Space Telescope Science Institute
DrizzlePac 2012 Handbook
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Table of Contents

The DrizzlePac Handbook
Chapter 1: Introduction to AstroDrizzle and DrizzlePac
1.1 What are AstroDrizzle and DrizzlePac?
1.2 What's new in AstroDrizzle?
1.2.1 Code Improvements
1.2.2 A Fundamentally Different Approach to Handling Image Distortions and Astrometry
1.2.3 Non-polynomial Distortions in ACS Data
1.2.4 Astrometric Operations Made Easier with the DrizzlePac Package
1.3 What’s Different about Data from the Archive
Chapter 2: Description of the Drizzle Algorithm
2.1 Image Reconstruction and Restoration Techniques
2.1.1 Interlacing
2.1.2 Shift-and-Add
2.1.3 Drizzle
2.2 Drizzle Concept
2.3 Weight Maps and Correlated Noise
2.3.1 Weight Maps
2.3.2 Weight Maps and Correlated Noise
The Calculation
2.4 Characteristics of Drizzled Data
2.4.1 Characteristics of Drizzled Data: Sampling
2.4.2 Characteristics of Drizzled Data: Photometry
AstroDrizzle Aperture Photometry Accuracy
2.4.3 Characteristics of Drizzled Data: Astrometry
Chapter 3: Astrometric Information in the Header
3.1 Introduction
3.2 How Distortions are Represented in AstroDrizzle
3.2.1 Design of the AstroDrizzle Distortion Representation
3.2.2 World Coordinate System
Correcting for Velocity Aberration
Updating the WCS Header Information
3.2.3 The Simple Image Polynomial (SIP) Convention
3.2.4 Optional Non-Polynomial Distortions
The D2IMFILE Reference File
3.3 Distortion Information in Pipeline Calibrated Images
3.3.1 New Image Structure
3.3.2 New Keywords
Naming a Polynomial Distortion Solution
SIP Convention Keywords
Paper IV Proposal
Detector to Image Correction
3.3.3 Final Products
3.4 The Headerlet
3.4.1 Introduction
Why Headerlets are Useful
Types of Information Contained in Headerlets
3.4.2 WCS Information in Archival Images
3.4.3 Storing Multiple WCSs in an image
WCSCORR Image Extension for Storing Multiple WCSs
Syntax for Alternate WCSs
3.4.4 Headerlet Structure
Headerlet Format
Headerlet’s PRIMARY Header
SIPWCS: A New WCS FITS Extension
3.4.5 Working with Headerlets
Creating a Headerlet
Applying a Headerlet
3.4.6 Python Syntax of the Headerlet Tasks
Software Requirements
Getting Headerlet Function Names
Creating a Headerlet Object
Applying a Headerlet Object
Accessing the Headerlet Extension using PyFITS
Other Headerlet Extension Functions
Chapter 4: DrizzlePac Package
4.1 DrizzlePac: An Overview
Software Requirements
4.2 AstroDrizzle: The New Drizzle Workhorse
Note Regarding WFC3/IR Images
Note Regarding WFC3/IR Images
Detailed Description of the AstroDrizzle Steps
4.2.1 Set-up and Initialize astrodrizzle Parameters
Parameter Details
4.2.2 Create a Static Mask Containing Permanent Bad Pixels
How the Static Mask is Used in Subsequent astrodrizzle Steps
Parameter Details
4.2.3 Perform Sky Subtraction
Parameter Details
4.2.4 Create Separately Drizzled and Registered Images
Parameter Details
4.2.5 Create a Median Image
Parameter Details
4.2.6 Blot Median Image
Parameter Details
4.2.7 Create a Cosmic Ray Mask for Each Image
4.2.8 Create a Final Distortion-Free Combined Image
Parameter Details
4.2.9 Overriding Instrument-Specific Parameters
Parameter Details
4.2.10 astrodrizzle Syntax in PyRAF and Python
4.2.11 astrodrizzle Parameters
4.2.12 A Note about Photometry and Weights in AstroDrizzle
