WFPC2 Reference File Descriptions
Reference files used by the WFPC2 calibration pipeline (CALWP2) were updated on a regular basis. Current reference files files can be retrieved from the HST archive by clicking on the links to the files in their respective pages, or downloaded directly from the uref directory.
Mask Tables (MASKFILE)
Flags defects in the CCD that degrade pixel performance but are stable over time.
The static mask reference file (.r0h/.r0d) contains a map of the known bad pixels and columns; the mask reference filename is recorded in the MASKFILE header keyword in the science header images (.d0h and.c0h). If this correction is requested, (MASKCORR=PERFORM), the mask is included in the calibration output data quality files; 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 pixels flagged as bad in the final data quality file (.c1h).
A to D Tables (ATODFILE)
Corrects value of each pixel for the analog-to-digital conversion error.
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. The precise gain conversion values for each chip are listed in the WFPC2 Instrument Handbook.
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; although the "true" DN values can never be recovered, one can statistically adjust for this systematic bias. Fortunately, the WFPC2 A/D converters are relatively well-behaved and the correction is small (the largest correction is about 1.8 to 2.0 DN for bit 12, i.e., 2048).
The A/D fix up is applied when the ATODCORR keyword is set to PERFORM; the calibration file (lookup table) used to correct for the A/D errors has the suffix.r1h. The file has four groups, one for each detector; each group has 4096 columns (i.e., all possible DN values for a 12 bit detector) and two rows. The first row contains a -1 in the first pixel, followed by the Bay 3 temperature values in the subsequent pixels; the next row (each row corresponds to one temperature) contains the A-to-D conversion corrections for each DN value3. The example below illustrates determining the A-to-D fix up for a DN value of 450, in WF2, for gain 7, assuming the ATODFILE has already been retrieved from the Archive. Note that the correction for a given DN value is found in the DN+1’th pixel (i.e., [DN+1,2]).
Used to remove any position-dependent bias pattern remaining after the pedestal level is removed.
The WFPC2 bias correction is performed in the pipeline in two steps: a pedestal level is removed and a bias image subtracted. The pedestal level is determined from the overscan columns in each science image; the specific values subtracted are documented in the bias-even / bias-odd science image header keywords. However, the value of the pedestal can also vary with position across the chip. Therefore, after the pedestal correction is performed, the pipeline removes any position-dependent bias pattern by subtracting a bias reference file. This reference file is typically generated from a stack of 120 bias frames (CCD readouts without an exposure); new bias reference files were typically installed in the pipeline about once a year.
Dark Images (DARKFILE)
Used to detect CCD counts caused by thermal processes at the interfaces between the silicon and oxide layers, as well as charged particle and secondary radiation events.
A dark correction is required to account for the thermally-induced dark current as well as a glow from the field flattening lens. The dark reference file (.r3h/.r3d) is generated from a combination of a superdark image (typically, a stack of 120 dark frames) and warm pixels identified from a smaller stack of individual dark frames (usually five). Prior to stacking, each dark frame is examined and regions affected by image anomalies, such as CTE residual images, are masked out. If a dark correction is requested (DARKCORR=PERFORM), the dark reference file, which was normalized to one second, is scaled by the DARKTIME keyword value and subtracted from the observation. By default, DARKCORR is set to “PERFORM” for all exposures longer than 10 seconds, and set to “OMIT” for shorter exposures.
Flatfield Images (FLATFILE)
Corrects for the large-scale illumination pattern and the pixel-to-pixel response function.
The number of electrons generated in a given pixel by a star of a given magnitude depends upon the quantum efficiency of that individual pixel as well as on any large scale vignetting of the field-of-view caused by the telescope and camera optics. To correct for these variations, the science image is multiplied by an inverse flatfield file. WFPC2 flatfields are currently generated from a combination of on-orbit data (so-called Earthflats, images of the bright Earth) and pre-launch ground data. The on-orbit data allow a determination of the large-scale illumination pattern while the pre-launch data are used to determine the pixel-to-pixel response function. The application of the flatfield file (extension .r4h) is controlled by the keyword FLATCORR.
Shutter Shading Images (SHADFILE)
Corrects the effects of the shutter motion in short (<10 seconds) exposures.
The finite velocity of the shutter produces uneven illumination across the field of view (thus the term "shutter shading"), resulting in a position-dependent exposure time. The effect is only significant, however, for exposures of a few seconds or less. The shutter shading calibration is applied by default to all exposures less than ten seconds. It has the form of an additive correction, scaled to the appropriate exposure time, that varies spatially across the detectors. The keyword switch is SHADCORR, and the shutter shading file name is stored in the keyword, SHADFILE.
