FOS Data Quality Flag Values
Flag
Value
Description
< 1100 Å or > 7000 Å)
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You will be most interested in the contents of the .c1*, .c0*, and .c2* files as these contain the flux-calibrated data, wavelengths and errors for an exposure. The easiest way to get a quick glance at your spectra is by using the routine splot or fwplot if you want to plot wavelength vs. flux (see "FOS and GHRS Spectra" on page 3-17). We recommend that you generate the FOS paper products for your data with the STSDAS routine pp_dads and obtain a variety of displays and tables which, as we shall see in "FOS Paper Products" on page 30-15, can facilitate the assessment of your data quality.
FOS File Name Suffixes
30.1.1 Uncalibrated (Raw) Data Files
Table 30.1 lists the science files that are used as required input to calfos. These files are described briefly below.
Science Data Files (.d0h/.d0d)
Science data files contain single-precision floating point values that represent the number of detected counts accumulated in each pixel. The number of data elements in the one-dimensional science data array depends on the observation mode; specifically, the number of diodes, the number of substeps, the number of Y steps, and the number of repeats (sometimes called slices or bins) used in the observation. The maximum number of data elements is 12288. The associated header file also provides information on the different steps to be performed during pipeline calibration processing, and the reference files and tables to be used in the calibration. Science Header Line (.x0h/.x0d)
The science header line (SHL) file is a one dimensional array with a length equal to a line of the science data. It contains a partial copy of the unique data log. Science Trailer Line (.d1h/.d1d)
The science trailer line (STL) file is also a one dimensional array containing the number of measurements rejected from the various combinations of x substeps, y steps, repeats, etc. The information in these files is used to compute the total effective exposure time per pixel which is later used to convert the counts into count rates. Data Quality Files (.q1h/.q1d)
The science data files, science header line files, and the science trailer files have corresponding data quality files that contain the flags for bad or suspect data. These raw data quality files have quality flags as follows:
Calibrated Wavelength Files (.c0h/.c0d)
These files contain single-precision floating point calibrated vacuum wavelengths corresponding to the center of each pixel of the science data. All FOS wavelength solutions assume the first pixel is pixel 0. Calibrated Flux Files (.c1h/.c1d)
These files contain single-precision floating point calibrated fluxes (in ergs sec-1 cm-2 Å-1) corresponding to each pixel of the science data. Calibrated Statistical Error Files (.c2h/.c2d)
These files contain the statistical errors of the original data. These files are calibrated in lock-step with the science data files. Errors caused by sky and background subtraction, flatfields, and sensitivity are not included in the error estimates. Calibrated Special Mode Data Files (.c3h/.c3d)
Data acquired in the rapid-readout, time-resolved, or spectropolarimetry modes require processing steps in addition to (or complementing) those used for standard ACCUM data. The calibrated data are then stored in special mode data files. For RAPID mode, the files contain the total flux, integrated over all pixels, and the associated statistical error for each readout. For TIME RESOLVED mode, the files contain the pixel-by-pixel average of all slices or bins, the difference between each slice or bin and the average, and the average propagated statistical errors. For POLARIMETRY mode, the file contains the Stokes I, Q, U, and V parameters, the linear and circular polarization, and the polarization position angle. The polarimetric quantities and the propagated errors are calculated for each of the separate pass directions, the combined pass direction data, and the combined pass direction corrected for interference and instrumental orientation. Calibrated Data Quality Files (.cqh/.cqd)
The quality flags in these files flag the bad pixel values in the calibrated files. The quality flags from the raw data are updated and additional flags are added for problems detected in the calibration process. The data quality flags are defined in Table 30.2.
Intermediate Calibrated Output Data Files (.c4* - c8*)
At most, six sets of intermediate calibrated output files are produced depending on the observation mode.
For case 2, measures were taken from the upper and lower sections of the aperture (always starting in the lower aperture section which has the more negative Y-base value) in approximate 10-second alternating segments, readout to memory at the interval appropriate to the data-taking mode (either ACCUM or RAPID). In the .d0* files the data are stored in groups that are twice as long as single aperture groups, e.g., the default paired aperture ACCUM group size is 4128 pixels. The first 2064 pixels correspond to data readout at the Y-base for data taken in YSTEP1 and the second 2064 pixels are from YSTEP2. YSTEP1, which can be either OBJ (STAR) or SKY, always corresponds to the lower aperture. (The mapping of OBJ (STAR) and SKY to upper and lower halves of the aperture was dependent upon user-specification.) Case 2 .c4* files contain twice as many groups as in case 1 with the odd-numbered group containing to the count rate spectrum groups for YSTEP1 and the even-numbered groups containing the count rate spectrum groups for YSTEP2.
PODPS Data Quality File (.pdq)
STScI staff performed a quick Data Quality Assessment (DQA) of the target acquisitions and the science observations. This assessment identified any target acquisition failures, guide star acquisition failures, other spacecraft anomalies, or instrumental problems; the assessment was recorded in the data quality keywords of the PDQ (Procedural Data Quality) file. PDQ files are archived to data class PDQ and can be retrieved from the HST Archive using StarView. The FOS paper products list each relevant PDQ file comment for every exposure. 30.1.5 Reference Files and Tables
The calibration pipeline used reference files and tables to flux calibrate FOS data. Data in the archive and the data that were sent to GOs were calibrated with the reference files that were available at the time of the observation. These are generally not the "best" files to use to calibrate these data today. StarView allows you to retrieve either the "used" or the "best" reference files. The "best" reference files reflect our accumulated experience of FOS calibrations and derive from the late 1997 FOS closeout calibration analyses. Naming and Structure of FOS Reference Files and Tables
The reference files and tables are typically referred to by the name of the Calibration Data Base System (CDBS) reference relation that holds their names. The suffixes of the reference tables and files are of the form .cyn, .rnh and .rnd where n represents a value from 0 to 9 and A to D (see Table 30.3). These files are maintained in the CDBS. The STScI web site also includes a catalog of these tables and files.
A thorough description of the internal header and data formats of FOS reference files and tables is beyond the scope of this document, but all this information can be found in rigorous detail in STScI document ICD-47 which is available from the FOS WWW page and in paper format from help@stsci.edu. A description of what each FOS reference file and table is used for is found in "Details of the FOS Pipeline Process" on page 31-13.
Except for some spectropolarimetric reference files, which are twice this length (for two pass directions), all reference files contain a vector of length:
