The Wide-Field Infrared Survey Telescope (WFIRST) mission is the top-ranked large space mission in the New Worlds, New Horizon (NWNH) Decadal Survey of Astronomy and Astrophysics. WFIRST will settle essential questions in both exoplanet and dark energy research and will advance topics ranging from galaxy evolution to the study of objects within the galaxy. The WFIRST mission uses a repurposed 2.4-m Forward Optical Telescope assembly (FOA), which, when completed with new aft optics will be an Integrated Optical Assembly (IOA). WFIRST is equipped with a Wide Field Instrument (WFI) and a Coronagraph Instrument (CGI). An Instrument Carrier (IC) meters these payload elements together and to the spacecraft bus (S/C). A distributed ground system receives the data, uploads commands and software updates, and processes the data. After transition from the study phase, Pre-Phase-A (a.k.a., “Cycle 6”) design to NASA Phase A formulation, a significant change to the IOA was initiated; including moving the tertiary mirror from the instrument package to a unified three-mirror anastigmat (TMA) placement, that provides a wide 0.28-sq° instrumented field of view to the Wide Field Instrument (WFI). In addition, separate relays from the primary and secondary mirror feed the Wide Field Instrument (WFI) and Coronagraph Instrument (CGI). During commissioning the telescope is aligned using wavefront sensing with the WFI. A parametric and Monte-Carlo analysis was performed, which determined that alignment compensation with the secondary mirror alone degraded performance in the other instruments. This led to the addition of a second compensator in the WFI optical train to alleviate this concern. This paper discusses the trades and analyses that were performed and resulting changes to the WFIRST telescope architecture.
The James Webb Space Telescope (JWST) project is an international collaboration led by NASA’s Goddard Space
Flight Center (GSFC) in Greenbelt, MD. JWST is NASA’s flagship observatory that will operate nearly a million miles
away from Earth at the L2 Lagrange point. JWST’s optical design is a three-mirror anastigmat with four main optical
components; 1) the eighteen Primary Mirror Segment Assemblies (PMSA), 2) a single Secondary Mirror Assembly
(SMA), 3) an Aft-Optics Subsystem (AOS) consisting of a Tertiary Mirror and Fine Steering Mirror, and 4) an
Integrated Science Instrument Module consisting of the various instruments for JWST. JWST’s optical system has been
designed to accommodate a significant amount of alignment capability and risk with the PMSAs and SMA having rigid
body motion available on-orbit just for alignment purposes. However, the Aft-Optics Subsystem (AOS) and Integrated
Science Instrument Module (ISIM) are essentially fixed optical subsystems within JWST, and therefore the cryogenic
alignment of the AOS to the ISIM is critical to the optical performance and mission success of JWST.
In support of this cryogenic alignment of the AOS to ISIM, an array of fiber optic sources, known as the AOS Source
Plate Assembly (ASPA), are placed near the intermediate image location of JWST (between the secondary and tertiary
mirrors) during thermal vacuum ground-test operations. The AOS produces images of the ASPA fiber optic sources at
the JWST focal surface location, where they are captured by the various science instruments. In this manner, the AOS
provides an optical yardstick by which the instruments within ISIM can evaluate their relative positions to and the
alignment of the AOS to ISIM can be quantified. However, since the ASPA is located at the intermediate image location
of the JWST three-mirror anastigmat design, the images of these fiber optic sources produced by the AOS are highly
aberrated with approximately 2-3μm RMS wavefront error consisting mostly of 3rd-order astigmatism and coma. This is
because the elliptical tertiary mirror of the AOS is used off of its ideal foci locations without the compensating
wavefront effects of the JWST primary and secondary mirrors. Therefore, the PSFs created are highly asymmetric with
relatively complex structure and the centroid and encircled energy analyses traditionally used to locate images are not
sufficient for ensuring the AOS to ISIM alignment.
A novel approach combining phase retrieval and spatial metrology was developed to both locate the images with respect
to the AOS and provide calibration information for eventual AOS to ISIM alignment verification. During final JWST
OTE and ISIM (OTIS) testing, only a single thru-focus image will be collected by the instruments. Therefore, tools and
processes were developed to perform single-image phase retrieval on these highly aberrated images such that any single
image of the ASPA source can provide calibrated knowledge of the instruments’ position relative to the AOS. This paper
discusses the results of the methodology, hardware, and calibration performed to ensure that the AOS and ISIM are
aligned within their respective tolerances at JWST OTIS testing.
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.
The WFIRST-AFTA Wide-Field Infrared Survey Telescope TMA optical design provides 0.28-sq°FOV Wide Field Channel at 0.11” pixel scale, operating at wavelengths between 0.76-2.0μm, including a spectrograph mode (1.35-1.95μm.) An Integral Field Channel provides a discrete 3”x3.15” field at 0.15” sampling.
The search spaces in nonlinear optimization phase retrieval problems are of high dimensionality, making them difficult to visualize. Using a simplified low-order model, we explore the shape of the phase retrieval error surfaces using two-dimensional (2D) slices to visualize the relationship between different aberrations. We show how different pairs of aberrations exhibit very different coupling with one another and different distributions and frequencies of local minima, and discuss how this relates to the phase retrieval capture-range problem.
In this paper we develop methods to use a linear optical model to capture the field dependence of wavefront aberrations
in a nonlinear optimization-based phase retrieval algorithm for image-based wavefront sensing. The linear optical model
is generated from a ray trace model of the system and allows the system state to be described in terms of mechanical
alignment parameters rather than wavefront coefficients. This approach allows joint optimization over images taken at
different field points and does not require separate convergence of phase retrieval at individual field points. Because the
algorithm exploits field diversity, multiple defocused images per field point are not required for robustness. Furthermore,
because it is possible to simultaneously fit images of many stars over the field, it is not necessary to use a fixed defocus
to achieve adequate signal-to-noise ratio despite having images with high dynamic range. This allows high performance
wavefront sensing using in-focus science data. We applied this technique in a simulation model based on the Wide Field
Infrared Survey Telescope (WFIRST) Intermediate Design Reference Mission (IDRM) imager using a linear optical
model with 25 field points. We demonstrate sub-thousandth-wave wavefront sensing accuracy in the presence of noise
and moderate undersampling for both monochromatic and polychromatic images using 25 high-SNR target stars. Using
these high-quality wavefront sensing results, we are able to generate upsampled point-spread functions (PSFs) and use
them to determine PSF ellipticity to high accuracy in order to reduce the systematic impact of aberrations on the
accuracy of galactic ellipticity determination for weak-lensing science.