WFC3 Reference File Images & Tables

Reference files used by the WFC3 calibration pipeline (CALWF3 and PyDrizzle/MultiDrizzle) are updated on a regular basis. Current reference files files can be retrieved from the HST archive, or downloaded directly from the iref directory.

WFC3 Bias Images
The bias reference image consists of an image of the additive stationary pattern in the electronic zero point of the UVIS CCDs. The structure of the electronic bias is removed from science images by subtracting the bias reference image.

WFC3 Dark Images (DRK)
The dark current reference image file consists of an image of the dark signal (i.e. the signal detected in the absence of photons from the sky). The dark reference image is subtracted from science images to remove the dark signal.

WFC3 Flat-Field Images
WFC3 can utilize up to three different flat-field images during calibration: the pixel-to-pixel flat (PFLTFILE), the delta flat (DFLTFILE), and the low-order flat (LFLTFILE). If more than one of the three types of flats is specified for a given science data set, each flat is applied in turn to the science data by calwf3.

WFC3 Shutter Shading Images (SHD)
This reference file corrects a UVIS CCD image for the differential exposure time across the detector that results from the shutter travel time as it opens to start the exposure. Each pixel in the reference image gives the additional time that a given pixel was exposed above the nominal exposure time.

WFC3 Linearity Correction Files (LIN)
The linearity correction file contains a set of coefficients for each pixel that generate a linear correction over the nonlinear range of the IR detector. The observed response of the detector is represented by two regimes. At count levels below the saturation threshold the detector response deviates from the incident flux in a manner that can be adequately represented by a seond-order polynomial. At high count level as saturation sets in, the response becomes highly nonlinear and is not correctable to sufficient scientific accuracy.

WFC3 Analog-to-Digital Tables (A2D)
This table provides the actual number of counts for each detected count in the image and allows for possible irregularities that might occur in the conversion such as were seen on the original WFPC. The conversion takes into account the gain setting, the amps used, and typically the exposure time of the observation.

WFC3 Bad Pixel Tables (BPX)
This reference file maintains a record of all known bad pixels for each WFC3 detector. These pixels change with time as some hot pixels are annealed and others appear. It is the job of the BPIXTAB to maintain the list of bad pixels applicable for a given time period. The DQ values associated with each pixel listed in the BPIXTAB are written to the DQ image extensions of the science data being processed by calwf3 and are combined with any previously existing DQ values. Bad pixels due to telemetry errors will be flagged during Generic Conversion.

WFC3 CCD Characteristics Tables (CCD)
Up to four detector amplifiers can be used for any given observation and each amplifier has its own read-out characteristics. However, only a single value for these characteristics can be commanded by the observer. This table (with columns as defined in Table 11-6) provides the conversion from the commanded values to the calibrated values for each amp. These calibrated values are then used during processing by calwf3 to insure that a pixel read out by an amp has been properly calibrated for that amp's readout characteristics. The characteristics affected are readout noise (READNSE), A-to-D gain (ATODGN), and bias level (CCDBIAS).

WFC3 Overscan Region Tables (OSC)
This table describes the overscan regions for each chip along with the regions to be used for determining the actual bias level of the observation. Each row corresponds to a specific configuration as given by the amps used (CCDAMP), the chip (CCDCHIP), and the on-chip binning mode (BINX, BINY).

WFC3 Cosmic Ray Rejection Tables (CRR)
This reference table contains all the basic parameters necessary for performing cosmic-ray rejection. The cosmic-ray rejection process requires a number of input parameters to control how the cosmic-rays are detected and removed. The process starts by creating a first guess for the CR- combined image either by median combining or minimum value combining the input CR-SPLIT exposures, as specified by INITGUES. Determination of the sky and noise values is controlled by the SKYSUB and SCALENSE values, respectively. Actual detection of the cosmic rays requires the specification of a threshold above which a pixel value is considered a cosmic ray (CRSIGMAS, CRTHRESH) and the distance from the detected pixel which the cosmic ray can affect other pixels (CRRADIUS). Once a pixel is determined to be affected by a cosmic ray, that pixel will be marked in the input image DQ array, if CRMASK is set to yes.

WFC3 Image Distortion Coefficients Tables (IDC)
This reference table contains a description of the geometric distortion models for the WFC3 UVIS and IR detectors. A detailed description of the contents and usage of the IDCTAB with HST instruments is found in ISR ACS 2001-008 (Hack & Cox 2001). Briefly, the IDCTAB contains the coefficients of a polynomial fit that is used to transform image coordinates from raw (distorted) space to an undistorted space. It is utilized by calwf3 to geometrically transform calibrated images from raw detector space to an undistorted space.