calwp2 -- Routine calibration of HST Wide Field Planetary Camera-II images.
calwp2 inname outname
The calwp2 task provides routine calibration in both the SOGS/PODPS Routine Science Data Processing (RSDP) pipeline and in the STSDAS package. CALWP2 performs a standard sequence of processing steps on individual WFPC2 camera observations designed to remove most of the known instrumental signature. The task performs the AtoD correction, determines and removes the global bias level, subtracts the bias image, and scales and subtracts the dark frame, applies the flat field frame and shutter shading correction. The input data quality file (DQF) will be updated with pixels observed to be saturated, and with the DQF of known bad pixels (static mask), and with the DQFs of any images used in the processing sequence. Histograms of the input science image, the image after AtoD correction, and the output calibrated image are generated. This program is controlled by setting keywords in the header of the input image. CALWP2 writes the values of the group parameters to an ASCII file with extension ".cgr"
The calwp2 task determines whether it is running in the pipeline by looking for an environment variable with the name PODPS_CLD (yes, upper case). In the PODPS environment the VMS logical name PODPS_CLD is always defined, and this shows up in IRAF as an environment variable. (The value is a VMS file name) PODPS_CLD is presumed to be not set for ordinary IRAF use outside the pipeline. When running within the pipeline, the task requires an input ASCII file, which contains all the group parameters of the input raw image. This ASCII file must have the same root name as the input image but an extension of ".dgr". Within the pipeline, calwp2 will update the group parameters in the input ASCII file with extension ".dgr". This makes it easier for DADS to get those group parameters for the input raw image.
The calwp2 task constructs a PHOTMODE based on the header keywords, including INSTRUMENT, DETECTOR, ATODGAIN, FILTNAM1, FILTNAM2 and LRFWAVE, and computes the grand throughput table from the GRAPH and COMP tables stored in the HST Calibration Data Base System (CDBS). It then computes the photometric parameters and populates the photometric header keywords of PHOTFLAM, PHOTZPT, PHOTPLAM, and PHOTBW. The PHOTMODE string and the four photometric header keywords are all stored in the header of the calibrated image. The grand throughput table is saved in a STSDAS table with extension of ".c3t".
The input WFPC2 image should be in the form provided by PODPS. This task is specific to WFPC2 data and requires a large number of keywords (see below) obtained from the WFPC2 image header. The first two input images listed below are always required, the others may be required depending upon the options selected.
- .D0H (input_image)
- The input WFPC2 image in GEIS format. This image can contain from 1 to 4 groups. The image header should conform to the standard WFPC2 header format. Keywords in the header are used to select the various processing steps and to specify the names of the necessary reference files. The image must be an unmodified short integer WFPC2 image for the saturation detection to be meaningful.
- The input WFPC2 data quality file (DQF) in GEIS format. This file must consist of short integers and have the same dimensions and number of groups as the input_image. Each pixel indicates the quality of the corresponding pixel in the input_image. Zero valued pixels indicate "good" pixels in the input_image and nonzero valued pixels flag various pathologies associated with the corresponding image pixels.
- The input WFPC2 Extracted Engineering Data file in STF format. This file contains the two "engineering" columns and, for FULL mode, the section of [9:14,11:790] is used to compute the global bias level. It is required when the BIASCORR option is selected (see below).
- The input WFPC2 Extracted Engineering Data DQF. It is required when the BIASCORR option is selected (see below).
- The Standard Header Packet. This file is not currently used by calwp2 but is part of the standard WFPC2 dataset.
- The input group parameter ASCII file. This file is used when running the task in the pipeline and will be updated upon task completion. This file must be arranged in such a way that the parameters are bound together by group.
OUTPUT IMAGES AND FILES
Each image is a two-dimensional GEIS format image containing the same number of groups as the input_image. The names of the output images are derived from the root name of the input_image and the following extensions.
- The processed output image. This image is in REAL format (with an option to provide FITS-like scaling to SHORT or LONG integers---see DATATYPE below). Extreme values resulting from input data values outside of the range of the WFPC2 AtoD converter (0 to 4095) are edited to limit the range of values in the output image. These pixels are flagged in the DQF.
- The DQF for the processed output image. This contains the result of bitwise ANDing the input DQF and all DQFs associated with reference files used in the processing of the input_image.
- The histograms of the "good" (i.e., not DQF-flagged) pixel values. This image consists of three rows and the same number of groups as the input image. The first row contains the histogram of the pixel values in the input image. The second row contains the histogram following the A-to-D correction step. The third row contains the histogram of the output (.C0H) image. The creation of this file is controlled by the keyword `DOHISTOS' in the input image header.
- The output throughput table generated according to the photomode string. The photomode string is constructed from the information in the header of the input science image. This grand throughput table is used for calculating the four photometric quantities listed in the header of the calibrated image.
- The output ASCII text file containing values of the header keywords of the output image.
PROCESSING STEPS and REFERENCE FILES
The calwp2 task performs the sequence of processing steps listed below. Most of these steps may be selected (or de-selected) by the values of certain keywords in the header of the input_image. If a processing step is selected, it may require that other keywords in the input_image header contain necessary data (e.g., the name of a reference file).
The following keywords select steps to be performed by calwp2. The steps are performed in the order indicated. A more complete description of the algorithms employed can be found in the "Calibration Reference Guide".
The acceptable input values for each switch are (PERFORM | OMIT | COMPLETE). COMPLETE has the same effect as OMIT. In the output image (.C0H), those keywords which were set to PERFORM in the input_image and for which processing was successfully performed, are set to COMPLETE.
