http://www.stsci.edu/ftp/instrument_news/WFPC2/Wfpc2_top.html
All of the installed reference files contain HISTORY keywords at the end of the header which can be viewed using the imhead task. These keywords contain more detailed information about how the file was created and installed in the database.
26.2.2 Calibration Steps
Application of the Static Mask
The static mask reference file (.r0h/.r0d) contains a map of the known bad pixels and columns. If this correction is performed (MASKCORR=PERFORM), the mask is included in the calibration output data quality files. The mask reference file is identified in the MASKFILE keyword. The science data themselves are not changed in any way; the STSDAS task wfixup can be used on the final calibrated science image (.c0h/.c0d) to interpolate across bad pixels flagged in the final data quality file (.c1h). A/D Correction
The analog-to-digital (A/D) converter takes the observed charge in each pixel in the CCD and converts it to a digital number. Two settings, or gains, of the A/D are used on WFPC2. The first converts a charge of approximately seven electrons to a single count (called a Data Number or DN), and the second converts a charge of approximately 14 electrons to a DN, also referred to as gain 15 for historical reasons. A/D converters work by comparing the observed charge with a reference and act mathematically as a "floor" function. However, these devices are not perfect, and some values are reported more (or less) frequently than they would be by a perfect device. One can adjust statistically for this bias; fortunately the WFPC2 A/D converters are relatively well-behaved and this is a small correction. The largest correction is about 1.8 to 2.0 DN for bit 12 (thus 2048). Bias Level Removal
The charges that are in each pixel sit on top of an electronic pedestal, or "bias" designed to keep the A/D levels consistently above zero. The mean level of the bias must be determined empirically using extended register (overscan) pixels which do not view the sky. The values of these pixels are placed in the extracted engineering files (.x0h/.x0d). The overscan area used to calculate the mean bias levels is [9:14,10:790], with BIASODD being determined from columns 10, 12, and 14 and a BIASEVEN being determined from columns 9, 11, and 13 (this surprising nomenclature is due to an offset in the .x0h file; even and odd are correctly oriented with respect to the data file columns). In observations before March 8, 1994, the pipeline used a larger part of the overscan region, resulting in oversubtraction of the bias level and possibly large negative pixel values. Separate even and odd bias levels were extracted only after May 4, 1994. See Chapter 27 for more information on how to deal with early WFPC2 data. The keyword BLEVCORR controls the subtraction of the bias in calwp2. Bias Image Subtraction
The value of the bias pedestal can vary with position across the chip. Therefore, once the mean bias level correction has been completed, the pipeline looks at the keyword BIASCORR. If it is set to PERFORM, then a bias image (.r2h) is subtracted from the data to remove any position-dependent bias pattern. The bias reference file is generated from a large set of A/D and bias-level corrected zero-length exposures. The correction consists of subtracting the bias file from the observation and flagging in the .c1h/.c1d file any bad pixels noted in the bias data quality file (.b2h/.b2d). Dark Image Subtraction
A dark correction is required to account for the thermally-induced dark current as well as a glow (see "Dark Glow" on page 27-4) from the field flattening lens. The dark reference file is generated from ten or more individual dark frames (long exposures taken with the shutter closed) that have each had the standard calibration corrections applied (ATODCORR, BLEVCORR, and BIASCORR). In addition, each frame is examined and regions affected by image anomalies (such as residual images, see Figure 27.2) are masked out. If a dark correction is performed, the dark reference file (which was normalized to one second) is scaled by the DARKTIME keyword value and subtracted from the observation. The keyword DARKCORR controls the subtraction of the dark file (.r3h). By default, DARKCORR is set to "PERFORM" for all exposures longer than 10 seconds, and to "OMIT" for shorter exposures.
Flatfield Multiplication
The number of electrons generated in a given pixel by a star of a given magnitude depends on the individual quantum efficiency of the pixel as well as any large scale vignetting of the field-of-view caused by the telescope and camera optics. To correct these variations in total quantum efficiency, the image is multiplied by an inverse flatfield file, which is currently generated from a combination of on-orbit data, used to determine the large-scale structure of the illumination pattern, and data taken before launch to determine the pixel-to-pixel response function. The application of the flatfield file (extension .r4h) is controlled by the keyword FLATCORR. Shutter Shading Correction
The finite velocity of the shutter produces a position-dependent exposure time. This effect is only significant for exposures of a few seconds or less, and is automatically removed from all exposures less than ten seconds. The keyword switch is SHADCORR, and the shutter shading file name is stored in the keyword SHADFILE. Creation of Photometry Keywords
Photometry keywords, which provide the conversion from calibrated counts to astronomical magnitude, are created using the STSDAS package synphot. (More information on synphot can be found in this document, and in the Synphot User's Guide, which is available via WWW.) These keywords are listed in Figure 26.3, below; the first two keywords are in the ASCII header (both .d0h and .c0h) while the last five keywords are group parameters (use the IRAF tasks imheader or hedit to examine the group keywords-see Chapters 2 and 3 for more details). This calibration step uses the HST graph and component tables (.tmg and .tmc) to determine the throughput for the appropriate WFPC2 observing mode and filter. The keyword switch for this step is DOPHOTOM, and the reference file keywords are GRAPHTAB and COMPTAB.
Figure 26.3: Photometry Keywords
DOPHOTOM= `YES ` / Fill photometry keywords: YES, NO, DONE
PHOTTAB = `ucal$u27s0301n.c3t' / name of the photometry calibration table
`PHOTMODE' / Photometry mode (for example, WFPC2,1, A2D7, F675W,CAL)
`PHOTFLAM' / Inverse Sensitivity (erg/sec/cm2/Å for 1 DN/sec)
`PHOTPLAM' / Pivot wavelength (angstroms)
`PHOTBW ` / RMS bandwidth of the filter (angstroms)
`PHOTZPT' / Photometric zeropoint (magnitude)
The bad pixels flagged in the .c1h file have not been fixed in the .c0h file. You may wish to use the STSDAS task wfixup to interpolate across bad pixels in your science image or to use the .c1h file to determine which pixels to discard in various analysis programs.
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Calibrated saturated pixels may have values significantly lower than 4095 due to bias subtraction and flatfielding. In general, data values above 3500 DN are likely saturated.
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