KEYWORDS: James Webb Space Telescope, Point spread functions, Stars, Target acquisition, Space telescopes, Coronagraphy, Mirrors, Detection and tracking algorithms, Optimization (mathematics), Image processing
We discuss new results of coronagraphic simulations demonstrating a novel mode for JWST that utilizes sub-pixel
dithered reference images, called Small-Grid Dithers, to optimize coronagraphic PSF subtraction. These sub-pixel
dithers are executed with the Fine Steering Mirror under fine guidance, are accurate to ~2-3 milliarcseconds (1-σ/axis),
and provide ample speckle diversity to reconstruct an optimized synthetic reference PSF using LOCI or KLIP. We also
discuss the performance gains of Small-Grid Dithers compared to the standard undithered scenario, and show potential
contrast gain factors for the NIRCam and MIRI coronagraphs ranging from 2 to more than 10, respectively.
KEYWORDS: Coronagraphy, Stars, James Webb Space Telescope, Point spread functions, Target acquisition, Detection and tracking algorithms, Space telescopes, Planets, Telescopes, Observatories
Coronagraphic Target Acquisition (TA) is an important factor that contributes to the contrast performance and typically depends on the coronagraph design. In the case of JWST, coronagraphic TAs rely on measuring the centroid of the star's point spread function away from the focal plane mask, and performing a small angle ma- neuver (SAM), to place the star behind the coronagraphic mask. Therefore, the accuracy of the TA is directly limited by the SAM accuracy. Typically JWST coronagraphic observations will include the subtraction of a reference (either a reference star, or a self-reference after a telescope roll). With such differential measurement, the reproducibility of the TA is a very important factor. We propose a novel coronagraphic observation concept whereby the reference PSF is first acquired using a standard TA, followed by coronagraphic observations of a reference star on a small grid of dithered positions. Sub-pixel dithers (5-10 mas each) provide a small reference PSF library that samples the variations in the PSF as a function of position relative to the mask, thus compen- sating for errors in the TA process. This library can be used for PSF subtraction with a variety of algorithms (e.g; LOCI or KLIP algorithms, Lafrenière et al. 2007; Soummer, Pueyo and Larkin 2012). These sub-pixel dithers are executed under closed-loop fine guidance, unlike a standard SAM that executes the maneuver in coarse point mode, which can result in a temporary target offset of 1 arcsecond and would bring the star out from behind the coronagraphic mask. We discuss and evaluate the performance gains from this observation scenario compared to the standard TA both for MIRI coronagraphs.
KEYWORDS: James Webb Space Telescope, Observatories, Data archive systems, Calibration, Space telescopes, Hubble Space Telescope, Space operations, Telecommunications, Telescopes, Data communications
The James Webb Space Telescope (JWST) will be a powerful space observatory whose four science instruments will
deliver rich imaging and multiplexed spectroscopic datasets to the astronomical and planetary science communities.
The ground segment for JWST, now being designed and built, will carry out JWST's science operations. The ground
segment includes:
software that the scientific community will use to propose and specify new observations;
systems that will schedule science and calibration observations in a way that respects physical and investigator-specified
constraints, while satisfying preferences for efficient observing, low background levels, and distributed
subscription across a year;
the infrastructure to regularly measure and maintain the telescope's wavefront;
orbit determination, ranging, and tracking;
communication via the Deep Space Network to command the observatory and retrieve scientific data;
onboard scripts that execute each observing program in an event-driven fashion, with occasional interruptions for
targets of opportunity or time-critical observations; and
a system that processes and calibrates the data into science-ready products, automatically recalibrates when calibrations
improve, and archives the data for timely access by the principal investigator and later worldwide access by the scientific
community.
This ground system builds on experience from operating the Hubble Space Telescope, while solving challenges that are
unique to JWST. In this paper, we describe the elements of the JWST ground system, how it will work operationally
from the perspective of the observatory itself, and how a typical user will interact with the system to turn their idea into
scientific discovery.
KEYWORDS: Stars, James Webb Space Telescope, Solar system, Observatories, Sensors, Target acquisition, Space operations, Space telescopes, Planets, Comets
The James Webb Space Telescope will provide a unique capability to observe Solar System objects such as Kuiper Belt
Objects, comets, asteroids, and the outer planets and their moons in the near and mid-infrared. A recent study developed
the conceptual design for a capability to track and observe these objects. In this paper, we describe how the requirements
and operations concept were derived from the scientific goals and were distributed among the Observatory and Ground
Segment components in order to remain consistent with the current event-driven operations concept of JWST. In the
event-driven operations concept, the Ground Segment produces a high-level Observation Plan that is interpreted by on-board
scripts to generate commands and monitor telemetry responses. This approach allows efficient and flexible
execution of planned observations; precise or conservative timing models are not required, and observations may be
skipped if guide star or target acquisition fails. The efficiency of this approach depends upon most observations having
large time intervals in which they can execute. Solar System objects require a specification of how to track the object
with the Observatory, and a guide star that remains within the field of view of the guider during the observation. We
describe how tracking and guiding will be handled with JWST to retain the efficient and flexible execution
characteristics of event-driven operations. We also describe how the implementation is distributed between the
Spacecraft, Fine Guidance Sensor, On-board Scripts, and Proposal Planning Subsystem, preserving the JWST operations
concept.
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