The Nancy Grace Roman Space Telescope, NASA's next flagship mission in astrophysics, is due for launch in May 2027 with an onboard Coronagraph Instrument (CGI) which will serve as a technology demonstrator for exoplanet direct imaging. The Roman Coronagraph will be capable of detecting and characterizing exoplanets and circumstellar disks in visible light at an unprecedented contrast level of ~108 or better at small separations. The instrument is equipped with six precision alignment mechanisms (PAMs) which enable ultra-stable, sub-micrometer positioning of optical elements such as coronagraphic masks, optical filters and polarizers. In order to achieve contrast level, which are 2 to 3 orders of magnitude better than state-of-the-art visible or near-infrared coronagraphs, the mechanisms need to be stable at sub-microradian levels during a typically 10 hour long science observation. We report here about the development of these mechanisms and present their performance test results from the qualification/flight acceptance test program. All PAM flight models were delivered in the year 2022 and integrated into the CGI flight instrument. Meanwhile CGI has successfully completed all testing at JPL and was shipped to NASA GSFC in May 2024 for final integration into the Roman spacecraft.
The Nancy Grace Roman Space Telescope is NASA’s flagship astrophysics mission planned for launch in 2026. The Coronagraph Instrument (CGI) on Roman will demonstrate the technology for direct imaging and spectroscopy of exoplanets around nearby stars. It will work with the 2.4-meter diameter telescope to achieve starlight suppression and point source detection limits that are 2–3 orders of magnitude deeper than previous space-based and groundbased coronagraphs by using active wavefront control with deformable mirrors. CGI has passed its Critical Design Review (CDR) in April of 2021, and System Integration Review (SIR) in June of 2022. We describe the status of CGI’s development and plans for the upcoming integration and testing phase.
The Nancy Grace Roman Space Telescope (formerly WFIRST) will be launched in the mid-2020s with an onboard Coronagraph Instrument which will serve as a technology demonstrator for exoplanet direct imaging. The Roman Coronagraph will be capable of detecting and characterizing exoplanets and circumstellar disks in visible light at an unprecedented contrast level of ~10-8 or better at small separations. Such a contrast level, which is 2 to 3 orders of magnitude better than state-of-the-art visible or near-infrared coronagraphs, raises entirely new challenges that will be overcome using a combination of hardware, calibration and data processing. In particular, the Roman Coronagraph will be the first space-based coronagraphic instrument with active low- and high-order wavefront control through the use of largeformat (48x48) deformable mirrors, and its electron-multiplying Charge Coupled Device (EMCCD) detector will enable faint signal detection in photon-counting mode. The Roman Coronagraph successfully passed its critical design review in April 2021 and its system integration review in June 2022. It is now well on its path to demonstrate many core technologies at the levels required for a future exo-Earth direct imaging mission.
The Nancy Grace Roman Space Telescope (formally WFIRST) will be launched in the mid-2020’s with an onboard coronagraph instrument which will serve as a technology demonstrator for exoplanet direct imaging. The Roman Coronagraph will be capable of detecting and characterizing exoplanets and circumstellar disks in visible light at an unprecedented contrast level of ~10-8 or lower. Such a contrast level, which is several magnitudes better than state-of-the-art visible or near-infrared coronagraphs, raises entirely new challenges that will be overcome using a combination of hardware, calibration and data processing. In particular, the Roman Coronagraph will be the first space-based coronagraphic instrument with real-time active wavefront control through the use of large format deformable mirrors, and its EMCCD detector will enable faint signal detection in photon-counting mode. The Roman Coronagraph instrument passed its critical design review successfully in April 2021, and is now well on its path to demonstrate many core technologies at the levels required for future exo-Earth direct imaging missions.
NASA’s Nancy Grace Roman Space Telescope (formerly known as WFIRST) is a flagship astrophysics mission planned for launch in 2025. The coronagraph instrument (CGI) on Roman will demonstrate the technology for direct imaging and spectroscopy of exoplanets around nearby stars. It will work with the 2.4-meter diameter telescope to achieve starlight suppression that is 2-3 orders of magnitude deeper than previous space-based and ground-based coronagraphs by using active wavefront control in space with deformable mirrors. CGI has passed its Preliminary Design Review (PDR) in September 2019 and is working toward the instrument Critical Design Review (CDR) in the spring of 2021. We describe the CGI engineering design going into CDR and the operational concept planned for CGI observations.
