ZWFS is known to be photon noise optimal for measuring low order aberrations. Recently,
ZWFS was selected as the baseline LOWFS technology on WFIRST for its sensitivity, accuracy,
and its ease of integration with the starlight rejection mask. In this paper, we present the
development of ZWFS sensor, including the algorithm description, sensitivity analysis, and some
early experimental model validation results from a fabricated ZWFS phase mask on a standalone
To maintain the required WFIRST Coronagraph starlight suppression performance in a realistic space environment,
a low order wavefront sensing and control (LOWFS/C) subsystem is necessary. The LOWFS/C uses the rejected
stellar light from coronagraph to sense and suppress the telescope pointing drift and jitter as well as the low order
wavefront errors due to changes in thermal loading on the telescope and the rest of the observatory. In this paper we
will present an overview of the low order wavefront sensing and control subsystem for the WFIRST Coronagraph
and describe the WFIRST Coronagraph LOWFS function, its design, and modeled performance. We will present
experimental results on a dedicated LOWFS/C testbed that show that the LOWFS/C subsystem not only can sense
pointing errors better than 0.2 mas but has also experimentally demonstrated closed loop pointing error suppression
with residuals better than 0.4 mas rms per axis for the vast majority of observatory reaction wheel speeds.
ATLAST is a particular realization of the Large Ultraviolet Optical Infrared telescope (LUVOIR), a ∼10-m diameter space telescope being defined for consideration in the 2020 Decadal Review of astronomy and astrophysics. ATLAST/LUVOIR is generally thought of as an ambient temperature (∼300 K) system, and little consideration has been given to using it at infrared wavelengths longward of ∼2 μm. We assess the scientific and technical benefits of operating such a telescope further into the infrared, with particular emphasis on the study of exoplanets, which is a major science theme for ATLAST/LUVOIR. For the study of exoplanet atmospheres, the capability to work at least out to 5.0 μm is highly desirable. Such an extension of the long wavelength limit of ATLAST would greatly increase its capabilities for studies of exoplanet atmospheres and provide powerful capabilities for the study of a wide range of astrophysical questions. We present a concept for a fiber-fed grating spectrometer, which would enable R=200 spectroscopy on ATLAST with minimal impact on the other focal planet instruments. We conclude that it is technically feasible and highly desirable scientifically to extend the wavelength range of ATLAST to at least 5 μm.
To maintain the required Wide-Field Infrared Survey Telescope (WFIRST) coronagraph performance in a realistic space environment, a low-order wavefront sensing and control (LOWFS/C) subsystem is necessary. The LOWFS/C uses the rejected stellar light from the coronagraph to sense and suppress the telescope pointing errors as well as low-order wavefront errors (WFEs) due to changes in thermal loading of the telescope and the rest of the observatory. We will present a conceptual design of a LOWFS/C subsystem for the WFIRST-AFTA coronagraph. This LOWFS/C uses a Zernike phase contrast wavefront sensor (ZWFS) with a phase shifting disk combined with the stellar light rejecting occulting masks, a key concept to minimize the noncommon path error. We will present our analysis of the sensor performance and evaluate the performance of the line-of-sight jitter suppression loop, as well as the low-order WFE correction loop with a deformable mirror on the coronagraph. We will also report the LOWFS/C testbed design and the preliminary in-air test results, which show a very promising performance of the ZWFS.
To maintain the required WFIRST Coronagraph starlight suppression performance in a realistic space environment, a low order wavefront sensing and control (LOWFS/C) subsystem is necessary. The LOWFS/C uses the rejected stellar light from coronagraph to sense and suppress the telescope pointing drift and jitter as well as the low order wavefront errors due to changes in thermal loading on the telescope and the rest of the observatory. In this paper we will present an overview of the low order wavefront sensing and control subsystem for the WFIRST Coronagraph. We will describe LOWFS/C’s Zernike wavefront sensor concept and control design, and present an overview of sensing performance analysis and modeling, predicted line-of-sight jitter suppression loop performance, as well as the low order wavefront error correction with the coronagraph’s deformable mirror. We will also report the LOWFS/C testbed design and the preliminary in-air test results, which show promising performance of the Zernike wavefront sensor and FSM feedback loop.
