Space Telescope Science Institute
DrizzlePac 2012 Handbook
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The DrizzlePac Handbook > Chapter 4: DrizzlePac Package > 4.3 AstroDrizzle in the Pipeline

Only ACS and WFC3 data are currently being processed using On-the-Fly Calibration (OTFR) with AstroDrizzle in the pipeline. Active imaging instruments that are AstroDrizzle-compatible are WFC3, ACS, and STIS. Drizzled NICMOS and WFPC2 data are stored in a Static Archive as MultiDrizzle products along with their flat-field calibrated files (flt.fits and c0m.fits, respectively). However, they are also compatible with AstroDrizzle for anyone who wishes to reprocess those images.
In the pipeline, data undergoes standard calibration (to produce flat-field calibrated files (flt.fits, and flc.fits for CTE-corrected ACS data). AstroDrizzle processing starts with the name of a single exposure, or the name of an association file containing the names of several exposures to be combined. A single exposure will generate an output file with the same rootname but with the suffix drz.fits. When an association file is provided as input to AstroDrizzle, all flt.fits images belonging to that association are drizzle-combined to create a product with the same rootname (with suffix drz.fits) as the association file. The AstroDrizzle products from the Archive, listed below with their respective file suffix, are:
drz.fits: These are the default drizzle multi-extension FITS products, created using flt.fits input files. This type of drizzled image has extension types SCI (for the science image extension), WHT (the weight image extension), and CTX (context image extension) in the final combined image
drc.fits: These drizzled multi-extension FITS products are generated from ACS CTE-corrected calibrated data with the suffix flc.fits.
.log: AstroDrizzle generates an ASCII file containing a log of all messages reported during processing; this file gets used as the basis for the final products trailer file during pipeline processing.
As part of pipeline processing, distortion information are integrated into the calibrated data (flt.fits and flc.fits for CTE-corrected ACS/WFC) from a set of reference files, and WCS information is then recomputed using this distortion information.
AstroDrizzle parameter settings for pipeline use are defined for different observing configurations in a reference table named by the header keyword MDRIZTAB1. Drizzle-combined data from the pipeline should generally be regarded as quick-look products, and users are strongly encouraged to reprocess their images using AstroDrizzle to see if the quality of data can be improved using different parameter values. Information on how to determine the quality of Archive drizzled products is available in Chapter 8.
How does AstroDrizzle know which images to combine in the pipeline? The association file contains images that were taken as a dither pattern, using the POS   TARG special requirement, or repeated exposures in the form of sub-exposures or CR-SPLIT observations. These observations types, taken in the same visit with the same guide star pairs, almost always have highly accurate offset values in the image header that could be used to align the images. (Exceptions, of course, could be due to a loss of lock on a guide star midway through the visit or other telescope pointing anomalies. So it is always useful to check the image quality for signs of anomalies.)
MDRIZTAB Reference File
The MDRIZTAB reference table contains AstroDrizzle task parameters optimized for a wide range of observations. AstroDrizzle uses this table to match the best parameter values with the type of observations being processed. Each instrument detector has its own MDRIZTAB reference table; in it, each row has AstroDrizzle task parameter settings optimized for the filters used and three ranges of input image numbers per association (one image, 2 to 6 images, and more than 6 images).
To understand the processing that takes place in the pipeline, it can be helpful to inspect the MDRIZTAB reference file; it can be identified by the image header keyword MDRIZTAB. This reference file specifies the values for most AstroDrizzle task parameters, and uses software default values for the rest of the parameters.
Table 4.16 shows an example of the MDRIZTAB contents for four types of observations. But first, outlined below are two summary lists of astrodrizzle parameters; the first list shows parameters with values that are the same for all observations processed in the pipeline, and therefore, those parameters do not need to be in the MDRIZTAB file. The second list shows parameter values that may have different values based on the type of observations.
Table 4.14: astrodrizzle Parameters With the Same Values During Pipeline Processing for All Observations
Set to True so astrodrizzle in the pipeline always uses the MDRIZTAB reference file for populating its parameters
Set to 10; distortion corrections are applied directly to every 10th pixel, and those in-between are interpolated. Tests have found that the interpolation values are highly accurate for a step size of 10.
Set to True to always update the headers with the latest distortion information
Set to 4096 so that all pixels flagged as cosmic rays by calibration software that was run before AstroDrizzle processing are treated as good pixels.
Set to False. This parameter makes backups of the input filesit is useful for reprocessing but not needed in the pipeline.
Table 4.15: astrodrizzle Parameters that Appear in the MDRIZTAB Table
driz_separate, driz_sep_outnx, driz_sep_outny, driz_sep_kernel, driz_sep_scale, driz_sep_pixfrac, driz_sep_rot, driz_sep_fillval, driz_sep_bits
median, median_newmasks, combine_type, combine_nsigma, combine_nlow, combine_nhigh, combine_lthresh, combine_hthresh, combine_grow
driz_combine, final_outnx, final_outny, final_kernel, final_scale, final_pixfrac, final_rot, final_fillval, final_bits
These parameter values depend on two primary conditions: the number of input exposures to be combined and the filters used for the exposures.
Many parameters use the task default values. Some parameter values are determined by specific image characteristics. For instance, sky subtraction is turned off for narrow-band and ramp filters because the image sky background is usually low. Associations with more than six images are drizzled to a finer scale and “pixfrac” to increase image resolution (more about that in Chapter 6).
For ACS data, the MDRIZTAB reference file specifies "final_bits=96" so that astrodrizzle in the pipeline treats any pixel flagged with bits 32 and 64 in the data quality array of the flat-field calibrated (flt.fits) files as good pixels. These flags are set by calacs and correspond to CTE tails of hot pixels in superdark DQ arrays and warm pixels in superdark DQ arrays.
Table 4.16 shows MDRIZTAB reference file contents for four different types of WFC observations. Four rows corresponding to those observation types were extracted from the MDRIZTAB table, and transposed for easier viewing. Here, the spectral element ANY refers to all wide-band filters.
Wide band, one image
Wide band, two images
Wide band, six image
F658N + CLEAR2L, two images



Default values in the astrodrizzle task serve as a starting point for reprocessing data; they’re not the same as those in the MDRIZTAB reference file.

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