Aligning and commissioning the James Webb Space Telescope's segmented mirrors after launch will last many months and involve the telescope itself, all science instruments, and all parts of the observatory ground system. In an effort to assess and demonstrate readiness of the complete end-to-end system - i.e. the flight optical telescope elements (OTE), the Integrated Science Instruments Module, the on-board operational scripts, and the ground processing infrastructure - we performed two operations tests during the JWST OTIS cryogenic campaign in 2017. They are the Wavefront Sensing and Control Demonstration activities at NASA Johnson Space Center (JSC), where we performed flight-like sensing and control using the flight software to command mirror moves and take measurements, and a "Shadow Mode test" at the Space Telescope Science Institute's Mission Operations Center (MOC), where we demonstrated processing of the JSC data through the entire ground system infrastructure. Overall, these tests demonstrated that the full system that will support OTE commissioning is soundly designed although still not fully mature. This paper focuses on the operations and systems testing aspects and some lessons learned. We also report on a series of Wavefront Rehearsals being held at the MOC that are providing additional opportunities to build team readiness in operating the ground system as a whole using high fidelity observatory simulators
The James Webb Space Telescopes segmented primary and deployable secondary mirrors will be actively con- trolled to achieve optical alignment through a complex series of steps that will extend across several months during the observatory's commissioning. This process will require an intricate interplay between individual wavefront sensing and control tasks, instrument-level checkout and commissioning, and observatory-level calibrations, which involves many subsystems across both the observatory and the ground system. Furthermore, commissioning will often exercise observatory capabilities under atypical circumstances, such as fine guiding with unstacked or defocused images, or planning targeted observations in the presence of substantial time-variable offsets to the telescope line of sight. Coordination for this process across the JWST partnership has been conducted through the Wavefront Sensing and Control Operations Working Group. We describe at a high level the activities of this group and the resulting detailed commissioning operations plans, supporting software tools development, and ongoing preparations activities at the Science and Operations Center. For each major step in JWST's wavefront sensing and control, we also explain the changes and additions that were needed to turn an initial operations concept into a flight-ready plan with proven tools. These efforts are leading to a robust and well-tested process and preparing the team for an efficient and successful commissioning of JWSTs active telescope.
Experience with the Hubble Space Telescope has shown that accurate models of optical performance are extremely
desirable to astronomers, both for assessing feasibility and planning scientific observations, and for data analyses
such as point-spread-function (PSF)-fitting photometry and astrometry, deconvolution, and PSF subtraction.
Compared to previous space observatories, the temporal variability and active control of the James Webb Space
Telescope (JWST) pose a significantly greater challenge for accurate modeling. We describe here some initial
steps toward meeting the community's need for such PSF simulations. A software package called WebbPSF
now provides the capability for simulating PSFs for JWST's instruments in all imaging modes, including direct
imaging, coronagraphy, and non-redundant aperture masking. WebbPSF is intended to provide model PSFs
suitable for planning observations and creating mock science data, via a straightforward interface accessible
to any astronomer; as such it is complementary to the sophisticated but complex-to-use modeling tools used
primarily by optical designers. WebbPSF is implemented using a new exible and extensible optical propagation
library in the Python programming language. While the initial version uses static precomputed wavefront
simulations, over time this system is evolving to include both spatial and temporal variation in PSFs, building
on existing modeling efforts within the JWST program. Our long-term goal is to provide a general-purpose PSF
modeling capability akin to Hubble's Tiny Tim software, and of sufficient accuracy to be useful to the community.
The Hubble Space Telescope is a Ritchie-Chrétien optical design with a main primary concave mirror followed by a
convex secondary. The focus is determined by the position of each of these two mirrors. The truss containing them is
made of graphite epoxy which has very low thermal expansion. Nevertheless, temperature variations do cause the mirror
separation to vary by several microns within an orbit. Additionally, outgassing of water vapor causes a long-term
shrinkage which soon after launch in 1990 varied by more than 2 microns per month. This necessitated adjusting the
position of the secondary mirror every few months. Currently this rate is greatly reduced and adjustments are needed less
than once per year.
The focus is monitored monthly to continually assess the need for such adjustments. The measurements have been used
to develop models to predict the focus at times between measurements to assist in the analysis of observations. Detailed
focus knowledge is of value in photometry, coronagraphy and image deconvolution. The various focus models that have
been applied so far are described with an evaluation of their performance. Continuing attempts to refine the model will
The Hubble Space Telescope's mission is summarized, with special emphasis placed on the Space Telescope Science Institute's unique experience with Hubble's behavior as an astronomical telescope in the environment of low earth orbit for over two decades. Historical context and background are given, and the project's early scientific expectations are described. A general overview of the spacecraft is followed by a more detailed look at the optical design, both as intended and as built. Basic characteristics of the complete complement of science instruments are also summarized. Experience with the telescope on-orbit is reviewed, starting with the major initial problems, solutions, human servicing missions, and the associated expansion of the observatory's capabilities over this time. Specific attention is then given to understanding Hubble's optical quality and pointing/jitter performance, two fundamental characteristics of a telescope. Experience with-and the important mitigation of-radiation damage and contamination is also related. Beyond the telescope itself, the advances in data reduction, calibration, and observing techniques are briefly discussed, as well as the subsequent emergence of highly accessible high-level archival science products. Hubble's scientific impact concludes the discussion.
Though the Hubble Space Telescope (HST) has proven to be a stable platform for astronomical observations
when compared with ground-based imaging, it features its own characteristic optical variations that have required
post-observation processing to support the full range of science investigations that should be possible with HST.
While the overall system focus has been monitored and adjusted throughout the life of the Observatory, the recent
installation of the Advanced Camera for Surveys and its High Resolution Camera has allowed us to use phase
retrieval techniques to accurately measure changes in coma and astigmatism as well. The aim of this current
work is to relate these measurements of wave front error back to characterizations more common to science
data analysis (e.g. FWHM, and ellipticity). We show how variations in these quantities over the timescales
observed may impact the photometric and astrometric precision required of many current HST programs, as well
as the characterization of barely-resolved objects. We discuss how improved characterization and modeling of
the point spread function (PSF ) variations may help HST observers achieve the full science capabilities of the
The focus of the Hubble Space Telescope (HST) has been monitored throughout the life of the observatory primarily by
phase retrieval techniques. This method generates and fits model Point Spread Functions (PSFs) to nearly in-focus stellar
images to solve for coefficients of the Zernike polynomials describing focus, and often coma and astigmatism. Here, we
discuss what these data from the ongoing monitoring strategies and special observations tell us about modes and
timescales observed in HST optical variations.
The development and tracking of Hubble Space Telescope science operations metrics will be described. In order for such metrics to be meaningful, they must be clearly linked to well-defined scientific contributions the observatory staff makes to the overall mission. The process for defining these contributions for HST, and then developing the appropriate metrics will be discussed. The process of developing and using metrics must take into account the fact that some may be more quantifiable than others. The fact that a metric is not easy to quantify does not necessarily detract from its importance or usefulness. Examples from the HST suite of metrics will be used to illustrate these situations. Operational metrics and data are also important at the subsystem level, to provide guidance in the process of trying to improve performance against the high-level science metrics. To the extent possible, the development of a system for capturing metric information should provide information useful at both these levels. These points will also be discussed in the context of examples from the HST suite of metrics. Our experiences to date with the collection and presentation of metric information will also be discussed.