4.2.13 AstroDrizzle Memory Usage
4.3 AstroDrizzle in the Pipeline
MDRIZTAB Reference File
4.4 The DrizzlePac Package
4.4.1 DrizzlePac tasks
4.4.2 Aligning Images with TweakReg and ImageFindPars
Overview of the TweakReg Software
tweakreg Task Syntax in PyRAF and Python
tweakreg Parameter Details
imagefindpars Parameter Details
Format of Exclusions Catalog
Format of Regions Files
A Note About “Difficult” Images
4.4.3 TweakBack
When to Use TweakBack
TweakBack Algorithm
tweakback Usage
4.4.4 Updating Images from the Pre-AstroDrizzle Archive
updatewcs Requirements
Using updatewcs
Using updatenpol for ACS images
Making WFPC2 Images Compatible with AstroDrizzle
4.4.5 Handling WCS Information With stwcs
More About updatewcs
More About Running updatenpol for ACS Images
4.4.6 blendheaders
Running blendheaders
Drizzled Image Header Summary Table
blendheaders Rules
Chapter 5: Introduction to the drizzlepac Interface
5.1 Introduction to TEAL and Python Interfaces
5.2 Starting drizzlepac in PyRAF and Python
5.3 Using the TEAL GUI
5.4 Using the PyRAF/Python Command-line
5.5 Configuration Files (cfg)
Chapter 6: Reprocessing with the DrizzlePac Package
6.1 Beyond the Standard Calibration Pipeline
6.2 Image Alignment
6.2.1 Alignment Error Sources
6.2.2 Processing Large Images
6.2.3 Using TweakReg for Image Alignment
Processing Steps Overview
Catalog Matching
6.2.4 Aligning Subsampled Images
6.3 Running AstroDrizzle
6.3.1 Sky Subtraction Considerations
6.3.2 Cosmic Ray Rejection
Image Alignment Requirement
6.3.3 Selecting the Optimal Scale and Pixfrac
6.3.4 Controlling the Bit Mask
Data Quality Flags
Drizzled Masks
Chapter 7: Examples: Using DrizzlePac for Combining Images
7.1 WFC3/UVIS: Optimizing Image Alignment for Multiple Visits
Summary of Steps
7.1.1 Description of the Data
7.1.2 Aligning Images with tweakreg
Logistics Parameters in tweakreg
Source Detection Parameters in imagefindpars
Source Matching Parameters in tweakreg
7.1.3 Combining the Images using astrodrizzle
Creating the Static Mask
Removing the Sky
Separately Drizzling Each Input Image onto a Common Output Frame
Creating the Median image
Computing Cosmic Ray Masks
Final Drizzle Combination
7.2 WFC3/IR: Optimizing Image Sampling for a Single Visit
Summary of Steps
7.2.1 Description of the Data
7.2.2 Recommendations on Selecting Optimal “Scale” and “Pixfrac” Parameter Values
7.2.3 Image Combination with AstroDrizzle
7.3 ACS/WFC: Optimizing the Image Alignment for Multiple Visits
Example Overview
7.3.1 Description of the Data
7.3.2 Aligning the Images Using tweakreg and imagefindpars
7.4 ACS/WFC: Optimizing the Image Sampling for a Single Visit
Summary of Steps
7.4.1 Description of the Data
7.4.2 Combining the Images using astrodrizzle
7.5 WFC3/UVIS: Aligning Images by Defining an Output Reference Frame
Summary of Steps
7.5.1 Description of the Data
7.5.2 Align Images for Each Filter Set Using tweakreg
7.5.3 Overplot Matched Sources Onto the Original flt.fits Image
7.5.4 Align the Header WCS of the Two Filter Images
7.5.5 Propagate Improved Solution to Original flt.fits Images with tweakback
7.5.6 Drizzle the Images to a Common Reference Frame
Chapter 8: Data Quality Checks and Troubleshooting Problems
8.1 Inspecting the Drizzled Products from MAST
Should the images be reprocessed? Are the pipeline drizzled products adequate for the science goals?
