Table of Contents
Acknowledgments ......................................................... ix
Chapter 1: Introduction ............................................. 1
1.1 How to Use The Multidrizzle Handbook .................. 1
1.2 What is New in Multidrizzle? ...................................... 2
Chapter 2: Introduction to Dithering ............. 3
2.1 What is Dithering? ......................................................... 3
2.2 Benefits of Dithering ..................................................... 4
2.3 Costs and Drawbacks of Dithering............................ 5
2.4 Observational Dither Strategies ................................. 5
2.4.1 No Dithering................................................................ 6
2.4.2 Simple Dithering ......................................................... 6
2.4.3 “Full” Dithering ............................................................ 6
2.4.4 Dithering for Parallel Images ...................................... 6
2.5 More Detailed Considerations .................................... 7
2.5.1 Data Quality Issues involved in Dithering ................... 7
2.5.2 How Many Dither Positions - 2, 3, 4 or more? ............ 8
2.5.3 Data with Inaccurate Offsets in Position or Roll
Angle ............................................................................. 12
2.5.4 How many Images to Obtain at each Dither
Location......................................................................... 12
2.5.5 Specific Instrument Related Issues .......................... 13
2.5.5.1 WFPC2............................................................. 13
2.5.5.2 ACS .................................................................. 14
2.5.5.3 NICMOS ........................................................... 15
2.5.5.4 STIS ................................................................. 17
2.5.5.5 WFC3 ............................................................... 19
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Chapter 3: How the Drizzle Algorithm
Works................................................................................. 23
3.1 Theory ............................................................................ 23
3.2 Image Reconstruction and Restoration
Techniques ...................................................................... 25
3.2.1 Interlacing ................................................................. 26
3.2.2 Shift-and-Add............................................................ 26
3.2.3 Drizzle....................................................................... 27
3.3 Weight Maps and Correlated Noise ........................ 27
3.3.1 Correlated Noise Details........................................... 28
3.3.1.1 Overview .......................................................... 28
3.3.1.2 The Calculation ................................................ 30
3.4 Image Fidelity After Drizzling.................................... 31
3.5 Photometric Accuracy After Drizzling ..................... 33
3.6 Astrometric Accuracy After Drizzling ...................... 34
Chapter 4: Astrometric Header
Information ................................................................... 35
4.1 HST Pointing Accuracy and Stability ...................... 35
4.1.1 HST Pointing Accuracy............................................. 35
4.1.2 HST Tracking Stability at a Single Location.............. 36
4.1.3 Precision of Commanded Offsets ............................. 37
4.1.4 Pointing Repeatability After Guide Star
Re-acquisition................................................................ 38
4.1.5 Roll Angle Repeatability Over Multiple Visits............ 38
4.2 Detector Plate scales and Geometric
Distortion .......................................................................... 39
4.2.1 Introduction ............................................................... 39
4.2.2 Summary of Detector Plate Scales........................... 40
4.2.3 Detector Distortion Models ....................................... 42
4.2.4 The IDC table............................................................ 47
4.2.5 SIP Coefficients in the image headers...................... 49
4.3 The CD Matrix .............................................................. 50
4.3.1 Definition of the CD Matrix........................................ 50
4.3.2 Computation of the CD Matrix .................................. 51
4.3.3 Updating the WCS Header Information .................... 51
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Chapter 5: Drizzle Software ................................. 53
5.1 Available Software ...................................................... 53
5.1.1 Original IRAF tasks................................................... 53
5.1.2 Currently supported tasks......................................... 54
5.1.3 Supplemental tasks .................................................. 55
5.2 Drizzle ............................................................................ 55
5.2.1 The IRAF Drizzle Task.............................................. 56
5.2.2 The IRAF Blot Task .................................................. 57
5.2.3 WCS-enabled Dither Tasks ...................................... 59
5.2.3.1 WCS-enabled Drizzle ....................................... 59
5.2.4 Coordinate Transformation Tasks ............................ 61
5.3 PyDrizzle ....................................................................... 62
5.3.1 Introduction ............................................................... 62
5.3.2 Parameters ............................................................... 62
5.3.3 Basic Examples ........................................................ 64
5.3.3.1 Running in Pure Python ................................... 64
5.3.3.2 Running in the PyRAF Environment................. 65
5.3.4 Output Product File Format ...................................... 66
5.4 Multidrizzle .................................................................... 67
5.4.1 Introduction ............................................................... 67
5.4.2 Software Requirements ............................................ 67
5.4.3 Data and Header Specifics ....................................... 67
5.4.3.1 Reference files ................................................. 68
5.4.4 Running under PyRAF.............................................. 69
5.4.5 Running under Python .............................................. 69
5.4.6 Running Multidrizzle ................................................. 70
5.4.6.1 Session Summary ............................................ 71
5.4.6.2 Intermediate Products ...................................... 71
5.4.6.3 Final Products .................................................. 72
5.4.7 Multidrizzle Parameters ............................................ 73
5.4.8 Initialization ............................................................... 79
5.4.8.1 General Explanation......................................... 79
5.4.8.2 Parameter Details............................................. 79
5.4.8.3 Basic Example.................................................. 82
5.4.9 Static Mask Creation................................................. 82
5.4.9.1 General Explanation......................................... 82
5.4.9.2 Parameter Details............................................. 83
5.4.9.3 Basic Examples................................................ 83
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5.4.10 Create Separate Drizzled Images........................... 83
5.4.10.1 General Explanation....................................... 83
5.4.10.2 Parameter Details........................................... 84