As part of its technology demonstration, the Nancy Grace Roman Space Telescope (RST) Coronagraph Instrument (CGI) will demonstrate point source spectroscopy and polarization measurements of disks. The specific implementation of spectroscopy is a zero-deviation Amici prism and a slit to be placed on the planet after high contrast has been achieved by CGI. The polarization optics are a set of Wollaston prisms so that orthogonal polarization states can be measured simultaneously. The CGI spectral characterization mode, being designed and built and Goddard Space Flight Center (GSFC), has a spectral resolution of R50 at its central wavelength and is designed to accommodate a 15% bandpass spanning 610785nm. In order to recover Stokes information, there are two sets of Wollaston prisms clocked 45 degrees with respect to one another with each measurement taken in series. The Wollaston design and optical elements are a contribution by the Japanese Aerospace Exploration Agency (JAXA), with final alignment and testing being done at GSFC. The spectroscopy mode is designed to target Methane absorption features around 730nm, keeping the spectral resolution as low as possible to improve the signal-to-noise ratio and hence reduce detection time. We highlight the requirements for these modes and address the challenge of on-orbit spectral calibration for a deployable slit in the presence of pointing drifts. Of unique interest is how the observatory error budget couples into good stellar spectrum calibration and subsequent speckle subtraction. We also provide further detail on the optomechanical design, its modeled performance, and operations concept. These performance metrics are simulated to demonstrate how a slit located at an arbitrary field point is homed onto the planet and converted to a calibrated spectrum.
To create a high contrast field of view with a shaped pupil coronagraph (SPC), the telescope pupil is imaged onto the shaped pupil mask, the shaped pupil mask is designed and fabricated through a design optimization based on a given pupil wavefront, and deformable mirrors (DM) in the coronagraph are modulated according to a nonlinear optimization procedure. Both the mask design optimization and the DM optimization require accurate knowledge of the image of the pupil at the shaped pupil plane for the calculation of Jacobians, and the evaluation of alignment tolerances. The shaped pupil coronagraphs demonstrated in the high contrast imaging testbed (HCIT) at JPL has heretofore assumed perfect imaging of the pupil onto the shaped pupil mask. Similarly, the design and performance modeling of the WFIRST CGI instrument makes the same assumption. In this paper we estimate and compare the image of the pupil mask at the shaped pupil plane in the case of the coronagraph testbed, and the image of the telescope pupil at the shaped pupil plane for the WFIRST CGI instrument. The estimated pupil images include field curvature and aberrations resulting from the layout of the parabolic mirror imaging system, which is optimized for the source image, not the image of the pupil. We also show the results of direct measurement of the image of the pupil mask at the shaped pupil plane in the coronagraph testbed.
Direct imaging of an Earth-like exoplanet requires starlight suppression with a contrast ratio on the order of 1×10-10 at small angular separations of 100 milliarcseconds or less in visible light. To aid the technology development to reach this capability and enable future exoplanet missions, we built a high contrast coronagraph testbed, titled the Decadal Survey Testbed (DST). As of early 2019, the testbed has repeatedly demonstrated a monochromatic contrast floor about 1×10-10, and broadband performance at 550 nm with 10% color band- width <4×10-10 . The testbed has also demonstrated open-loop contrast drift rates of around 10-10/hour, temperature drift stabilities of <10 milliKelvins/day, passive pointing stability of around 0.1 λ/D per day on the occulting mask, and rms pointing jitter around 0.005 λ/D. This paper focusses primarily on the testbed hardware description, and a companion paper by Seo et al. details the experimental results.
KEYWORDS: Coronagraphy, Exoplanets, Space telescopes, James Webb Space Telescope, Infrared telescopes, Visible radiation, Aerospace engineering, Space operations, Telescopes, Near infrared
Direct imaging of an Earth-like exoplanet requires starlight suppression with a contrast ratio on the order of 1 ♦ 10-10 at small angular separations of 100 milliarcseconds or less in visible light with more than 50 nm bandwidth. To our knowledge, the technology needed to achieve the contrast and stability has not been demonstrated as of January 2019. The science requirements for near future NASA missions such as James Webb Space Telescope’s (JWST) Near Infrared Camera (NIRCam) coronagraph and Wide Field Infrared Space Telescope (WFIRST) Coronagraph Instrument (CGI) are at least 10 times short. To investigate and guide the technology to reach this capability, we built a high contrast coronagraph testbed at NASA’s Jet Propulsion Laboratory (JPL). Titled the Decadal Survey Testbed (DST), state-of-art testbed is based on the accumulated experience of JPL’s High Contrast Imaging Testbed (HCIT) team. Currently, the DST hosts a Hybrid Lyot Coronagraph (HLC) with an unobscured, circular pupil. The DST also has two deformable mirrors and is equipped with the Low Order Wavefront Sensing and Control (LOWFS/C) subsystem to sense and correct the dynamic wavefront disturbances. In this paper, we present up-to-date progress of the testbed demonstration. As of January 2019, we repeatedly obtain convergence below 4 × 10-10 mean contrast with 10% broadband light centered at 550 nm in a 360 degrees dark hole with a working angle between 3 λ/D and 9 λ/D. We show the key elements used in the testbed and the performance results with associated analysis.
The WFIRST Coronagraph Instrument will perform direct imaging of exoplanets via coronagraphy of the host star. The Phase B optical design fits within a new allocated instrument enclosure and accommodates both Hybrid Lyot and Shaped Pupil coronagraphs. It provides optical path and space for accessible focal planes for occulting masks and field stops. It provides accessible pupil planes for shaped pupil and Lyot masks. It accommodates ten active optical assemblies, including one fast steering mirror, on focusing mirror, two deformable mirrors, and six precision alignment mechanisms. We present the optical design and analyses for the Direct Imaging channel, including polarization imaging. We also present the performance analysis of pupil imaging for starlight illumination and diffused light illumination of the pupil.
