5.3 AstroDrizzle in the Pipeline

In the pipeline, data undergoes standard calibration to produce flat-field calibrated files (flc.fits/flt.fits). 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 drc.fits/drz.fits. When an association file is provided as input to AstroDrizzle, all flc.fits/flt.fits images belonging to that association are drizzle-combined to create a product with the same rootname (with suffix drc.fits/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.
  
  • The weight extension contains an image that is is a relative weight map of the output pixels, and is essentially an effective exposure time map. 
  • The context extension contains an image that is a map of the output images, and a record of which images contributed to each pixel.
• drc.fits: These drizzled multi-extension FITS products are generated from ACS/WFC or WFC3/UVIS 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 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 MDRIZTAB. 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 tuned to the specific scientific needs by 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, 4, 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.

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).