5.4.10.3 Basic Example................................................ 86
5.4.11 Create Median Image ............................................. 87
5.4.11.1 General Explanation....................................... 87
5.4.11.2 Parameter Details........................................... 87
5.4.11.3 Basic Example................................................ 88
5.4.12 Blot Median Image.................................................. 89
5.4.12.1 General Explanation....................................... 89
5.4.12.2 Parameter Details........................................... 89
5.4.12.3 Basic Example................................................ 90
5.4.13 Remove Cosmic-rays ............................................. 90
5.4.13.1 General Information........................................ 90
5.4.13.2 Parameter Details........................................... 91
5.4.13.3 Basic Example................................................ 91
5.4.14 Create Final Cleaned, Combined Product .............. 91
5.4.14.1 General Explanation....................................... 91
5.4.14.2 Parameter Details........................................... 92
5.4.14.3 Basic Example................................................ 94
5.4.15 Override Instrument Specific Parameters............... 95
5.4.15.1 General Information........................................ 95
5.4.15.2 Parameter Details........................................... 96
5.5 Optimizing the Drizzle Parameters
for Your Data ................................................................... 96
5.5.1 Creating Custom Association Tables........................ 96
5.5.1.1 Merging Association Tables ............................. 96
5.5.1.2 Creating Association Tables from Scratch ....... 96
5.5.2 Buildasn .................................................................... 97
5.5.2.1 Parameters....................................................... 97
5.5.2.2 Example Usage ................................................ 98
5.5.2.3 Shift Files.......................................................... 99
5.5.3 Alignment................................................................ 102
5.5.3.1 Alignment Error Sources ................................ 102
5.5.3.2 Visit Alignment................................................ 103
5.5.3.3 Inter-visit alignment ........................................ 103
5.5.3.4 Combining large mosaics or data
from multiple programs................................................ 103
5.5.3.5 User Defined Shifts ........................................ 104
5.5.3.6 Tweakshifts .................................................... 104
5.5.4 Sky Subtraction....................................................... 109
5.5.4.1 Introduction..................................................... 109
5.5.4.2 Methodology....................................................110
5.5.4.3 Parameter Details............................................110
5.5.4.4 Basic Example................................................. 111
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5.5.5 Cosmic-Ray Rejection ............................................. 111
5.5.5.1 Image Alignment Requirement........................112
5.5.6 Selecting the Optimal Scale and Pixfrac..................112
5.5.6.1 Choosing the drizzle.scale Parameter.............113
5.5.6.2 Choosing the drizzle.pixfrac Parameter ..........114
5.5.7 Selecting the ‘Bits’ Parameter..................................114
5.5.7.1 Data Quality Flags for Bad Pixels:
The ‘Bits’ Parameter.....................................................114
5.5.7.2 Using the 'Bits' Parameter to Update
the Masks .....................................................................115
Chapter 6: Real-World Examples ..................117
6.1 Limitations of Pipeline Processing .........................117
6.2 ACS ...............................................................................118
6.2.1 Introduction ..............................................................118
6.2.2 Example 1: Optimizing the Image Sampling
for Single Visit ............................................................. 120
6.2.3 Example 2: Optimizing the Image Alignment
for Multiple Visits ......................................................... 128
6.2.4 Example 3: Optimizing the Cosmic-Ray Rejection
for an ACS Mosaic with Sub-pixel Dithers................... 132
6.2.4.1 Improving the Alignment................................. 134
6.2.4.2 A Quick-way to Produce a Combined
Image .......................................................................... 137
6.2.4.3 Creating the Median ....................................... 137
6.2.4.4 Computing the Cosmic Ray Masks ................ 140
6.2.4.5 Final Drizzle.................................................... 143
6.2.4.6 Optimizing Scale and Pixfrac
for Subsampled Data................................................... 145
6.3 NICMOS ...................................................................... 146
6.3.1 Introduction ............................................................. 146
6.3.2 NICMOS Specific issues......................................... 147
6.3.3 Initial Setup ............................................................. 147
6.3.4 The Default Parameters.......................................... 149
6.3.5 Step 1: Static Mask................................................. 152
6.3.6 Step 2: Sky Subtraction .......................................... 153
6.3.7 Step 3: Drizzle separate images
and Refining Offsets................................................... 154
6.3.8 Step 4: Create Median Image................................. 156
6.3.9 Step 5: Blot back the median image ....................... 160
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6.3.10 Step 6: Remove cosmic rays with DERIV or
DRIZ_CR?................................................................... 161
6.3.11 Step 7: Drizzle final combined image ................... 162
6.3.12 Other Filters .......................................................... 163
6.3.13 Examining the Results .......................................... 164
6.4 WFPC2 ........................................................................ 169
6.4.1 Introduction ............................................................. 169
6.4.2 General tips ............................................................ 170
6.4.3 NGC 2440 example ................................................ 171
6.4.4 Data calibration and image registration .................. 172
6.4.5 Image combination and cleaning ............................ 173
6.4.6 Output and data quality........................................... 174
6.5 STIS.............................................................................. 175
6.5.1 Drizzling STIS Data ................................................ 175
6.6 WFC3 ........................................................................... 177
6.7 Scripting ....................................................................... 178
6.7.1 Introduction ............................................................. 178
6.7.2 The Quick and Easy Way ....................................... 179
Chapter 7: Troubleshooting .............................. 183
7.1 Interpreting your results ........................................... 183
7.1.1 General Issues........................................................ 183
7.1.2 Instrument Specific Information .............................. 184
7.1.2.1 ACS ................................................................ 184
Chapter 8: References ........................................... 187
Index ...................................................................................... 191