WFIRST Coronagraph Instrument (CGI) uses its Low Order Wavefront Sensing and Control (LOWFS/C) subsystem to maintain the coronagraph contrast in presence of wavefront disturbances from the WFIRST observatory. The Occulting Mask Coronagraph (OMC) dynamic testbed is used to demonstrate LOWFS/C performance together with the coronagraphs. We have demonstrated that LOWFS/C can maintain coronagraph contrast to better than 10-8 in presence of WFIRST-like LoS and low order WFE disturbances for both Hybrid Lyot Coronagraph (HLC) and Shaped Pupil Coronagraph (SPC) modes. However, in our previous tests the LoS disturbances simulated on the testbed were from the modeling of a single reaction wheel with a quasi-static wheel speed. In this paper we will describe the LOWFS/C LoS performance test in which the injected LoS disturbances are derived from all six reaction wheels on WFIRST, each with an independent varying wheel speed profile modeled for a typical coronagraph observational scenario. We will describe the LoS disturbances profile and testbed implementation approach. We will present the LOWFS/C LoS feedback and feed forward loops testbed performance under these realistic LoS disturbances in which we have demonstrated the LoS control that meets the CGI requirement.
Direct imaging of Earth-like exoplanets in reflected starlight requires high contrast imaging systems, such as coronagraph, with starlight suppression better than 1e-10 over a broad spectral band. Such deep contrast can only be achieved by means of wavefront control techniques (e.g. speckle nulling, electric field conjugation, stroke minimization…) where deformable mirrors correct for low and mid-frequency optical aberrations. In space, the coronagraph with wavefront control will create high contrast regions using a target star before pointing to a science star. While re-pointing the space-telescope, the surface stability of the deformable mirrors will play a very important role in maintaining the high contrast regions. In the present work, we report an optical characterization and stability analysis of MEMS deformable mirrors for high contrast imaging using a vortex coronagraph in vacuum to simulate space conditions. We created high contrast regions using electric field conjugation and monitored its evolution for several hours using pair-wise estimation.
In order to validate required operation of the proposed Wide-Field InfraRed Survey Telescope (WFIRST) coronagraph instrument, we have built a testbed in Jet Propulsion Laboratory (JPL), which is analogous to the baseline WFIRST coronagraph instrument architecture. Since its birth in 2016, this testbed, named as Occulting Mask Coronagraph (OMC) testbed, has demonstrated several crucial technological milestones: Broadband high contrast demonstration in both Hybrid Lyot Coronagraph (HLC) and Shape Pupil Coronagraph (SPC) modes while the Low Order Wavefront Sensing and Control (LOWFS/C) subsystem senses and corrects the dynamic flight-like wavefront disturbances. In this paper, we present up-to-date progress of HLC mode demonstration in the OMC testbed. While injecting the flight-like low photon flux starlight with expected Line of Sight (LoS) and Wavefront Error (WFE) perturbation to the OMC testbed, we demonstrate generating high contrast dark hole images. We first study the expected photon flux in actual flight environment, and estimate detection noise and estimation accuracy of the complex electric field if the wavefront sensing algorithm is used based on the pair-wise difference imaging. Then, we introduce our improved scheme to mitigate this photon-starved flight-like low flux environment. As a result, we generate a dark hole that meets the WFIRST raw contrast requirements using the 2nd magnitude star light. We establish the key ideas, describe test setups, and demonstrate test results with data analysis.
To maintain the required performance for the WFIRST Coronagraph Instrument (CGI) in a realistic space environment, a Low Order Wavefront Sensing and Control (LOWFS/C) subsystem is necessary. The WFIRST CGI LOWFS/C subsystem will use the Zernike wavefront sensor, which has a phase-shifting disk combined with the coronagraph’s focal plane mask, to sense the low-order wavefront drift and line-of-sight (LoS) error using the rejected starlight. The dynamic tests on JPL’s Occulting Mask Coronagraph (OMC) Testbed have demonstrated that LOWFS/C can maintain coronagraph contrast to better than 10-8 in presence of WFIRST-like line of sight and low order wavefront disturbances in both Shaped Pupil Coronagraph (SPC) and Hybrid Lyot Coronagraph (HLC) modes. However, the previous dynamic tests have been done using a bright source with photon flux equivalent to stellar magnitude of MV = -3.5. The LOWFS/C technology development on the OMC testbed has since then concentrated in evaluating and improving the LOWFS/C performance under the realistic photon flux that is equivalent to WFIRST Coronagraph target stars. Our recent testbed tests have demonstrated that the LOWFS/C can work cohesively with the stellar light suppression wavefront control, which brings broad band coronagraph contrast from ~1x10-6 to 6x10-9, while LOWF/C is simultaneously suppressing the WFIRST like LoS and low order wavefront drift disturbances on a source that photon flux is equivalent to a MV = 2 star. This lab demonstration mimics the CGI initial dark hole establish process on a bright reference star. We have also demonstrated on the testbed that LOWFS/C can maintain the coronagraph contrast by suppressing the WFIRST like line-of-sight disturbances on a fainter MV = 5 star. This mimics scenario of CGI science target observations. In this paper we will present the recent dynamic testbed performance results of LOWFS/C LoS loops and low order wavefront error correction loop on the flight like photon flux.
