Staying Sharp: Keeping Hubble and Webb Seeing Clearly

Hubble WFS&C

Over Hubble’s long mission, we have found that the primary and secondary mirrors do not change shape enough to be noticed in the PSF. Tips, tilts, and off-center motions of these mirrors are also so small that they go unnoticed. We do, however, see significant changes in focus caused by variations in the distance between the primary and secondary mirrors. These disparities are caused by small temperature changes in the truss as Hubble orbits the earth and points in different directions (see Fig. 5). The small but measurable focus errors due to temperature changes have been reasonably well understood and modeled, and no mirror control is performed to compensate for this effect, which occurs over hours to days.

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A more significant effect on Hubble’s focus is the long-term, gradual shrinkage of the entire truss due to outgassing of water and other molecules from the graphite-epoxy material. This outgassing has caused Hubble’s focus to trend throughout its mission, as shown in Figure 6. If left uncorrected, by now this effect would have built up a wavefront error of many times the design specification. To compensate, we have commanded dozens of focus adjustments, each time backing the secondary mirror away from the primary mirror. These adjustments are typically 3–4 microns every few years, based on monthly measurements of the PSF.

For Hubble, the phase retrieval method used to determine wavefront error from the observed PSF expresses the result in terms of a polynomial of aberration terms. The software is typically set to solve for only certain “low-order” aberrations like focus, astigmatism, and sometimes coma, but among these, only the focus error varies enough to require periodic corrections. The Hubble phase retrieval operates on science images that are very close to being “in focus.” The sharp star images formed by the telescope are also undersampled at the detector, meaning that the PSF falls on fewer pixels than needed to optimally represent it (Fig. 7). This can present a challenge to a fitting algorithm like phase retrieval, but due in large part to the limited parameters that are being fit, we can accurately determine the amount of defocus even in a sharp Hubble PSF.

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Webb WFS&C

Unlike Hubble’s truss, the large, cold, carbon-composite structure holding Webb’s secondary mirror and 18 primary mirror segments is not expected to shrink significantly over time. The structure will, however, unavoidably deform when repointing of the telescope changes the angle of the sunshield with respect to the sun. Over the mission of five or more years, the sunshade material will degrade, causing temperatures to rise, which will produce a long-term evolution of the PSF. We may also find the PSF affected by unanticipated mechanical changes in the structure. The Webb WFS&C program is designed to address these issues and deliver an optimal PSF to the science instruments.

Compared with Hubble, the more complex telescope design of Webb requires the phase-retrieval algorithm to extract much more information about the wavefront from the observed PSF in order to guide more complex control of the mirrors. Instead of simply determining a single parameter (i.e., focus aberration), which scales directly with secondary mirror despace, the sophisticated algorithms developed to support Webb will produce a map of the best fitting observed wavefront, compare that with the required or a desired target map, then determine the set of commands needed to control the positions and orientations (“poses”) of each of the 18 segments to achieve that target wavefront. The relationship between the mirror poses and the wavefront is much more complicated for Webb than with Hubble, and sometimes the solutions will be degenerate, with various sets of adjustments producing indistinguishable PSFs. In such cases, practical considerations— economy of motion, conservation of the actuators, and ensuring that mirror segments avoid contact with each other—will influence the final choice of mirror commands.

Star images taken through NIRCam’s special lenses will increase the amount of information measurable in the PSF by introducing a precisely known defocus. This spreads the PSF out over more pixels, facilitating a more accurate and detailed phase retrieval (see Fig. 8).

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Without corrections, Webb’s total wavefront error could wander outside of its specification over periods of days to weeks, depending on the amount of thermal perturbations. Therefore, the current plan is to correct the aberrations as needed on an approximately biweekly timescale, based on wavefront sensing every two days. Figure 9 shows one particular model of possible fluctuations of the wavefront error during a year of representative science observations. As was the case with Hubble, much about Webb’s stability will not be known until it is operating on orbit. We will continue to refine models before launch, and utilize a control scheme that is flexible enough to adapt as we better understand behavior in flight and develop optimal control cadences.

Webb’s initial optical alignment

We have described how Webb’s WFS&C program will maintain the optical quality of the PSF during routine science operations, and compared the program to Hubble’s. The first test of these WFS&C procedures will be the initial alignment of the optics after the backplane assembly and secondary-mirror supports are deployed, when gross alignment errors at the millimeter level will be removed. These are very large adjustments compared with the tens of nanometers of fine control typically expected for the routine WFS&C program to follow. Nevertheless, the same wavefront sensing and mirror management tools will be used in both cases. The one-time WFS&C program after deployment will also involve special operations to support the unique conditions and scenarios that are part of the observatory commissioning.

The approach to performing Webb’s WFS&C resulted from a joint effort of NASA/GSFC, NASA/JPL, Ball Aerospace and Technology, Northrop Grumman Aerospace Systems, and the Institute. Ball will also be responsible for the initial alignment of the telescope.

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PSF modeling tools

The Institute provides tools to the community to model Hubble and Webb PSFs:

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