4.5 MultiDrizzle
MultiDrizzle, developed in its original form by A. Koekemoer (Koekemoer et al. 2002, HST Calibration Workshop 2002, p.381), is a general purpose tool for drizzle combination of images. It provides a single-step interface to the complex suite of tasks in the STSDAS dither package. These tasks include, in order: initial image registration, creation of a cleaned median image, transformation back to the input image plane, creation of cosmic ray masks, and final drizzling. (For a general introduction, refer to the HST Dither Handbook.) The goal of MultiDrizzle is to provide a high-level task which has an extensive suite of user-adjustable parameters which, if left at their default values, will allow the task to perform all steps in a single operation with no user intervention. At the same time, the parameters allow the user a large amount of flexibility in controlling the relevant aspects of these steps, in case the default parameters are not sufficient for specific scientific applications.
The current version of MultiDrizzle uses the image header world coordinate system (WCS) to deduce the image-to-image offsets. However, user-supplied offsets may also be applied in a flexible manner. In the future, tasks to ease the determination of such shifts will be made available. MultiDrizzle processing with default parameters may eventually be part of pipeline processing, if a detailed assessment shows that this is feasible.
MultiDrizzle and PyDrizzle are both Python scripts which can only be run within the PyRAF environment. MultiDrizzle calls PyDrizzle to manage the images, to compute the drizzle parameters, and to control the operation of both the drizzle and blot tasks. While the latest release of STSDAS includes PyDrizzle, MultiDrizzle must be obtained separately (see below). It is recommended that the complete current STSDAS package is installed to avoid potential missing components or mismatched versions. STSDAS and PyRAF are available from:
http://www.stsci.edu/resources/software_hardware.
Because many of these components are evolving rapidly, subsequent updates will be available from the same site.
While the software has been well tested and is fairly stable, it is still periodically being improved. We therefore refer readers to the MultiDrizzle web site for the most up-to-date software:
http://stsdas.stsci.edu/pydrizzle/multidrizzle
and to the ACS Drizzling web site for additional documentation:
http://www.stsci.edu/hst/acs/analysis/drizzle.
The following sections describe in detail the tasks performed by MultiDrizzle and give an example of its use. A help document describing the numerous MultiDrizzle parameters can be accessed by typing 'help multidrizzle' from within PyRAF. HST Dither Handbook is currently under revision to include documentation for MultiDrizzle and several detailed ACS drizzling examples. Version 3.0 is expected to be released by mid-2004.
4.5.1 Reprocessing with MultiDrizzle
In Section 4.4, we described several occasions when off-line interactive processing with PyDrizzle is required, for example, when images are taken across multiple visits, when fine-tuned image registration is required, or when the user wishes to specify which DQ flags should be considered good. MultiDrizzle will address these same issues, while at the same time providing additional capabilities such as cosmic ray rejection and sky subtraction. Because it allows access to an extensive suite of user-adjustable parameters, MultiDrizzle gives users more flexibility in selecting the parameters that best suit their observations. This is all made possible within a single, easy-to-use interface.
MultiDrizzle creates many large files and does a considerable amount of processing. It is hence recommended that a modern fast machine with several GBytes of free space be used if possible. In the example to follow, each of the input FLT files is ~168MB and the final combined output product is 310MB. Many large intermediate files are also created but may be cleaned up if required.
The only calibration reference file required by MultiDrizzle is the distortion table (IDCTAB) discussed earlier in this chapter. This table describes the ACS distortion in terms of polynomial coefficients. The appropriate reference file is recorded in the image header keyword IDCTAB. This file should be obtained from the STScI archive (via Starview) and placed in some local directory. Before starting MultiDrizzle the environment variable "jref" must be set appropriately to point to this directory (see Example 1 in Section 4.4.4 for more information). Once the FLT files have been collected and the 'jref' environment established, PyRAF should be started and the stsdas, analysis, and dither packages loaded.
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The IDCTAB reference file in the image header must NOT be modified prior to running MultiDrizzle or PyDrizzle. To make use of an updated distortion solution, users are advised to re-retrieve their data from the archive such that the CD-matrix keywords in the image header are correctly populated.
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To avoid potential problems, it is recommended that the parameters for both MultiDrizzle and PyDrizzle be reset to the default values:
pyraf> unlearn pydrizzle
pyraf> unlearn multidrizzle
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4.5.2 Tasks Performed by MultiDrizzle
MultiDrizzle starts with the flat fielded, but still distorted, FLT files created by CALACS in the pipeline. While these are the only mandatory input, there are many additional optional inputs including lists of improved registration information. In default mode MultiDrizzle performs the following steps in order, although the user has full control via the parameter file, of which steps are to be performed or omitted. Each step is described in detail in the example which follows in Section 4.6.
- StaticMask: Create a mask of bad pixels, using information from the images themselves and associated data quality files,
- SkySub: Carry out sky subtraction on each individual input image,
- Driz_Separate: Calculate relative image shifts based on header information in the world coordinate system keywords (optionally supplemented by user-supplied shift information) and drizzle the input images onto a series of separate output images,
- Median: Combine these images to create a clean median image,
- Blot: Transform, or blot, the median image back to the frame of the input images,
- Driz_cr: Create a cosmic ray mask for each input image based on comparison between the blotted median and its derivative and the original input image,
- Driz_Combine: Perform the final drizzle combination step, applying the cosmic ray masks.
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A warning to users: MultiDrizzle modifies the original calibrated images, for example, when updating the image DQ array with the static pixel mask and when performing the sky subtraction. When reprocessing, we advise retaining a copy of the original files in a separate directory.
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4.5.3 Specifying Shifts Between Images
By default, MultiDrizzle uses the world coordinate system (WCS) information in the image headers to align the images. While dithers applied to a target within a single visit of HST are usually accurately reflected by the WCS, this is not the case for multiple visits which normally require guide star re-acquisitions and may utilize different guide stars. Even for nominally back-to-back exposures that are part of a CR-SPLIT association, offsets large enough to degrade final combinations do sometimes occur. As a result, it is essential to accurately determine image-to-image shifts, and possibly rotations, before running MultiDrizzle.
These (delta) shifts may be determined by separately drizzling each image onto a common WCS frame. This is performed by the 'driz_separate' task in step 3. Object lists derived for each drizzled image may then be matched and fit to derive a single shift for each image. More details on this topic and a description of the shift file formats are given in Section 4.4.3.
Once the shifts have been determined, MultiDrizzle must be rerun from the beginning with the shift file specified in the 'shiftfile' parameter. MultiDrizzle will update the association table to reflect these 'delta' offsets and will now use both the header WCS information plus the shift file to appropriately align images.