Should the images be reprocessed? Are the pipeline drizzled products adequate for the science goals?
8.1.1 Examine the Drizzled Science Image
Do the drizzled products look “clean?”
Are there any irregularities (or discontinuities) in the sky background?
Are the PSFs “round” and “narrow,” as expected?
Are there unusual patterns or clusters of bright pixels repeated across the image?
Were the observations dithered?
Were observations obtained in multiple visits?
Why is there a Moiré pattern in the sky background?
8.1.2 Verify the Image Header Sky Keyword
Does the MDRIZSKY header keyword seem correct when compared to an independent estimate using other software tools?
8.1.3 Examine the Data Quality Array
Did the pipeline flag excessive numbers of pixels as 4096 (cosmic ray DQ flag)?
8.1.4 Examine the Drizzled Weight Image
Is the mean value of the weight image approximately equal to the total exposure time?
Does there appear to be an imprint of the target in the weight image?
8.2 Verifying tweakreg Solutions
8.2.1 Examine the Fit Residuals
Does the solution make sense?
Is the fit RMS small?
8.2.2 Examine the Astrometric Residuals
Is there any remaining slope in residuals plot?
8.2.3 Examine the Vector Plot
Are there any obvious “flow” patterns in the vectors?
Are sources detected over the entire field of view (for sparse fields or star clusters)ハ If not, are they detected in the regions expected (for example, a small galaxy which does not fill the image)?
Are there “clumps” of sources detected near very bright stars?
8.2.4 Examine the 2-Dimensional Histogram
Is the fit crosshair centered on the peak?
8.2.5 Verify the Number of Matches Used to Compute the Solution
Does tweakreg crash with the message “not enough matches found?”
8.3 Inspecting Drizzled Products after Reprocessing
Are the reprocessing task parameters optimal?
Are the reprocessing task parameters optimal?
8.3.1 Examine the Drizzled Science Image
8.3.2 Examine the Drizzled Weight Image
Are there “holes” in the final weight image?
Appendix A: Plate Scales and Polynomial Distortions
A.1 Introduction
A.2 Detector Plate Scales
A.3 Effects of Velocity Aberration
A.4 Velocity Aberration for ACS
A.5 Detector Distortion Models
A.5.1 ACS
Time-dependent Distortion for ACS
A.5.2 WFC3
Appendix B: HST Pointing Accuracy and Stability
B.1 HST pointing stability
B.2 HST Tracking Stability at a Single Location
B.3 Precision of Commanded Offsets
B.3.1 Pointing Repeatability After Guide Star Re-acquisition
B.3.2 Roll Angle Repeatability Over Multiple Visits
Appendix C: Observational Dithering Options for Drizzling Data
C.1 Dithering Strategies
C.1.1 What is Dithering?
C.1.2 Benefits of Dithering
C.1.3 Costs and Drawbacks of Dithering
C.2 Selecting the Right Dither Strategy
C.2.1 Dealing with Cosmic Rays, Hot Pixels, Undersampling, and Photometric Accuracy
C.2.2 A Top-level View of Dithering Strategies
C.2.3 Selecting the Number of Dither Pointings and Step Sizes
Integer-Spaced Dither Steps
Subpixel Dithering
C.2.4 Data with Inaccurate Offsets in Position or Roll Angle
C.2.5 How Many Images to Obtain at Each Dither Location
C.2.6 Specific Instrument-related Issues
Image Mode Dithering
Spectroscopic Mode Dithering
The Effect of WFPC2 Geometric Distortion on Dither Offsets
The Exact Relationship Between POS TARGs and WFPC2 CCD Rows and Columns
Appendix D: Previous Drizzle Software Packages
D.1 Introduction
D.2 First Generation Drizzle IRAF Tasks
D.3 Second Generation Dither tasks
D.4 WCS-enabled Dither Tasks
D.5 WCS-enabled Drizzle
D.6 Coordinate Transformation Tasks
D.7 Supplemental tasks
Papers and Instrument Science Reports
Instrument Pages
Data Analysis
Observing Campaigns

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