Space Telescope Science Institute
ACS Data Handbook V7.1
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Table of Contents

ACS Data Handbook
How to Use this Handbook
Handbook Structure
Typographic Conventions
Visual Cues
Chapter 1: ACS Overview
1.1 Instrument Design and Capabilities
1.1.1 Detectors
1.1.2 ACS Optical Design
1.1.3 ACS Geometric Distortion
1.1.4 ACS Performance after Servicing Mission 4
1.2 Basic Instrument Operations
1.2.1 Target Acquisitions
1.2.2 Typical ACS Observing Strategies
1.2.3 Data Storage and Transfer
1.2.4 Parallel Operations
Chapter 2: ACS Data Structure
2.1 Types of ACS Files
2.1.1 Data Files and Suffixes
2.1.2 Association Tables
2.1.3 Trailer Files
2.2 ACS File Structure
2.3 Data Storage Requirements
2.4 Headers and Keywords
Chapter 3: ACS Calibration Pipeline
3.1 On The Fly Reprocessing (OTFR)
3.2 Pipeline Overview
3.2.1 calacs: Image Calibration
calacs and Single Exposures
calacs and Combining of Sub-Exposures
calacs and Dithered Exposures
3.2.2 AstroDrizzle Processing in the Pipeline:
3.2.3 When is OTFR not Appropriate?
3.3 Structure of calacs
3.4 calacs Processing Steps
3.4.1 acsccd
doDQI - Bad Pixel Determination
doBias - Bias Image Subtraction
toElectrons - Unit Conversion From DN to Electrons
doBlev - Bias level Correction
doNoise - Error Array Initialization
Final Output From acsccd
3.4.2 acscte-Pixel-Based CTE Corrections
3.4.3 acsrej
Cosmic Ray Rejection Table
3.4.4 acs2d
doDQI - Bad Pixel Determination
doNonLin - Linearity Correction for MAMA Data
doNoise - Error Array Initialization
doDark - Dark Image Subtraction
doFlash - Post-Flash Subtraction
doFlat - Flat-Field Image(s) Correction
doShad - Shutter Shading File Correction (skipped)
doPhot - Photometry Keyword Calculation
doStat - Image Statistics Determination
3.4.5 acssum
3.5 Manual Recalibration of ACS Data
3.5.1 Requirements for Manual Recalibration
Software Requirements
Data Retrieval
Setting up “jref”
Using Non-default Reference files and Calibration Switches
Post-SM4 WFC Image Artifact Correction for WFC Sub-Array Images
Bypassing the PHOTCORR Step
3.5.2 calacs Examples
Example 1: Reprocessing a Single Exposure Using a Different Bias File
Example 2: Reprocessing Multiple Exposures Taken with “CR-SPLIT” Within an Association
Example 3: Combining Exposures From Multiple Associations
Example 4: Reprocessing Images taken as part of a Dither Pattern
Chapter 4: ACS Data Processing Considerations
4.1 Read Noise and A-to-D Conversion
4.1.1 Read Noise
4.1.2 A-to-D Conversion
4.2 Bias Issues
4.2.1 Bias Calibration
4.2.2 Bias Jump
4.2.3 Post-SM4 WFC Signal-dependent Bias Shift
4.2.4 Bias Sub-Arrays for WFC and HRC
4.3 Dark Current, Hot Pixels, and Cosmic Rays
4.3.1 Dark Current
4.3.2 Hot Pixels
4.3.3 Cosmic Rays
Fractional Coverage
Electron Deposition
Attached Pixels
4.4 Flat-Field Reference Files
4.4.1 Ground Flats (P-flats)
4.4.2 L-flats
4.4.3 Pipeline Flat-Field Files
4.4.4 Earth Flat Verification for the HRC
4.4.5 Sky Flats for the WFC
4.5 Image Anomalies
4.5.1 Dust Motes
4.5.2 Optical Ghosts and Scattered Light
4.5.3 Cross-Talk
4.5.4 Scattered Earth Light
4.5.5 HRC Polarimetry
4.6 ACS/WFC CCD Detector Charge Transfer Efficiency (CTE)
4.6.1 The Issue
4.6.2 Improving CTE: Considerations Before Making the Observations
4.6.3 Improving CTE: Post-Observation Image Restoration
4.7 Generic Detector and Camera Properties
4.7.1 Gain Calibration
4.7.2 Full Well Depth
High Resolution Camera
Wide Field Camera
4.7.3 Linearity at Low to Moderate Intensity
High Resolution Camera
Wide Field Camera
4.7.4 Linearity Beyond Saturation
High Resolution Camera
Wide Field Camera
4.7.5 Shutter Stability
Chapter 5: ACS Data Analysis
5.1 Photometry
5.1.1 Photometric Systems, Units, and Zeropoints
5.1.2 Aperture and Color Corrections
Color Correction
5.1.3 Pixel Area Maps
Pixel Area Map Concept Illustration
5.1.4 PSF
PSF Field Dependence
PSF Long Wavelength Artifacts
5.1.5 CTE
Pixel-Based CTE Correction
Photometric CTE Correction
CTE Correction Cookbook
The Relationship Between Field-Dependent Charge Diffusion and CTE
Internal CTE Measurements
5.1.6 Red Leak
5.1.7 UV Contamination
5.2 Astrometry
5.2.1 Coordinate Transformations
5.2.2 Absolute and Relative Astrometry
5.2.3 Impact of Guide Star Failure
5.3 Polarimetry
5.3.1 Absolute Calibration
5.3.2 Instrumental Issues
5.3.3 Flats
5.3.4 Polarization Calibration
5.4 Coronagraphy
5.4.1 Flat Fielding
5.4.2 PSF Subtraction
5.4.3 Photometry
5.4.4 Ghosting
5.5 Ramp Filters
5.5.1 Introduction
5.5.2 Existing Ground Calibrations
5.5.3 In-Flight Calibrations
5.6 Spectroscopy with the ACS Grisms and Prisms
5.6.1 What to Expect from ACS Slitless Spectroscopy Data
Bright Stars
Resolution and Object Size
Zeroth Order
Extra Field Objects
5.6.2 Pipeline Calibration
5.6.3 Slitless Spectroscopy Data and Dithering
5.6.4 Extracting and Calibrating Slitless Spectra
Preparing the aXe Reduction
aXe Tasks
aXedrizzle, for Processing G800L Data
Background Subtraction
Output Products: Extracted Spectra
Handling the aXe Output
Tuning the Extraction Process
5.6.5 Accuracy of Slitless Spectra Wavelength and Flux Calibration
Wavelength Calibration
Flux Calibration

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