WFIRST-AFTA design makes use of an existing 2.4m telescope for direct imaging of exoplanets. To maintain the high contrast needed for the coronagraph, wavefront error (WFE) of the optical system needs to be continuously sensed and controlled. Low Order Wavefront Sensing (LOWFS) uses the rejected starlight from an immediate focal plane to sense wavefront changes (mostly thermally induced low order WFE) by combining the LOWFS mask (a phase plate located at the small center region with reflective layer) with the starlight rejection masks, i.e. Hybrid Lyot Coronagraph (HLC)’s occulter or Shaped Pupil Coronagraph (SPC)’s field stop. Zernike wavefront sensor (ZWFS) measures phase via the phase-contrast method and is known to be photon noise optimal for measuring low order aberrations. Recently, ZWFS was selected as the baseline LOWFS technology on WFIST/AFTA for its good sensitivity, accuracy, and its easy integration with the starlight rejection mask. In this paper, we review the theory of ZWFS operation, describe the ZWFS algorithm development, and summarize various numerical sensitivity studies on the sensor performance. In the end, the predicted sensor performance on SPC and HLC configurations are presented.
This paper presents results of the feedback control design for JPL's Fast Steering Mirror (FSM) for the WFIRST- AFTA coronagraph instrument. The objective of this controller is to cancel jitter disturbances in the beam from the spacecraft to a pointing stability of 0.4 masec over the duration of the observation using a momentum- compensated FSM. The plant model for the FSM was characterized experimentally, and the sensor model is based on simulated modeling. The control approach is divided between feedback compensation of low frequency attitude control system (ACS) drift, and feedforward cancellation of high frequency tonal disturbances originating from reaction wheel excitation of the telescope structure. This paper will present various aspects of the controller design, plant characterization, sensor modeling, disturbance estimation, performance simulation, and preliminary experimental testing results.
NASA’s WFIRST-AFTA mission concept includes the first high-contrast stellar coronagraph in space. This coronagraph will be capable of directly imaging and spectrally characterizing giant exoplanets similar to Neptune and Jupiter, and possibly even super-Earths, around nearby stars. In this paper we present the plan for maturing coronagraph technology to TRL5 in 2014-2016, and the results achieved in the first 6 months of the technology development work. The specific areas that are discussed include coronagraph testbed demonstrations in static and simulated dynamic environment, design and fabrication of occulting masks and apodizers used for starlight suppression, low-order wavefront sensing and control subsystem, deformable mirrors, ultra-low-noise spectrograph detector, and data post-processing.
Large aperture telescope commonly features segment mirrors and a coarse phasing step is needed
to bring these individual segments into the fine phasing capture range. Dispersed Fringe Sensing
(DFS) is a powerful coarse phasing technique and its alteration is currently being used for JWST.
An Advanced Dispersed Fringe Sensing (ADFS) algorithm is recently developed to improve the
performance and robustness of previous DFS algorithms with better accuracy and unique
solution. The first part of the paper introduces the basic ideas and the essential features of the
ADFS algorithm and presents the some algorithm sensitivity study results. The second part of the
paper describes the full details of algorithm validation process through the advanced wavefront
sensing and correction testbed (AWCT): first, the optimization of the DFS hardware of AWCT
to ensure the data accuracy and reliability is illustrated. Then, a few carefully designed algorithm
validation experiments are implemented, and the corresponding data analysis results are shown.
Finally the fiducial calibration using Range-Gate-Metrology technique is carried out and a
<10nm or <1% algorithm accuracy is demonstrated.
The search for Earth-mass planets in the habitable zones of nearby Sun-like stars is an important goal of astrophysics.
This search is not feasible with the current slate of astronomical instruments. We propose a new concept for microarcsecond
astrometry which uses a simplified instrument and hence promises to be low cost. The concept employs a
telescope with only a primary, laser metrology applied to the focal plane array, and new algorithms for measuring image
position and displacement on the focal plane. The required level of accuracy in both the metrology and image position
sensing is at a few micro-pixels. We have begun a detailed investigation of the feasibility of our approach using
simulations and a micro-pixel image position sensing testbed called MCT. So far we have been able to demonstrate that
the pixel-to-pixel distances in a focal plane can be measured with a precision of 20 micro-pixels and image-to-image
distances with a precision of 30 micro-pixels. We have also shown using simulations that our image position algorithm
can achieve accuracy of 4 micro-pixels in the presence of λ/20 wavefront errors.