The Shaped Pupil Coronagraph (SPC) is one of the two operating modes of the baseline coronagraph instrument for the proposed WFIRST mission. While in SPC mode, multiple sets of shaped pupil masks and focal plane masks would be available for various imaging tasks. The disk science mask set (SPC-DSM) is designed for exozodiacal disk science. With a 360 degree high contrast field of view, extending up to 20 λ/D, the SPC-DSM provides a powerful tool to study exozodiacal dust clouds associated with stellar debris disks to gain insight of the exoplanet formation and stellar disk dynamics. We will describe the performance verification and demonstration of the SPC-DSM coronagraph as tested in the high contrast imaging testbed (HCIT) at JPL. The goal of the testbed demonstration is an average contrast of 5e-9 over a 10% bandwidth centered at 565nm, in a field of view extending from 6.5 λ/D to 20 λ/D. We will discuss electric field conjugation, performance metrics, and model agreement as applied to the SPC-DSM.
NASA WFIRST mission has planned to include a coronagraph instrument to find and characterize exoplanets. Masks are needed to suppress the host star light to better than 10-8 – 10-9 level contrast over a broad bandwidth to enable the coronagraph mission objectives. Such masks for high contrast coronagraphic imaging require various fabrication technologies to meet a wide range of specifications, including precise shapes, micron scale island features, ultra-low reflectivity regions, uniformity, wave front quality, etc. We present the technologies employed at JPL to produce these pupil plane and image plane coronagraph masks, and lab-scale external occulter masks, highlighting accomplishments from the high contrast imaging testbed (HCIT) at JPL and from the high contrast imaging lab (HCIL) at Princeton University. Inherent systematic and random errors in fabrication and their impact on coronagraph performance are discussed with model predictions and measurements.
The Shaped Pupil Coronagraph (SPC) is one of the two operating modes of the WFIRST coronagraph instrument. The SPC provides starlight suppression in a pair of wedge-shaped regions over an 18% bandpass, and is well suited for spectroscopy of known exoplanets. To demonstrate this starlight suppression in the presence of expected onorbit input wavefront disturbances, we have recently built a dynamic testbed at JPL analogous to the WFIRST flight instrument architecture, with both Hybrid Lyot Coronagraph (HLC) and SPC architectures and a Low Order Wavefront Sensing and Control (LOWFS/C) subsystem to apply, sense, and correct dynamic wavefront disturbances. We present our best up-to-date results of the SPC mode demonstration from the testbed, in both static and dynamic conditions, along with model comparisons. HLC results will be reported separately.
Hybrid Lyot Coronagraph (HLC) is one of the two operating modes of the Wide-Field InfraRed Survey Telescope (WFIRST) coronagraph instrument. Since being selected by National Aeronautics and Space Administration (NASA) in December 2013, the coronagraph technology is being matured to Technology Readiness Level (TRL) 6 by 2018. To demonstrate starlight suppression in presence of expecting on-orbit input wavefront disturbances, we have built a dynamic testbed in Jet Propulsion Laboratory (JPL) in 2016. This testbed, named as Occulting Mask Coronagraph (OMC) testbed, is designed analogous to the WFIRST flight instrument architecture: It has both HLC and Shape Pupil Coronagraph (SPC) architectures, and also has the Low Order Wavefront Sensing and Control (LOWFS/C) subsystem to sense and correct the dynamic wavefront disturbances. We present upto-date progress of HLC mode demonstration in the OMC testbed. SPC results will be reported separately. We inject the flight-like Line of Sight (LoS) and Wavefront Error (WFE) perturbation to the OMC testbed and demonstrate wavefront control using two deformable mirrors while the LOWFS/C is correcting those perturbation in our vacuum testbed. As a result, we obtain repeatable convergence below 5 × 10−9 mean contrast with 10% broadband light centered at 550 nm in the 360 degrees dark hole with working angle between 3 λ/D and 9 λ/D. We present the key hardware and software used in the testbed, the performance results and their comparison to model expectations.
