45.2 Calibration of WF/PC-1 Data
WF/PC-1 science data was calibrated in the pipeline using calwfp, available under the hst_calib package. By using this IRAF/STSDAS task, the data can be recalibrated using the same software used in the routine calibration pipeline.1
The calibration software takes as input the raw WF/PC-1 data file pairs (Table 44.1 on page 44-2) d0h/d0d, q0h/q0d, x0h/x0d, q1h/q1d and any necessary calibration reference images or tables. The software determines which calibration steps to perform by looking at the values of the calibration switches (e.g., MASKCORR, BIASCORR, etc.) in the header of the raw data (.d0h) file. Likewise, it selects the reference files to use in the calibration of the data by examining the reference file keywords (e.g., MASKFILE, BIASFILE, BIASDFILE, etc.). The appropriate values of the calibration switches and reference file keywords depend on the instrumental configuration used, the date when the observations were taken, and any special pre-specified constraints. They were initially set in the headers of the raw data file in the RSDP pipeline during generic conversion; for recalibrating, they can be redefined (using hedit for example) and calwfp run on the raw files.
To determine what calibration steps the pipeline applied to the data and which calibration reference files were used to calibrate the data, examine the values of the calibration switches in the header of the raw (or calibrated) data. Prior to calibration, the calibration switches will have the value YES, NO, or PERFORM. After calibration, the switches for completed steps will have been assigned the value COMPLETE in the header keywords of the calibrated data file.
As with other header keywords, the calibration keywords can be viewed using any of several tasks, for example, imhead, hedit, or hselect. Alternately, the chcalpar task in the STSDAS tools package can be used to view the calibration keywords directly.
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 calwfp, 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.
| Switch | Processing Step | Reference File |
|---|---|---|
| MASKCORR | 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. | mask file (r0h) |
| ATODCORR | Correct the value of each pixel for the analog-to-digital conversion error using information in the A/D lookup reference file (ATODFILE). | atod file (r1h) |
| BLEVCORR | 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 bias levels exhibit column-wise pattern that changes over time. | extracted engineering file (x0h/ q1h) |
| BIASCORR | Subtract bias image reference file (BIASFILE) from the input science image and update output data quality file with bias image data quality (BIASDFIL). | bias file, bias DQF file (r2h/b2h) |
| PREFCORR | Correct for uneven illumination pattern produced by preflash by subtracting preflash reference file (PREFFILE) scaled by preflash lamp exposure (PREFTIME keyword). If no preflash was done, a correction called CTE fixup is done to correct charge transfer problem across gate. | preflash file, preflash DQF file (r3h/b3h) |
| PURGCORR | Remove residual effects of superpurge, which removes residual image from highly overexposed source. This step was not implemented in pipeline. | purge file (r4h/b4h) |
| DARKCORR | 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). | dark file (r5h/b5h) |
| FLATCORR | Correct for pixel-to-pixel gain variation by multiplying by inverse flatfield image. | flatfield file (r6h/b6h) |
| DOSATMAP | Create an output data quality file (.c3h) that flags pixels that saturated the A/D converter. This is redundant because saturated pixels are flagged in the DQF (.c1h). | |
| DOPHOTOM | Determine absolute sensitivity using throughputs in photometry calibration table (PHOTTAB). This step does not change science data values. | phottab (cw0) |
| DOHISTOS | Create 3-row image (.c2h) for each group. Row 1 is a histogram of raw science values, row 2 the A/D corrected data, row 3 the calibrated image. |
1 Online details about the WF/PC-1 calibration pipeline are available from within the IRAF help system using the opt=sys argument, type: help wfpc opt=sys
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