The SIM-Lite astrometric interferometer will search for Earth-size planets in the habitable zones of nearby stars. In this
search the interferometer will monitor the astrometric position of candidate stars relative to nearby reference stars over
the course of a 5 year mission. The elemental measurement is the angle between a target star and a reference star. This is
a two-step process, in which the interferometer will each time need to use its controllable optics to align the starlight in
the two arms with each other and with the metrology beams. The sensor for this alignment is an angle tracking CCD
camera. Various constraints in the design of the camera subject it to systematic alignment errors when observing a star of
one spectrum compared with a start of a different spectrum. This effect is called a Color Dependent Centroid Shift
(CDCS) and has been studied extensively with SIM-Lite's SCDU testbed. Here we describe results from the simulation
and testing of this error in the SCDU testbed, as well as effective ways that it can be reduced to acceptable levels.
In the course of fulfilling its mandate, the Spectral Calibration Development Unit (SCDU) testbed for SIM-Lite produces
copious amounts of raw data. To effectively spend time attempting to understand the science driving the data, the team
devised computerized automations to limit the time spent bringing the testbed to a healthy state and commanding it,
and instead focus on analyzing the processed results. We developed a multi-layered scripting language that emphasized
the scientific experiments we conducted, which drastically shortened our experiment scripts, improved their readability,
and all-but-eliminated testbed operator errors. In addition to scientific experiment functions, we also developed a set of
automated alignments that bring the testbed up to a well-aligned state with little more than the push of a button. These
scripts were written in the scripting language, and in Matlab via an interface library, allowing all members of the team to
augment the existing scripting language with complex analysis scripts. To keep track of these results, we created an easilyparseable
state log in which we logged both the state of the testbed and relevant metadata. Finally, we designed a distributed
processing system that allowed us to farm lengthy analyses to a collection of client computers which reported their results
in a central log. Since these logs were parseable, we wrote query scripts that gave us an effortless way to compare results
collected under different conditions. This paper serves as a case-study, detailing the motivating requirements for the
decisions we made and explaining the implementation process.
SIM-Lite missions will perform astrometry at microarcsecond accuracy using star light interferometry. For typical
baselines that are shorter than 10 meters, this requires to measure optical path difference (OPD) accurate to tens of
picometers calling for highly accurate calibration. A major challenge is to calibrate the star spectral dependency
in fringe measurements - the spectral calibration. Previously, we have developed a spectral calibration and
estimation scheme achieving picometer level accuracy. In this paper, we present the improvements regarding the
application of this scheme from sensitivity studies. Data from the SIM Spectral Calibration Development Unit
(SCDU) test facility shows that the fringe OPD is very sensitive to pointings of both beams from the two arms of
the interferometer. This sensitivity coupled with a systematic pointing error provides a mechanism to explain the
bias changes in 2007. Improving system alignment can effectively reduce this sensitivity and thus errors due to
pointing errors. Modeling this sensitivity can lead to further improvement in data processing. We then investigate
the sensitivity to a model parameter, the bandwidth used in the fringe model, which presents an interesting trade
between systematic and random errors. Finally we show the mitigation of calibration errors due to system drifts
by interpolating instrument calibrations. These improvements enable us to use SCDU data to demonstrate that SIM-Lite missions can meet the 1pm noise floor requirement for detecting earth-like exoplanets.
SIM Lite is a space-borne stellar interferometer capable of searching for Earth-size planets in the habitable zones of
nearby stars. This search will require measurement of astrometric angles with sub micro-arcsecond accuracy and optical
pathlength differences to 1 picometer by the end of the five-year mission. One of the most significant technical risks in
achieving this level of accuracy is from systematic errors that arise from spectral differences between candidate stars and
nearby reference stars. The Spectral Calibration Development Unit (SCDU), in operation since 2007, has been used to
explore this effect and demonstrate performance meeting SIM goals. In this paper we present the status of this testbed
and recent results.