To maintain the required performance of WFIRST Coronagraph in a realistic space environment, a Low Order Wavefront Sensing and Control (LOWFS/C) subsystem is necessary. The LOWFS/C uses a Zernike wavefront sensor (ZWFS) with the phase shifting disk combined with the starlight rejecting occulting mask. For wavefront error corrections, WFIRST LOWFS/C uses a fast steering mirror (FSM) for line-of-sight (LoS) correction, a focusing mirror for focus drift correction, and one of the two deformable mirrors (DM) for other low order wavefront error (WFE) correction. As a part of technology development and demonstration for WFIRST Coronagraph, a dedicated Occulting Mask Coronagraph (OMC) testbed has been built and commissioned. With its configuration similar to the WFIRST flight coronagraph instrument the OMC testbed consists of two coronagraph modes, Shaped Pupil Coronagraph (SPC) and Hybrid Lyot Coronagraph (HLC), a low order wavefront sensor (LOWFS), and an optical telescope assembly (OTA) simulator which can generate realistic LoS drift and jitter as well as low order wavefront error that would be induced by the WFIRST telescope’s vibration and thermal changes. In this paper, we will introduce the concept of WFIRST LOWFS/C, describe the OMC testbed, and present the testbed results of LOWFS sensor performance. We will also present our recent results from the dynamic coronagraph tests in which we have demonstrated of using LOWFS/C to maintain the coronagraph contrast with the presence of WFIRST-like line-of-sight and low order wavefront disturbances.
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.
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
LOWFS testbed.
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.
The most recent concept for the Wide Field Infrared Survey Telescope (WFIRST) Design Reference Mission (DRM) features an instrument that will perform exoplanet detection via coronagraphy of the host star. This observatory is based on the existing Astrophysics Focused Telescope Asset’s (AFTA) 2.4-meter telescope. The WFIRST/AFTA Coronagraph Instrument combines the Hybrid Lyot and Shaped Pupil Coronagraphs to meet the science requirements. The cycle 5 optical design fits the required enclosure and accommodates both coronagraphic architectures. We present the optical performance including throughput of both the imaging and the IFS channels, the wavefront error at the first pupil, and polarization effects from optical coatings.
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.
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.
The WFIRST/AFTA 2.4 m space telescope currently under study includes a stellar coronagraph for the imaging and the spectral characterization of extrasolar planets. The coronagraph employs two sequential deformable mirrors (DMs) to compensate for phase and amplitude errors in creating dark holes. DMs are critical elements in high contrast coronagraphs, requiring precision and stability measured in picometers to enable detection of Earth-like exoplanets. Working with a low-order wavefront-sensor the DM that is conjugate to a pupil can also be used to correct low-order wavefront drift during a scientific observation. However, not all actuators in a DM have the same gain. When using such a DM in low-order wavefront sensing and control subsystem, the actuator gain calibration errors introduce highspatial frequency errors to the DM surface and thus worsen the contrast performance of the coronagraph. We have investigated the effects of actuator gain calibration errors and the actuator command digitization errors on the contrast performance of the coronagraph through modeling and simulations, and will present our results in this paper.
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.
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.
KEYWORDS: Calibration, Sensors, Monte Carlo methods, James Webb Space Telescope, Signal detection, Error analysis, Detection and tracking algorithms, Space telescopes, Wavefronts, Telescopes
Dispersed Fringe Sensing (DFS) is an elegant method of coarse phasing segmented mirrors. DFS performance
accuracy is dependent upon careful calibration of the system as well as other factors such as internal optical
alignment, system wavefront errors, and detector quality. Novel improvements to the algorithm have led to
substantial enhancements in DFS performance. In this paper, we present Advanced DFS, an advancement of
the DFS algorithm, which allows the overall method to be less sensitive to calibration errors. This is achieved
by correcting for calibration errors, which appear in the fitting equations as a signal phase term. This paper will
outline a brief analytical explanation of the improvements, results of advanced DFS processed simulations and
experimental advanced DFS results.
Phase retrieval is an image-based wavefront sensing process, used to recover phase information from defocused
stellar images. Phase retrieval has proven to be useful for diagnosis of optical aberrations in space telescopes,
calibration of adaptive optics systems, and is intended for use in aligning and phasing the James Webb Space
Telescope. This paper describes a robust and accurate phase retrieval algorithm for wavefront sensing, which has
been successfully demonstrated on a variety of testbeds and telescopes. Key features, such as image preprocessing,
diversity adaptation, and prior phase nulling, are described and compared to other methods. Results demonstrate
high accuracy and high dynamic range wavefront sensing.
The Advanced Wavefront Sensing and Control Testbed (AWCT) is built as a versatile facility for developing and
demonstrating, in hardware, the future technologies of wavefront sensing and control algorithms for active optical
systems. The testbed includes a source projector for a broadband point-source and a suite of extended scene targets, a
dispersed fringe sensor, a Shack-Hartmann camera, and an imaging camera capable of phase retrieval wavefront
sensing. The testbed also provides two easily accessible conjugated pupil planes which can accommodate active optical
devices such as fast steering mirror, deformable mirror, and segmented mirrors. In this paper, we describe the testbed
optical design, testbed configurations and capabilities, as well as the initial results from the testbed hardware
integrations and tests.
