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MultiDrizzle Overview

MultiDrizzle automates and simplifies the detection of cosmic-rays and the combination of dithered observations using the Python scripting language and PyRAF, the Python-based interface to IRAF. It supports combining images and correcting for distortion in all active HST imaging instruments: WFC3, ACS, STIS, NICMOS, and the recently retired WFPC2.  MultiDrizzle was developed by the Science Software Branch at the Space Telescope Science Institute.

The MultiDrizzle software (Koekemoer et al. 2002, HST Calibration Workshop, p. 337) has been incorporated into the HST Archive Pipeline. This means that when ACS associations are requested from the Archive, the drizzled image product will be delivered ("*_drz.fits") which consists of a cosmic-ray cleaned, combined image using the "drizzle" software (Fruchter, A. S. & Hook, R. N., 2002, PASP 114, 144). This image product will be created for all ACS associations, which are defined as groups of exposures within a single visit, obtained using the recommended dither patterns or CR-SPLITs as specified in the Phase II observing instructions.

An example of the cleaned MultiDrizzle product from the ACS Archive Pipeline, for a dithered 4-exposure observation of the nearby galaxy NGC 4594. The top panels show the final full-frame drizzled and weight images (left and right, respectively). The bottom left panel shows a close-up of the output image from MultiDrizzle, while the bottom right panel shows the sum of all the accumulated cosmic rays originally present in the exposures.

In addition, all WFPC2 images will be distortion corrected using MultiDrizzle on each image separately. This will also produce a drizzled image product ("*_drz.fits") that only removes distortion and mosaics all the chips into a single field-of-view.  No cosmic-ray rejection will be done on WFPC2 data as it requires the combination of multiple exposures, a step not taken in the pipeline.

The distortion model applied to ACS and WFPC2 data in the latest version of MultiDrizzle (Version 3.2.1 and later) corrects for:
  • time-dependent distortion (ACS)
  • time-dependent chip-to-chip offsets (WFCP2)
  • fixed-column corrections (WFPC2 and ACS)
  • velocity aberration (ACS)
  • non-polynomial residuals(ACS)
This full distortion correction solution has been calibrated by the ACS team and documented in the ISR ACS 2007-08. Initial testing indicates alignment between images taken in different visits with an RMS of about 0.05 pixels after applying this full solution to ACS/WFC data.  More details on how to turn on this time-dependent correction can be found under Instrument-specific behavior.

MultiDrizzle news
Improved distortion solutions for ACS

Resetting DQ bits


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