. It is computed by calcos and written to the science data header of the output corrtag file before spectral extraction is performed.
The shift at each wavelength is
λHelio = λObs [1 - (VHelio/c)]
where λHelio is the corrected wavelength (), c is the speed of light in km/s and λObs is the wavelength before the Heliocentric correction.
The velocity vector of the Earth is computed in the J2000 equatorial coordinate system, using derivatives of low precision formulae for the Sun’s coordinates in the Astronomical Almanac. The algorithm does not include Earth-Moon motion, Sun-barycenter motion, or the Earth-Sun light-time correction.
3.4.19 Finalization (making the sum files)
The final data products for spectroscopic and NUV imaging are different and are discussed separately.
SPECTROSCOPY: Once the processing is complete, an x1d file is written for each spectroscopic exposure. This file includes spectra from both segments A and B for the FUV detector, and from all three stripes for the NUV detector (see, Section 2.4.3). In addition, one or more x1dsum files are created. This is done even if only one spectrum was obtained.
The x1dsum files differ from the x1d files in one important respect. When an x1dsum file is created the DQ_WGT array (Section 3.4.19) is used to determine whether a point is good or bad. When only a single file contributes to the x1dsum file, if DQ_WGT = 0 for a pixel, then the counts, net and flux arrays for that point are set to zero. If the x1dsum or x1dsum<n> (for FPPOS observations) includes several x1d files, then, for each point in the spectrum, only those files with a DQ_WGT = 1 at that point are included (weighted by the individual exposure times), and the DQ_WGT array in the x1dsum file is updated to reflect the number of individual spectra which contributed to the point. If the updated value of DQ_WGT for a particular point is 0, then the value of the spectrum at that point is set to 0 in the x1dsum file.
NUV IMAGING: The end product for NUV imaging is an fltsum file. Like the x1dsum files, an fltsum file is created even if only one exposure is processed. However, since no shifting is performed for imaging observations (see, Figure 3.5), the fltsum file is a simple exposure time weighted mean of the individual flt files (and it is identical to the flt file if only one exposure contributed to it). The DQ flags image of the fltsum file and, for that matter, all of the individual flt images, are identical. This is because the only data which make it into an flt or counts image are free of temporal or event flags (see, Section 2.7). Consequently, in the absence of shifting, all of their spatial flags should be identical.

Table of Contents Previous Next Print