The baseline wavefront sensing and control for James Webb Space Telescope (JWST) includes the Dispersed Hartmann
Sensors (DHS) for segment mirror coarse phasing. The two DHS devices, residing on the pupil wheel of the JWST's
Near Infrared Camera (NIRCam) Short Wavelength Channel (SWC), can sense the JWST segment mirror pistons by
measuring the heights of 20 inter-segment edges from the dispersed fringes. JWST also incorporates two identical
grisms in the NIRCam's Long Wavelength Channel (LWC). The two grisms, designed as the Dispersed Fringe Sensor
(DFS), are used as the backup sensor for JWST segment mirror coarse phasing. The versatility of DFS enables a very
flexible JWST segment coarse phasing process and the DFS is designed to have larger piston capture range than that of
DHS, making the coarse phasing more robust. The DFS can also be a useful tool during JWST ground integration and
test (I&T). In this paper we describe the DFS design details and use the JWST optical model to demonstrate the DFS
coarse phasing process during flight and ground I&T.
NASA's Technology Readiness Level (TRL)-6 is documented for the James Webb Space Telescope (JWST) Wavefront
Sensing and Control (WFSC) subsystem. The WFSC subsystem is needed to align the Optical Telescope Element
(OTE) after all deployments have occurred, and achieves that requirement through a robust commissioning sequence
consisting of unique commissioning algorithms, all of which are part of the WFSC algorithm suite. This paper identifies
the technology need, algorithm heritage, describes the finished TRL-6 design platform, and summarizes the TRL-6 test
results and compliance. Additionally, the performance requirements needed to satisfy JWST science goals as well as the
criterion that relate to the TRL-6 Testbed Telescope (TBT) performance requirements are discussed.
The one-meter Testbed Telescope (TBT) has been developed at Ball Aerospace to facilitate the
design and implementation of the wavefront sensing and control (WFSC) capabilities of the
James Webb Space Telescope (JWST). We have recently conducted an "end-to-end"
demonstration of the flight commissioning process on the TBT. This demonstration started with
the Primary Mirror (PM) segments and the Secondary Mirror (SM) in random positions,
traceable to the worst-case flight deployment conditions. The commissioning process detected
and corrected the deployment errors, resulting in diffraction-limited performance across the
entire science FOV. This paper will describe the commissioning demonstration and the WFSC
algorithms used at each step in the process.
KEYWORDS: Stars, James Webb Space Telescope, Exoplanets, Sensors, Signal to noise ratio, Planets, Spectroscopy, Optical filters, Jupiter, Spatial resolution
The near-infrared camera (NIRCam) on the James Webb Space Telescope (JWST) will incorporate 2 identical
grisms in each of its 2 long wavelength channels. These transmission gratings have been added to assist with
the coarse phasing of the JWST telescope, but they will also be used for slitless wide-field scientific observations
over selectable regions of the λ = 2.4 − 5.0 μm wavelength range at spectroscopic resolution R ≡ λ/δλ ≃ 2000.
We describe the grism design details and their expected performance in NIRCam. The grisms will provide point-source
continuum sensitivity of approximately AB = 23 mag in 10,000 s exposures with S/N = 5 when binned
to R = 1000. This is approximately a factor of 3 worse than expected for the JWST NIRSpec instrument, but
the NIRCam grisms provide better spatial resolution, better spectrophotometric precision, and complete field
coverage. The grisms will be especially useful for high precision spectrophotometric observations of transiting
exoplanets. We expect that R = 500 spectra of the primary transits and secondary eclipses of Jupiter-sized
exoplanets can be acquired at moderate or high signal-to-noise for stars as faint as M = 10 − 12 mag in 1000 s of
integration time, and even bright stars (V = 5 mag) should be observable without saturation. We also discuss
briefly how these observations will open up new areas of exoplanet science and suggest other unique scientific
applications of the grisms.
KEYWORDS: Coronagraphy, Stars, Planets, James Webb Space Telescope, Point spread functions, Wavefronts, Telescopes, Space telescopes, Sensors, Diffraction
The expected stable point spread function, wide field of view, and sensitivity of the NIRCam instrument on the James
Webb Space Telescope (JWST) will allow a simple, classical Lyot coronagraph to detect warm Jovian-mass companions
orbiting young stars within 150 pc as well as cool Jupiters around the nearest low-mass stars. The coronagraph can also
be used to study protostellar and debris disks. At λ = 4.5 μm, where young planets are particularly bright relative to their
stars, and at separations beyond ~0.5 arcseconds, the low space background gives JWST significant advantages over
ground-based telescopes equipped with adaptive optics. We discuss the scientific capabilities of the NIRCam
coronagraph, describe the technical features of the instrument, and present end-to-end simulations of coronagraphic
observations of planets and circumstellar disks.
