James Webb Space Telescope's Fine Guidance Sensor Achieves Star Lock
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Chapter 1: Overview of the Fine Guidance Sensor
Recently, the Canadian Space Agency (CSA) announced that the Fine Guidance Sensor (FGS) of the James Webb Space Telescope (JWST) has successfully locked onto a designated guide star in tracking mode. This component is vital for the operational efficiency of JWST, aiding engineers in the precise alignment of the telescope's 18 hexagonal mirror segments. This alignment phase commenced shortly after the Near-Infrared Camera (NIRCam) detected its first light, leading to NASA's unveiling of the initial, unfocused image captured by the telescope.
The initial image from James Webb depicts a star represented by 18 distinct points of light. Credit: NASA
The FGS, developed by the Canadian Space Agency, serves as a mission-critical tool. While JWST employs this technology, it is not exclusive to it; other space telescopes also utilize FGS for enhanced guidance. The sensor provides high-accuracy pointing data to the telescope’s attitude control systems, ensuring precise targeting.
The FGS is designed to "lock on" to celestial objects such as stars, enabling consistent tracking as the JWST maneuvers. This capability is essential for obtaining sharp images and accurate data, maintaining the telescope's focus during observations.
Section 1.1: Comparison with Hubble's FGS
Unlike the Hubble Space Telescope, which uses three fine guidance sensors—two for targeting and one for positional measurements—JWST's FGS adopts a more compact approach. By integrating the FGS, NIRCam, and Slitless Spectrograph into a single unit, JWST reduces mass and volume while still achieving distinct functionality for each instrument.
The FGS/NIRISS at NASA Goddard prior to its module integration. Credit: NASA
Section 1.2: Functions of the Fine Guidance Sensor
The JWST's FGS performs three critical functions. First, it captures images for target acquisition by comparing the brightness and positions of observed objects against a catalog of known celestial bodies.
Secondly, it identifies pre-selected guide stars, ensuring that observations are accurately oriented. This process mirrors the guidance systems used in ground-based telescopes, which also lock onto designated guide stars before proceeding to other targets.
Lastly, the FGS supplies the attitude control system with measurements of guide stars at a rate of 16 times per second. This rapid data collection enables the telescope to maintain stable targeting with milliarcsecond precision.
“Alongside its rapid speed, the FGS must exhibit remarkable precision. Its ability to detect minute changes in the pointing to a celestial body is akin to a person in New York City detecting the blink of an eye from someone located 500 kilometers (311 miles) away at the Canadian border!” – René Doyon, Principal Investigator for FGS/NIRISS, Université de Montréal; and Nathalie Ouellette, Outreach Scientist for Webb, Université de Montréal.
A recently released image showcasing the early alignment of JWST. Credit: NASA/STScI/J. DePasquale
Chapter 2: Future Prospects and Updates
The team responsible for JWST is diligently working on aligning the primary mirror's 18 segments, a crucial yet time-intensive step in the telescope's setup post-launch. NASA aims to reveal the first focused images by summer, with additional updates expected as preparations for observations continue.
The first video titled "MOSAIC!! Webb's Fine Guidance Sensor Provides a Preview" discusses the significance of the FGS and its role in the mission.
The second video, "James Webb Space Telescope Locks its First Target! First Live Images? Aliens?" explores the telescope's initial target acquisition and what it means for future discoveries.
For ongoing updates about JWST's progress, click here.