Space Telescope Science Institute
FGS Instrument Handbook
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Table of Contents

Fine Guidance Sensor Instrument Handbook for Cycle 22
List of Figures
List of Tables
Chapter 1: Introduction
1.1 Purpose
1.2 Instrument Handbook Layout
1.3 The FGS as a Science Instrument
1.4 Technical Overview
1.4.1 The Instrument
1.4.2 Spectral Response
1.4.3 The S-Curve: The FGS’s Interferogram
1.4.4 FGS1r and the AMA
1.4.5 Field of View
1.4.6 Modes of Operation
Position Mode
Transfer Mode
1.5 Planning and Analyzing FGS Observations
1.5.1 Writing an FGS Proposal
1.5.2 Data Reduction
1.6 FGS Replacement in SM4
Chapter 2: FGS Instrument Design
2.1 The Optical Train
2.1.1 The Star Selectors
2.1.2 The Interferometer
2.2 FGS Detectors
2.3 HST’s Spherical Aberration
2.4 The FGS Interferometric Response
2.4.1 The Ideal S-Curve
2.4.2 Actual S-Curves
HST’s Spherical Aberration
Field Dependence and Temporal Stability of the S-Curves
2.5 The FGS1r Articulated Mirror Assembly
2.6 FGS Aperture and Filters
2.6.1 FOV and Detector Coordinates
2.6.2 Filters and Spectral Coverage
Filter Bandpasses
2.7 FGS Calibrations
Chapter 3: FGS Science Guide
3.1 The Unique Capabilities of the FGS
3.2 Position Mode: Precision Astrometry
3.3 Transfer Mode: Binary Stars and Extended Objects
3.3.1 Observing Binaries: The FGS vs. HST’s Cameras
3.3.2 Transfer Mode Performance
3.4 Combining FGS Modes: Determining Stellar Masses
3.5 Angular Diameters
3.6 Relative Photometry
3.7 Moving Target Observations
3.8 Summary of FGS Performance
3.9 Special Topics Bibliography
3.9.1 STScI General Publications
3.9.2 Position Mode Observations
3.9.3 Transfer Mode Observations
3.9.4 Miscellaneous Observations
3.9.5 Web Resources
Chapter 4: Observing with the FGS
4.1 Position Mode Overview
4.1.1 The Position Mode Visit
4.1.2 The Position Mode Exposure
4.2 Planning Position Mode Observations
4.2.1 Target Selection Criteria
Near Neighbors
Target Field
Reference Stars
Check Stars
4.2.2 Filters
PUPIL Not Recommended for Position Mode
FESTIME and Signal-to-Noise
4.2.3 Background
4.2.4 Position Mode Exposure Time Calculations
4.2.5 Exposure Strategies for Special Cases
Observing Binaries and Extended Sources in Position Mode
4.2.6 Sources Against a Bright Background
4.2.7 Crowded Field Sources
4.3 Position Mode Observing Strategies
4.3.1 Summary of Position Mode Error Sources
4.3.2 Drift and Exposure Sequencing
4.3.3 Cross Filter Observations
4.3.4 Moving Target Observation Strategy
4.4 Transfer Mode Overview
4.4.1 The FGS Response to a Binary
4.4.2 The Transfer Mode Exposure
4.5 Planning a Transfer Mode Observation
4.5.1 Target Selection Criteria
4.5.2 Transfer Mode Filter and Color Effects
4.5.3 Signal-to-Noise
4.5.4 Transfer Mode Exposure Time Calculations
4.6 Transfer Mode Observing Strategies
4.6.1 Summary of Transfer Mode Error Sources
4.6.2 Drift Correction
4.6.3 Temporal Variability of the S-Curve
4.6.4 Background and Dark Counts Subtraction
4.6.5 Empirical Roll Angle Determination
4.6.6 Exposure Strategies for Special Cases: Moving Targets
Chapter 5: FGS Calibration Program
5.1 Position Mode Calibrations and Error Sources
5.1.1 Position Mode Exposure Level Calibrations
Star Selector Encoder Fine Bit Errors
Position Centroiding
PMT Sensitivities and Position Centroid Adjustment
Differential Velocity Aberration
Optical Field Angle Distortion (OFAD) Calibration
Lateral Color
5.1.2 Position Mode Visit Level Calibrations
Cross Filter Calibrations
5.1.3 Position Mode Epoch-Level Calibrations
Plate Scale and Relative Distortion Stability
Errors Associated with Plate Overlays
5.2 Transfer Mode Calibrations and Error Sources
Background and Dark Subtraction
PMT Sensitivity Differences
Instrumental Stability
Interferometric Response and Source Color
Transfer Mode Scale as a Function of HST Roll Angle
5.3 Linking Transfer and Position Mode Observations
5.4 Cycle 21 Calibration and Monitoring Program
5.4.1 Active FGS1r Calibration and Monitoring Programs
5.5 Special Calibrations
Chapter 6: Writing a Phase II Proposal
6.1 Phase II Proposals: Introduction
6.1.1 Required Information
6.1.2 STScI Resources for Phase II Proposal Preparation
6.2 Instrument Configuration
Spectral Element
6.2.1 Optional Parameters for FGS Exposures
6.3 Special Requirements
6.3.1 Visit-Level Special Requirements
Spacecraft Orientation ORIENT:
Timing Requirements:
6.3.2 Exposure-Level Special Requirements
6.4 Overheads
6.4.1 Pos Mode Overheads
6.4.2 Trans Mode Overhead
6.5 Proposal Logsheet Examples
6.5.1 Parallax program using FGS1r in Pos mode
6.5.2 Determining a binary’s orbital elements using FGS1r in Trans mode
6.5.3 Mass determination: Trans and Pos mode
6.5.4 Faint binary with Trans mode: special background measurements
Chapter 7: FGS AstrometryData Processing
7.1 Data Processing Overview
7.2 Exposure-Level Processing
7.2.1 Initial Pipeline Processing
7.2.2 Observing Mode Dependent Processing
Position mode
Transfer mode
7.3 Visit-Level Processing
7.3.1 Position Mode
HST Oscillations: Using the Guide Star Data
Drift Correction
7.3.2 Transfer Mode
Guide Star Data
Drift Correction
Transfer/Position Mode Bias
7.4 Epoch-Level Processing
7.4.1 Parallax, Proper Motion, and Reflex Motion
7.4.2 Binary Stars and Orbital Elements
Appendix A: Target Acquisitionand Tracking
A.1 FGS Control
A.2 Target Acquisition and Position Mode Tracking
A.2.1 Slew to the Target
A.2.2 Search
A.2.3 CoarseTrack
A.2.4 FineLock
A.3 Transfer Mode Acquisition and Scanning
A.4 Visit Level Control
Appendix B: FGS1r PerformanceSummary
B.1 FGS1r’s First Three Years in Orbit
B.2 Angular Resolution Test
B.2.1 Test Results: The Data
B.2.2 Test Results: Binary Star Analysis
B.3 FGS1r’s Angular Resolution: Conclusions
B.4 FGS1r: Second AMA Adjustment
B.5 FGS1r: Third AMA Adjustment
Glossary of Terms

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