The values of the calibration switches in the headers of the raw and calibrated data indicate what calibration steps the pipeline applied to the data and which calibration reference files were used. Calibration switches will have the value PERFORM, OMIT, or COMPLETE, depending on whether the step has yet to be performed, is not performed during the processing of this dataset, or was completed. For convenience, these keywords are reported in the dataset information page of the new-style paper products.
There are history records at the bottom of the header file of the calibrated data (as
well as the calibration reference file headers). These history records sometimes
contain important information regarding the reference files used to calibrate the
data in the pipeline.
The flow chart below summarizes the sequence of calibration steps performed by calwp2, including the input calibration reference files and tables, and the output data files from each step. The purpose of each calibration step is briefly described in the accompanying table; a more detailed explanation is provided in the following section.
Figure 26.2: Pipeline Processing by calwp2
Calibration Steps and -Reference Files Used for WFPC2 -Pipeline -Processing
Update the data quality file using the static bad pixel mask reference file (maskfile), which flags defects in the CCD that degrade pixel performance and that are stable over time.
Correct the value of each pixel for the analog-to-digital conversion error using information in the A/D lookup reference file (atodfile).
Subtract the mean bias level from each pixel in the science data. Mean values are determined separately for even column pixels (group parameter BIASEVEN) and odd column pixels (BIASODD) because the bias level exhibits a column-wise pattern that changes over time.
Subtract bias image reference file (biasfile) from the input science image and update output data quality file with bias image data quality (biasdfil).
Correct for dark current by scaling dark image reference file and subtracting it from science data. Dark image is multiplied by total dark accumulation time (keyword DARKTIME).
Correct for pixel-to-pixel gain variation by multiplying by flatfield image.
Remove shading due to finite shutter velocity (exposures less than 10 seconds)
Determine absolute sensitivity using throughputs in synphot tables identified in GRAPHTAB and COMPTAB. This step does not change science data values.
Create 3-row image (.c2h) for each group. Row 1 is a histogram of raw science values, row 2 the A/D correction, row 3 the calibrated image. (Optional, default is not to produce them.)