WF4 Correction Tables (WF4TFILE)
Used to correct low CCD bias levels and lowered count levels (low CCD gain) on the WF4 detector.
The WF4 CCD anomaly is characterized by low or zero CCD bias levels, lowered count levels on the WF4 detector (i.e., low CCD gain), and faint horizontal background streaks. To correct the first two effects, a new processing step has been added to the WFPC2 calibration pipeline. It rescales each pixel using a gain correction that depends on the observed pixel value and the bias level of the image. Internal VISFLAT observations have been used to derive the corrections, which are tabulated into separate reference files for gain 7 and gain 15 data. After correction, the WF4 images show normal bias levels. Photometric tests using the standard star GRW+70D5824 indicate that the corrections are generally accurate to ~ 0.01 magnitude, with lower accuracy of ~ 0.02 magnitude in some infrequent cases. In most cases, the photometric properties of the corrected WF4 images are essentially indistinguisable from normal images taken in the other WFPC2 CCDs.
Please see ISR WFPC2 2009-003 'Pipeline Correction of Images Impacted by the WF4 Anomaly' for more information on the creation of these tables.
Distortion Correction Tables
Used in drizzling WFPC2 images. Contains the best distortion models for all four chips.
The image distortion correction table consists of a set of world coordinate information and fits to a distortion function that are used to construct a rectified, linearized 2-D image. Each row of the table contains the chip, filter, reference pixel and plate scale for the chip, and the coefficients to the fits to the distortion functions. Possible function types include CHEBYSHEV, LEGENDRE, and POWER_SERIES.
Inter-chip Offsets Table
Used in conjunction with the IDC table; contains the secular relative orientations and positions of the chips.
Along with the 'normal shifting of the chips due to outgassing, etc., the relative positions of the four CCDs in the focal plane shifted by about 0.01 arcsecond for each 1 degree C reduction in the electronics bay temperature that were used in mitigating the low bias levels (change in the CCD gain) in WF4. These shifts may have had a small effect on projects which required accurate chip-to-chip astrometry and so this table is used to account for these shifts.
34 Row Displacement Correction File (DEGEOFILE)
Corrects for the reduced sensitivity seen in approximately every 34th row in the detectors.
Approximately every 34th row on the WFPC2 detectors possesses a reduced sensitivity; these features are likely due to a manufacturing defect that resulted in the affected rows being somewhat narrower than the rest. The pipeline flatfields contain these features and produce calibrated images appropriate for surface brightness measurements. Point source photometry, however, can suffer 1-2% errors; and, the narrow rows have a significant effect on astrometry, causing periodic errors of up to 0.03 pixel. This file is used in correcting the reduction in sensitivity.
IDT Superbias Files
Early bias files created by the IDT to remove any position-dependent bias pattern remaining after the pedestal level is removed.
These files behave exactly like the default files for a given useafter date. Please see the HISTORY section to see how the files were generated.
IDT Superdark Files
Early dark files created by the IDT; behave like regular pipeline darks, may have better noise characteristics.
These files behave exactly like the default files for a given useafter date, though they may better noise properties and alone do not account for hot pixels. Please see the HISTORY section to learn how these files were generated.
IDT Deltadark Files
Used with IDT Superdark and Superbias files to remove hot pixels in early WFPC2 data.
Please see the Deltadark Section of the IDT Reference file memo for more information on these files.
Correction Flats from Erich Karkoschka
Correction flat to be multiplied by the 'previous' flatfield to produce a less noisy revised flatfield.
Improved flatfields were created by averaging flatfields of similar wavelength in such a manner that wavelength-dependent features were mostly preserved, yet the noise of the flatfield was significantly lowered.
These improved flatfields are called 2001-flatfields. The ratios of the 2001-flatfields / 2000-flatfields have been computed and are available to observers via the HST archive system. These correction flats can be multiplied into calibrated data for the purpose of reducing noise contributed by the flatfields.
Please see Instrument Science Report WFPC2 2001-07 for more information on the generation and use of these file.
Pixel Area Correction File
Used to restore the proper total counts of the target.
NOTE:Do not use this correction file if you plan to use Multidrizzle, as all geometric distortion corrections, including the pixel area correction, are properly accounted for in the software.
Geometric distortion near the edges of the chips results in a change of the surface area covered by each pixel. The flatfielding corrects for this distortion so that surface photometry is unaffected. However, integrated point-source photometry using a fixed aperture will be affected by 1 to 2% near the edges, with a maximum of about 4-5% in the corners. The counts measured for a star centered at a given pixel position must be multiplied by the value of these same pixels in the correction image (simply multiplying your image by the correction image and then measuring the counts is easiest). A small residual effect, due to the fact that the aperture radius differs from the nominal size, depends on the aperture used and is generally well below 1%.