Chapter 4:
Distortion Correction and Drizzling of ACS Images
4.1 ACS Geometric Distortions
4.1.1 Distortion Model
4.1.2 On-orbit Calibration Programs
4.1.3 Results
4.1.4 Distortion Reference Files (IDCTAB & DGEOFILE)
4.2 Combination of Dithered Data
4.2.1 Introduction to Dithering
4.2.2 Introduction to Drizzle
4.2.3 Overview of Drizzle-Related Software Tools
4.2.4 Association Table Usage
4.3 PyDrizzle
4.4 MultiDrizzle
4.4.1 Steps Performed by MultiDrizzle
4.4.2 Multidrizzle in the ACS Pipeline
4.4.3 Data Quality Flags for Bad Pixels: The 'Bits' Parameter
4.4.4 MultiDrizzle Data Products
4.5 Post-Pipeline Reprocessing
4.5.1 Example 1: Running PyDrizzle on a Single Exposure
4.5.2 Example 2: Running PyDrizzle on an Association file
4.5.3 Example 3: Running MultiDrizzle with Different Output Image Options
4.5.4 Example 4: Re-Running MultiDrizzle to Improve Sky Subtraction
4.6 Refining the Shifts: Manually and with Tweakshifts
4.6.1 Delta Shifts
4.6.2 Absolute Shifts
4.6.3 Constructing the 'shiftfile'
4.6.4 Example 5: Improving the Image Registration for Different Visits
4.6.5 Tweakshifts
4.7 MultiDrizzle Details: Example Application
to the Tadpole Galaxy
4.8 Creating Custom Association Tables
This chapter describes the geometric distortion of the ACS and presents the available tools and methods for removing the distortion and combining dithered datasets into clean, undistorted, and photometrically faithful output products.
The first section describes how the distortion has been quantified and how this information is held in reference files which are used by the calibration pipeline. Plots of the distortion field are given for the three ACS detectors.
The remaining sections give an overview of the drizzle method and discuss the higher level PyDrizzle and MultiDrizzle software tools available for distortion correction and image combination.
