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.
Association 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.
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.
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.
