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26.1 Overview of Pipeline Calibration

In this section we provide an overview of the steps comprising the routine pipeline calibration. The flow of the data through the pipeline is presented in schematic form in Figure 26.2. The pipeline calibration software (which the user can find as calwp2 in the STSDAS hst_calib package) takes as input the raw WFPC2 data file pairs (see Table 25.1) .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 your 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 of the observation, and any special pre-specified constraints. They were initially set in the headers of the raw data file in the OPUS pipeline during generic conversion; if reprocessing is necessary, they can be redefined by editing the raw header (.d0h) using the task chcalpar and then running calwp2 on the raw files.

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.

As with other header keywords, the calibration keywords can be viewed using, for example, imhead or hedit. Alternately, you can use the chcalpar task in the STSDAS tools package 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 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

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.

maskfile
(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).

atodfile
(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 the bias level exhibits a column-wise pattern that changes over time.

blevfile, blevdfil
(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).

biasfile, biasdfil
(r2h/b2h)

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).

darkfile, darkdfil
(r3h/b3h)

FLATCORR

Correct for pixel-to-pixel gain variation by multiplying by flatfield image.

flatfile, flatdfil
(r4h/b4h)

SHADCORR

Remove shading due to finite shutter velocity (exposures less than 10 seconds)

shadfile (r5h)

DOPHOTOM

Determine absolute sensitivity using throughputs in synphot tables identified in GRAPHTAB and COMPTAB. This step does not change science data values.

DOHISTOS

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.)

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.	maskfile (`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).	atodfile (`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 the bias level exhibits a column-wise pattern that changes over time.	blevfile, blevdfil (`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).	biasfile, biasdfil (`r2h`/`b2h`)
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).	darkfile, darkdfil (`r3h`/`b3h`)
FLATCORR	Correct for pixel-to-pixel gain variation by multiplying by flatfield image.	flatfile, flatdfil (`r4h`/`b4h`)
SHADCORR	Remove shading due to finite shutter velocity (exposures less than 10 seconds)	shadfile (`r5h`)
DOPHOTOM	Determine absolute sensitivity using throughputs in synphot tables identified in GRAPHTAB and COMPTAB. This step does not change science data values.
DOHISTOS	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.)

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stevens@stsci.edu