|Space Telescope Science Institute|
|WFC3 Data Handbook v. 3.0|
Science data taken in orbit by WFC3 are received from the Space Telescope Data Capture Facility and sent to the STScI OPUS pipeline, where the data are unpacked, keyword values extracted from the telemetry stream, and the science data reformatted and repackaged into raw (uncalibrated) FITS files by the Generic Conversion process (see Section 1.1.1 of the Introduction to the HST Data Handbooks). All WFC3 science data products are two-dimensional images stored in Multi-Extension FITS format files. For each exposure taken with WFC3, there is one FITS file with a unique 9-character rootname followed by a 3-character suffix: rootname_xxx.fits. The rootname identifies the observation and the suffix denotes what type of file it is (see Chapter 5 of the Introduction to the HST Data Handbooks for more details on HST file names).2.1.1 Data Files and SuffixesThe suffixes used for WFC3 raw and calibrated data products are described in Table 2.1 and closely mimic those used by ACS and NICMOS.Table 2.1: WFC3 data file suffixes.
The initial input files to the calibration program calwf3 are the raw files (raw) from Generic Conversion and the association (asn) table, if applicable, for the complete observation set.For UVIS images, a temporary file, with the suffix "_blv_tmp", is created once bias levels are subtracted and the overscan regions are trimmed. This file will be renamed with the "_flt" suffix after the standard calibrations (flat fielding, dark subtraction, etc.) are complete. The "_blv_tmp" files serve as input for cosmic ray rejection, if required. For UVIS CR-SPLIT and REPEAT-OBS exposures, a temporary CR-combined image (crj_tmp) is created and then renamed with the "_crj" suffix once basic calibrations of that image are complete.An intermediate MultiAccum (ima) file is the result after all calibrations are applied (dark subtraction, linearity correction, flat fielding, etc.) to all of the individual readouts of the IR exposure. A final step in calwf3 processing of IR exposures produces a combined image from the individual readouts, which is stored in an flt output product file.AstroDrizzle is used to correct all WFC3 images for geometric distortion, whether they are taken as single exposures or as part of an association. For CR-SPLIT and REPEAT-OBS, AstroDrizzle supersedes the calwf3 cosmic-ray rejection processing and uses the individual flt files directly as input, performing cosmic-ray rejection in the process of producing the final drizzled image from multiple exposures (see Table 2.2). This has significant advantages in cases where small numbers of CR-SPLIT images were obtained at a small number of different dither positions, because AstroDrizzle will use all the information from all the flt files to produce the best cosmic-ray rejection. The resulting drizzled images should generally be useful for science, although subsequent reprocessing off-line may be desirable in some cases to optimize the data for specific scientific applications.
For multiple exposures, AstroDrizzle supersedes the calwf3 cosmic-ray rejection processing. It uses the flt files produced by calwf3 as input, and performs cosmic-ray rejection in the process of producing the final distortion-corrected drizzled image.
For further information on drizzle please refer to the DrizzlePac Handbook for http://documents.stsci.edu/hst/HST_overview/documents/DrizzlePac/DrizzlePac.cover.html.2.1.2 Auxiliary Data FilesAssociation tables are useful for keeping track of the complex set of relationships that can exist between exposures taken with both WFC3 channels, especially with REPEAT-OBS, CR-SPLIT, and dithered exposures. Images taken at a given dither position may be additionally CR-SPLIT into multiple exposures (e.g., UVIS observations). In these cases, associations are built to describe how each exposure relates to the desired final product. As a result, WFC3 association tables can be used to create one or more science products from the input exposures, just like ACS associations. The relationships defined in the association tables determine how far through the calibration pipeline the exposures are processed and when the calibrated exposures get combined into sub-products for further calibration.The format of WFC3 association tables closely resembles the ACS and NICMOS association format, with three primary columns: MEMNAME, MEMTYPE, and MEMPRSNT. The column MEMNAME gives the name of each exposure making up the association and output product name(s). The column MEMTYPE specifies the role that the file has in the association. WFC3 uses the same set of MEMTYPES as ACS to provide support for multiple products. These MEMTYPES are summarized in Table 2.3.Table 2.3: Exposure types in WFC3 associations.The suffix "n" is appended to the MEMTYPE to denote multiple sets are present within a single association.
A sample association table for a two-position dithered observation with CR-SPLIT=2 is presented in Table 2.4 This example shows how both MEMNAME and MEMTYPE are used to associate input and output products. The MEMTYPE for each component of the first CR-SPLIT exposure, IxxxxxECQ and IxxxxxEGQ, are given the type EXP-CR1. The sub-product Ixxxxx011 is designated in the table with a MEMTYPE of PROD-CR1. The last digit of the product filename corresponds to the output product number in the MEMTYPE. A designation of zero for the last digit in the filename is reserved for the dither-combined product.The column MEMPRSNT indicates whether a given file already exists. For example, if cosmic ray rejection has not yet been performed by calwf3, the PROD-CRn files will have a MEMPRSNT value of "no". The sample association table in Table 2.4 shows the values of MEMPRSNT prior to calwf3 processing.Table 2.4: Sample association table ixxxxx010_asn.
Each task used by calwf3 creates messages during processing that describe the progress of the calibration and are sent to STDOUT. In calibration pipelines written for other HST instruments, trailer files were created by simply redirecting the STDOUT to a file. Because multiple output files can be produced in a single run of calwf3, creating trailer files presents a unique challenge. Each task within calwf3 must decide which trailer file should be appended with comments and automatically open, populate, and close each trailer file.calwf3 will always overwrite information in trailer files from previous runs of calwf3 while preserving any comments generated by Generic Conversion. This ensures that the trailer files accurately reflect the most recent processing performed. The string "CALWF3BEG" will mark the first comment added to the trailer file. If a trailer file already exists, calwf3 will search for this string to determine where to append processing comments. If it is not found, the string will be written at the end of the file and all comments will follow. Thus any comments from previous processing are overwritten and only the most current calibrations are recorded.As each image is processed, an accompanying trailer file with the "*_trl.fits" suffix will be created. Further processing with calwf3 will concatenate all trailer files associated with an output product into a single file. Additional messages will then be appended to this concatenated file. This duplicates some information across multiple trailer files but ensures that for any product processed within the pipeline, the trailer file will contain processing comments from all the input files.Linking trailer files together can result in multiple occurrences of the "CALWF3BEG" string. Only the first, however, determines where calwf3 will begin overwriting comments if an observation is reprocessed.The support files contain information about the observation and engineering data from the instrument and spacecraft that was recorded at the time of the observation. A support file can have multiple FITS image extensions within the same file. Each extension holds an integer (16-bit) image containing the data that populates the *_spt.fits header keyword values.