Hubble Space Telescope tracking systems
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Hubble Space Telescope Tracking Systems: Precision Pointing and Control
Fine Guidance Sensors (FGS) and Pointing Control System
The Hubble Space Telescope (HST) relies on its Fine Guidance Sensors (FGS) and a sophisticated Pointing Control System to achieve extremely precise pointing and stability, which are essential for high-quality astronomical observations. The FGS units can measure angles as small as 0.003 arc-seconds and provide the fine pointing information needed for the telescope's control system, allowing HST to maintain a pointing stability of 0.007 arc-seconds root mean square (rms) or less, a requirement more stringent than any previous spacecraft 568.
Tracking Modes: Fine Lock and Coarse Track
HST's FGS operates in two main tracking modes: Fine Lock and Coarse Track. Fine Lock mode uses a two-axis interferometer to achieve pointing errors of less than 3 milli-arcseconds rms, while Coarse Track mode, which uses a pseudo-quadrature detector, maintains errors within 20–25 milli-arcseconds rms under ideal conditions 1310. However, due to limitations in the control electronics and optical aberrations, Fine Lock is typically limited to brighter stars (V = 14.5 magnitude), and Coarse Track is used more frequently for fainter guide stars, which can compromise some scientific observations 1310.
Hardware and Software Components
The FGS hardware includes beam steering Star Selector servos, photon-counting photomultiplier tube detectors, and a 24-bit microprocessor running the control firmware . The Pointing Control System integrates data from the FGS and rate gyros, processes it with a digital computer, and commands reaction wheels to adjust the telescope's orientation. Improvements to the rate gyros and reaction wheels have been made to reduce noise and vibration, further enhancing pointing precision .
Challenges: Jitter, Aberrations, and Environmental Effects
HST's tracking performance is affected by several factors. Mechanical shocks from the solar array during orbital day/night transitions induce jitter, and the telescope's spherically aberrated optics reduce the signal-to-noise ratio in the control loop, especially in Coarse Track mode 135. Despite these challenges, the FGS and control system have demonstrated robust performance, with on-orbit data showing that pointing and tracking requirements are generally met or exceeded .
Moving Target and Solar System Object Tracking
The HST is also capable of tracking moving targets, such as planetary satellites and comets, with apparent motions up to 0.21 arc-seconds per second. The Pointing Control System accounts for velocity aberration and parallax, maintaining tracking accuracy under 0.03 arc-seconds at maximum rates 27. Special timing and data processing techniques are used to handle the unique challenges of tracking solar system objects with a sampled data, multi-rate digital control system 27.
Calibration and Performance Improvements
Calibration of internal FGS distortions and alignment between sensors has led to improved guide star and target acquisitions. Analytical studies and on-orbit tests have shown that the FGS can adapt to unexpected conditions, such as optical aberrations and solar panel jitter, ensuring continued high performance .
Conclusion
The Hubble Space Telescope's tracking systems, centered around its Fine Guidance Sensors and advanced Pointing Control System, enable the precise pointing and stability required for groundbreaking astronomical research. Despite challenges from hardware limitations and environmental disturbances, ongoing calibration, software improvements, and robust system design have ensured that HST continues to meet its demanding tracking and pointing requirements 1356+1 MORE.
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Solar Object Tracking For The Hubble Space Telescope
The Hubble Space Telescope's Pointing Control System must accurately track solar system targets with motion up to 0."21/s, achieving pointing accuracies and line of sight jitter levels significantly lower than ground-based telescopes.
DOI
Acquisition, pointing, and tracking performance of the Hubble Space Telescope fine-guidance sensors
The Hubble Space Telescope's Fine Guidance Sensors (FGS) meet and sometimes exceed requirements for acquisition, pointing, and tracking performance, with improvements in guide star acquisition and target acquisition through calibration techniques.
DOI