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 are extracted from the telemetry stream, and the science data reformatted and repackaged into raw (uncalibrated) FITS files by the Generic Conversion process (see
of the Introduction to the HST Data Handbooks
All WFC3 science data products are two-dimensional images that are 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
of the Introduction to the HST Data Handbooks
for more details on HST
is a single image file, the atomic unit of HST
is a collection of files having a common rootname.
is a dataset created by combining a subset of the exposures in an association.
is a dataset created by combining sub-products of an association.
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
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.
Processing of IR exposures results in an intermediate MultiAccum (ima
) file, which is a file that has had all calibrations 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.
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
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 MultiDrizzle
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.
Table 2.2: The calwf3
input and output products.
Association tables are useful for keeping track of the complex set of relationships
that can exist between exposures taken with WFC3, especially with REPEAT-OBS
and dithered exposures, for both the UVIS and IR channels. 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,
. 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
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
are used to associate input and output products. The MEMTYPE
for each component of the first CR-SPLIT
, are given the type EXP-CR1
. The sub-product Ixxxxx011
is designated in the table with a MEMTYPE
. 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
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.
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
” 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
” 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.