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.
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Improved distortion solutions for ACS
More...
Resetting DQ bits
More...
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