The most stringent astrometric performance requirements on NASA's SIM(Space Interferometer
Mission)-Lite mission will come from the so-called Narrow-Angle (NA) observing scenario,
aimed at finding Earth-like exoplanets, where the interferometer chops between the target star
and several nearby reference stars multiple times over the course of a single visit. Previously,
about 20 pm NA error with various shifts was reported1. Since then, investigation has been under
way to understand the mechanisms that give rise to these shifts. In this paper we report our
findings, the adopted mitigation strategies, and the resulting testbed performance.
The SIM Lite Astrometric Observatory is to perform narrow angle astrometry to search for Earth-like planets, and global
astrometry for a broad astrophysics program, for example, mapping the distribution of dark matter in the Galaxy. The
new SIM Lite consists of two Michelson interferometers and one star tracking telescope. The main six-meter baseline
science interferometer observes a target star and a set of reference stars. The four-meter baseline interferometer (guide-1)
monitors the attitude of the instrument in the direction of a target star. The Guide-2 telescope (G2T) tracks a bright star
to monitor the attitude of the instrument in the other two orthogonal directions. A testbed has been built to demonstrate
star-tracking capability of the G2T concept using a new interferometric angle metrology system. In the presence of
simulated 0.2 arcsecond level of expected spacecraft attitude control system perturbations, the measured star-tracking
capability of the G2T testbed system is less than 43 micro-arcsecond during single narrow angle observation.
The Space Interferometer Mission (SIM) consists of three interferometers (science, guide1, and
guide2) and two optical paths (metrology and starlight). The system requirements for each
interferometer/optical path combination are different and sometimes work against each other. A
diffraction model is developed to design and optimize various masks to simultaneously meet the
system requirements of three interferometers. In this paper, the details of this diffraction model
will be described first. Later, the mask design for each interferometer will be presented to
demonstrate the system performance compliance. In the end, a tolerance sensitivity study on the
geometrical dimension, shape, and the alignment of these masks will be discussed.
A corner cube (CC) articulation model has been developed to evaluate the SIM internal metrology (IntMet) optical delay bias (with the accuracy of picometer) due to the component imperfections, such as vertex offset, reflection coating index error, dihedral error, and surface figure error at each facet. This physics-based and MATLAB-implemented geometric optics model provides useful guidance on the flight system design, integration, and characterization. The first portion of this paper covers the CC model details. Then several feature of the model, such as metrology beam footprint visualization, roofline straddling/crossing analysis, and application to drive the sub-system design and the error budget flow-down, are demonstrated in the second part.
A corner cube model is developed to calculate the SIM internal metrology optical delay bias (with the accuracy of picometer) due to the component imperfections, such as vertex offset, coating index error, dihedral error, and gimbal offset. This physics-based and Matlab-implemented ray-trace model provides useful guidance on the flight system design, integration, and characterization. In this paper, the details of the corner cube model will be described first. Then the sub-nanometer level model validation through the MAM testbed will be presented. Finally several examples of the model application, such as the metrology delay bias minimization, design parameter error budget (or tolerance) allocation, and the metrology beam prints visualization, will be shown.
The Space Interferometer Mission (SIM) flight instrument will not undergo a full performance, end-to-end system test on the ground due to a number of constraints. Thus, analysis and physics-based models will play a significant role in providing confidence that SIM will meet its science goals on orbit. The various models themselves are validated against the experimental results of severl "picometer" testbeds. In this paper we describe a set of models that are used to predict the magnitude and functional form of a class of field-dependent systematic errors for the science and guide interferometers. This set of models is validated by comparing predictions with the experimental results obtained from the MicroArcsecond Metrology (MAM) testbed and the Diffraction testbed (DTB). The metric for validation is provided by the SIM astrometric error budget.
A high efficiency, low cost, low voltage operation liquid crystal on silicon beam steering device with multiple angles addressing capability is developed. Currently, seven steering angles with as high as 92.7% efficiency are achieved within 2.85 v. The device's design consideration, fabrication process as well as the characterization results are described.
A multiple-angle liquid crystal blazed grating beam deflector has been developed. It consists of a stack of liquid crystal blazed gratings where each layer can deflect incident light with very high efficiency into one of two different directions depending on the driving condition. Four steering angles (10.8 degrees, 7.2 degrees, 3.6 degrees, 0 degrees) with about 70% diffraction efficiency are demonstrated with 15 V. The device's working principle, design considerations, fabrication process, and characterization results are described.