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DrizzlePac 2012 Handbook
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The DrizzlePac Handbook > Chapter 7: Examples: Using DrizzlePac for Combining Images > Introduction

Introduction
Data and related files for the examples in this chapter are available at the DrizzlePac website. Please check the website latest news and additional resources for using DrizzlePac.
This chapter contains five examples covering a variety of observation types that may likely be encountered by users. These include data analysis goals such as creating combined distortion-free images, optimizing image resolution for subpixel dithered data, improving cosmic ray removal, and aligning images taken using different filters or detectors.
Table 7.1 contains a summary of the examples in this chapter. Users who are unfamiliar with drizzling are encouraged to try the first example (Section 7.1) where each processing step is accompanied by detailed explanations.
A special note to users working with older data
ACS and WFC3 data retrieved before AstroDrizzle was installed in the Archive’s on-the-fly recalibration (OTFR) pipeline have to be updated to insert astrometric information needed to run tasks in the drizzlepac package. Section 4.4 has detailed information on how to update older WFC3, ACS, and WFPC2 images.
Since AstroDrizzle became part of the OTFR pipeline, ACS and WFC3 products now contain significant changes geared towards simplifying the handling of astrometric information. Distortion correction reference files that used to be required for running MultiDrizzle are now used behind-the-scenes in OTFR processing. In the pipeline, AstroDrizzle extracts distortion correction information that is relevant to the images from reference files and inserts them directly into the data file headers.
Distortion corrections that are now incorporated in the new pipeline products can be described as follows:
1.
Linear distortion corrections, including velocity aberration and detector-related distortions, are built into the CD matrix
2.
Coefficients for distortion corrections that can be expressed as higher order polynomials are now stored as values to SIP header keywords
3.
Non-polynomial distortions for ACS images are stored as look-up tables attached to the image file as FITS extensions
Section 4.4 has detailed information on how to update older WFC3, ACS, and WFPC2 images. Users who wish to update their data should retrieve the relevant distortion correction reference files named in the image header keywords in order to manually update their data.
 
Users trying these examples on their home machines may notice small differences in their results compared to the example results archived in the DrizzlePac website. This could be due to different software versions since these examples were prepared in the test
Table 7.1: A Summary of the Examples in this Chapter
WFC3/UVIS: Optimizing Image Alignment for Multiple Visits
Align and combine three WFC3/UVIS images taken at different visits and orientations.
Three full-frame UVIS F606W images of Omega Centauri, each with exposure time of 40 sec., each taken in a different visit with different roll angle and offset.
WFC3/IR: Optimizing Image Sampling for a Single Visit
Optimize image sampling, using final_pixfrac and final_scale settings in astrodrizzle, for WFC3/IR images.
Spiral galaxy NGC 3370 in F160W, observed with a WFC3-IR-DITHERBOX-MIN pattern. All observations in the same visit with same orientation.
ACS/WFC: Optimizing Image Alignment for Multiple Visits
Align and combine three ACS/WFC images taken at different visits and orientations.
Three F606W images of the dense star field NGC 104, each with exposure time 339 sec. The images are CTE-corrected flat-field calibrated files with suffix flc.fits.
ACS/WFC: Optimizing Image Sampling for a Single Visit
Optimize image sampling, using final_pixfrac and final_scale settings in astrodrizzle, for ACS/WFC images.
A two-point dither line with subpixel dithers at each point: four images of galaxy NGC 4449 taken with the F555W filter. The images are CTE-corrected flat-field calibrated files with suffix flc.fits.
WFC3/UVIS: Aligning Images in Different Filters by Defining an Output Reference Frame
Two sets of drizzled WFC3/UVIS images, each for a different filter, are aligned by WCS and in pixel space.
Observations of galaxy M83 in F814W and F438W, each filter set taken in different visits. There are three images per filter.
Data and associated files for all examples in this chapter are available at the
DrizzlePac website.

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