Space coronagraphy is a promising method for direct imaging of planetary systems orbiting the nearby stars. The High
Contrast Imaging Testbed is a laboratory facility at JPL that integrates the essential hardware and control algorithms
needed for suppression of diffracted and scattered light near a target star that would otherwise obscure an associated
exo-planetary system. Stable suppression of starlight by a factor of 5×10−10 has been demonstrated consistently in
narrowband light over fields of view as close as four Airy radii from the star. Recent progress includes the extension of
spectral bandwidths to 10% at contrast levels of 2×10−9, with work in progress to further improve contrast levels,
bandwidth, and instrument throughput. We summarize recent laboratory results and outline future directions. This
laboratory experience is used to refine computational models, leading to performance and tolerance predictions for
future space mission architectures.
KEYWORDS: Coronagraphy, Light sources, Photomasks, Algorithm development, Wavefronts, Modeling and simulation, Near field optics, Phase measurement, Data modeling, Optical components
The performance of the high-contrast imaging testbed (HCIT) at JPL is investigated through optical modeling and
simulations. The analytical tool is an optical simulation algorithm developed by combining the HCIT's optical model
with a speckle-nulling algorithm that operates directly on coronagraphic images, an algorithm identical to the one
currently being used on the HCIT to actively suppress scattered light via a deformable mirror. It is capable of
performing full three-dimensional end-to-end near-field diffraction analysis on the HCIT's optical system. By
conducting extensive speckle-nulling optimization, we clarify the HCIT's capability and limitations in terms of its
contrast performance under various realistic conditions. Considered cases include non-ideal occulting masks, such as a
mask with parasitic phase-delay errors (i.e., a not band-limited occulting mask) and one with damped ripples in its
transmittance profiles, as well as the phase errors of all optics. Most of the information gathered on the HCIT's optical
components through measurement and characterization over the last several years at JPL has been used in this analysis to
make the predictions as accurate as possible. Our simulations predict that the contrast values obtainable on the HCIT
with narrow-band (monochromatic) illumination at 785nm wavelength are Cm=1.58x10-11 (mean) and C4=5.11x10-11(at
4λ/D), in contrast to the measured results of Cm~6×10-10 and C4~8×10-10, respectively. In this paper we report our
findings about the monochromatic light performance of the HCIT. We will describe the results of our investigation
about the HCIT's broad-band performance in an upcoming paper.
The one-meter Testbed Telescope (TBT) has been developed at Ball Aerospace to facilitate the
design and implementation of the wavefront sensing and control (WFS&C) capabilities of the
James Webb Space Telescope (JWST). The TBT is used to develop and verify the WFS&C
algorithms, check the communication interfaces, validate the WFS&C optical components and
actuators, and provide risk reduction opportunities for test approaches for later full-scale
cryogenic vacuum testing of the observatory. In addition, the TBT provides a vital opportunity
to demonstrate the entire WFS&C commissioning process. This paper describes recent WFS&C
commissioning experiments that have been performed on the TBT.
The Shaped Pupil Coronagraph (SPC) is a high-contrast imaging system pioneered at Princeton for detection of extra-solar earthlike planets. It is designed to achieve 10-10 contrast at an inner working angle of 4λ/D. However, a critical requirement in attaining this contrast level in practice is the ability to control wavefront phase and amplitude aberrations to at least λ/104 in rms phase and 1/1000 rms amplitude, respectively. Furthermore, this has to be maintained over a large spectral band. The High Contrast Imaging Testbed (HCIT) at the Jet Propulsion Lab (JPL) is a state-of-the-art facility for studying high contrast imaging systems and fine wavefront control methods. It consists of a vacuum chamber containing a configurable coronagraph setup with a Xinetics deformable mirror. In this paper, we present the results of testing Princeton's SPC in JPL's HCIT. In particular, we present the achievement of 4x10-8 contrast using a speckle nulling algorithm, and demonstrate that this contrast is maintained across wavelengths of 785, 836nm, and for broadband light having 10% bandwidth around 800nm.
The James Webb Space Telescope (JWST) Coarse Phase Sensor utilizes Dispersed Hartmann Sensing (DHS)1 to measure the inter-segment piston errors of the primary mirror. The DHS technique was tested on the Keck Telescope. Two DHS optical components were built to mate with the Keck optical and mechanical interfaces. DHS images were acquired using 20 different primary mirror configurations. The mirror configurations consisted of random segment pistons applied to 18 of the 36 segments. The inter-segment piston errors ranged from phased (approximately 0 μm) to as large as ±25 μm. Two broadband exposures were taken for each primary mirror configuration: one for the DHS component situated at 0°, and one for the DHS component situated at 60°. Finally, a "closed-loop" DHS sensing and control experiment was performed. Sensing algorithms developed by both Adaptive Optics Associates (AOA) and the Jet Propulsion Laboratory (JPL)2 were applied to the collected DHS images. The inter-segment piston errors determined by the AOA and JPL algorithms were compared to the actual piston steps. The data clearly demonstrates that the DHS works quite well as an estimator of segment-to-segment piston errors using stellar sources.