The following switches are available:
- Include the Static Bad Pixel Mask in the output DQF. The name of the Static Bad Pixel Mask must be provided in the keyword MASKFILE and must be in the same format as the DQF.
- Replace each pixel with the appropriate value from the A-to-D correction lookup table. The name of the lookup table must be provided as the value of the `ATODFILE' keyword. The A-to-D file may consist of multiple lookup tables, whereupon the table whose temperature is closest to the value of the input image header keyword `UBAY3TMP' is selected.
- The global bias level is determined from the extended register pixels in the extracted engineering data file (.X0H) and subtracted from each pixel in the input_image. (The names of these files must be provided in the keywords BLEVFILE and BLEVDFIL.)
- The bias image reference file is subtracted from the input_image. The names of the bias image and its DQF must be provided in the keywords BIASFILE and BIASDFIL.
- The dark image reference file is scaled and subtracted from the input_image. The dark image is multiplied by the total "dark" accumulation time (obtained from the keyword DARKTIME in the input_image header and expressed in seconds) and then subtracted from the input_image. The names of the Dark image and its DQF must be provided in the keywords DARKFILE and DARKDFIL.
- The input_image is corrected for variations in gain between pixels by the application of a flat field image. The input_image is multiplied by the flat field image. The flat field image should be normalized and inverted in preparation for this step. The names of the flat field image and its DQF must be provided in the keywords FLATFILE and FLATDFIL.
- Relatively short exposures for the WFPC2 show evidence of shutter shading, resulting from the finite time of flight of the shutter blade. This gives rise to uneven illumination across the field of view. The input science image is corrected for the shutter shading by means of an additive correction that varies spatially according to the shutter shading reference file which is scaled for the appropriate exposure time. The name of the shutter shading reference file must be provided in the keyword SHADFILE.
- Fill in the photometry keywords `PHOTFLAM', `PHOTZPT', `PHOTPLAM', and `PHOTBW' with values selected from the grand throughput table line that matches the `PHOTMODE' keyword. The PHOTMODE keyword itself is constructed during `calwp2' processing from values of several other keywords which specify the camera, chip, and filter(s) used during the observation. The throughput table is constructed during the processing from the GRAPHTAB and COMPTAB. The reference graph and comp tables must be provided in the keywords `GRAPHTAB' and `COMPTAB' if this option is selected. The name of the throughput table file (.C3T) is stored in the keyword `PHOTTAB'.
- Create an image of three rows containing histograms of the pixel values in the input image, following A-to-D correction, and in the output (`.C0H') image. This file has the extension `.C2H'.
- Select the data type of the output (.C0H) image. The allowed options are REAL, SHORT, LONG corresponding to real*4, short integer, and long integer pixels.
The following table associates the filename extensions for the various reference files. Note that the filenames specified for the reference files are prepended with something like: "wref$", or "wcal$". These are for use as IRAF logical pathnames, since it is often impractical to have all needed WF/PC reference files in the same directory (or even the same disk) as the images to be calibrated. For example, type "show wcal" to display the current setting for the "wcal" environment variable, and "set wcal = home$" if the ".x0h" file is in your IRAF login directory. See the on-line help for "set" for further information.
Table 1. Reference Header File Extensions Extension(s) | Reference type(s) ------------------------------------------------- .r0h | Bad pixel mask .r1h | A-to-D Correction Lookup Table .x0h, .q1h | Engineering file & DQF .r2h, .b2h | Bias image & DQF .r3h, .b3h | Dark image & DQF .r4h, .b4h | Flat-field image & DQF .r5h .b5h | Shutter shading correction image
DATA QUALITY FLAG VALUES
The DQF pixel values provide an estimate of the "quality" of each pixel in the image with which the DQF is associated. The following DQF pixel values are currently recognized by calwp2:
- GOODPIXEL (0)
- Unflagged pixel. Only pixels with this data quality value are included in the determination of the bias level and the statistics reported in the header of the output (.C0H) image.
- SOFTERROR (1)
- Reed-Solomon error in data transmission. This pixel is part of a packet of data in which one or more pixels may have been corrupted during data transmission.
- CALIBDEFECT (2)
- Defect in a calibration file used in processing the image. If a pixel is flagged bad in any of the calibration files applied to the image (e.g., dark, flat), this flag is set for that pixel.
- STATICDEFECT (4)
- Recognized long term defect in the WFPC2 camera. A DQF of static defects is maintained in the STScI CDBS database and includes such defects as blocked columns and dead pixels.
- ATODSAT (8)
- The AtoD converter was saturated. The actual signal in the pixel is unrecoverable but known to be greater than or equal to the AtoD full scale value.
- DATALOST (16)
- Missing data. The pixel was lost during readout or data transmission.
- BADPIXEL (32)
- Generic "bad" pixel. This value flags a bad pixel which does not fall into one of the above classifications.
- OVERLAP (64)
- This value flags pixels that lie outside the region that is corrected for the spherical aberration of the OTA.
- inname [file name]
- The input rootname of the WFPC2 dataset. The input filename consist of this rootname and the extension described below.
- (outname) [file name]
- The output rootname of the WFPC2 dataset. The output filename consist of this rootname and the extension described below. If no output name is given, it defaults to the input rootname.
1. Calibrate the WFPC2 observation in the data set with the rootname of "u00ug201t", and use the same rootname for the output files. In this case, the engineering data (.X0H) file is in the same directory as the raw data (.D0H) file: home$wfdata.
cl> set wcal=home$wfdata/ cl> calwp2 u00ug201t u00ug201t
Current version (188.8.131.52): 6 Dec 1994, by J.-C. Hsu