Dispersed Fringe Sensing (DFS) is an efficient and robust method for coarse phasing of a segmented primary mirror
such as the James Webb Space Telescope (JWST). In this paper, modeling and simulations are used to study the effect
of segmented mirror aberrations on the DFS fringe image, its signals, and the piston detection accuracy. The simulations
show that due to the pixilation spatial filter effect from DFS signal extraction the effect of wavefront error is reduced. In
addition, the DFS algorithm is more robust against wavefront aberration when the multi-trace DFS approach is used.
We have also studied the JWST Dispersed Hartmann Sensor (DHS) performance in presence of wavefront aberrations
caused by the gravity sag and we have used the scaled gravity sag to explore the JWST DHS performance relationship
with the level of the wavefront aberration. As a special case of aberration we have also included the effect from line-of-sight
jitter in the JWST modeling study.
Knowledge of wavefront amplitude is as important as the knowledge of phase for a coronagraphic high contrast imaging system. Efforts have been made to understand various contributions to the amplitude variation in Terrestrial Planet Finder's (TPF) High Contrast Imaging Testbed (HCIT). Modeling of HCIT with as-built mirror surfaces has shown an amplitude variation of 1.3% due to the phase-amplitude mixing for the testbed's front-end optics. Experimental measurements on the testbed have shown the amplitude variation is about 2.5% with the testbed's illumination pattern having a major contribution to the low order amplitude variation.
KEYWORDS: Visibility, Image segmentation, Point spread functions, James Webb Space Telescope, Diffraction, Sensors, Modulation, Wavefronts, Mirrors, Signal processing
Dispersed Fringe Sensing (DFS) is an efficient and robust method for coarse phasing of a segmented primary mirror such as the James Webb Space Telescope (JWST). Results from testbed experiments and modeling have shown that among the many factors that affect the performance of DFS, the diffraction from segment aperture and the interference between the segment wavefronts have the most intrinsic influence on the DFS performance. In this paper, modeling and simulations based on diffraction are used to study the formation of DFS fringe and fringe properties such as visibility. We examine the DFS piston detection process and explore the limitation of DFS wavefront piston detection accuracy and the DFS dynamic range under different segment aperture geometries, aperture orientations, and image samplings.
Predictions of contrast performance for the Eclipse coronagraphic telescope are based on computational models that are tested and validated with laboratory experience. We review recent laboratory work in the key technology areas for an actively-corrected space telescope designed for extremely high contrast imaging of nearby planetary systems. These include apodized coronagraphic masks, precision deformable mirrors, and coronagraphic algorithms for wavefront sensing and correction, as integrated in the high contrast imaging testbed at JPL. Future work will focus on requirements for the Terrestrial Planet Finder coronagraph mission.
The Wavefront Control Testbed (WCT) was created to develop and test wavefront sensing and control algorithms and software for the segmented James Webb Space Telescope (JWST). Last year, we changed the system configuration from three sparse aperture segments to a filled aperture with three pie shaped segments. With this upgrade we have performed experiments on fine phasing with line-of-sight and segment-to-segment jitter, dispersed fringe visibility and grism angle; high dynamic range tilt sensing; coarse phasing with large aberrations, and sampled sub-aperture testing. This paper reviews the results of these experiments.
The Northrup-Grummann/Ball/Kodak team is building the James Webb Space Telescope (JWST), scheduled for launch in 2011. Part of Ball’s responsibility is to develop the wavefront sensing and control (WFS&C) algorithms and software that will be used to provide the level of imaging performance needed to support the mission’s science objectives. Wavefront sensing on JWST differs from that performed on many ground-based telescopes in that it is conducted entirely within the focal plane of it’s chief science camera, the Near Infrared Camera (NIRCam). In a sense, the complexity of a conventional wavefront sensor is eliminated, in favor of rather complex image processing performed on the ground, to extract the wavefront information. This paper will describe the algorithms being developed for JWST. Specifically, we will describe algorithms for the coarse alignment of the primary mirror segments and the secondary mirror, the coarse phasing of the primary mirror segments, and the fine phasing of the entire telescope. We will also present algorithms for monitoring the wavefront quality throughout the JWST mission.
The James Webb Space Telescope (JWST) will use image-based wavefront sensing to align the telescope optics and achieve diffraction-limited performance at 2 µm. The Phase Retrieval Camera (PRC) is a high-accuracy, image-based wavefront sensor that was built for the optical characterization of JWST technology-demonstrator mirrors. Recently, experiments with the PRC were performed at the NASA Marshall Space Flight Center to measure the cryogenic surface figure of the beryllium Advanced Mirror System Demonstrator (AMSD). This paper describes the results of these experiments. Using the Modified Gerchberg-Saxton phase retrieval algorithm (JWST’s baseline method for fine-phasing), the PRC measured wavefront aberrations that were as large as 10 waves peak-to-valley (wavefront) in the optical system. A comparison between the PRC results and measurements acquired with an Instantaneous Phase Interferometer will also be presented.
The Terrestrial Planet Finder (TPF) mission is planning to launch a visible coronagraphic space telescope in 2014. To achieve TPF science goals, the coronagraph must have extreme levels of wavefront correction (less than 1 Å rms over controllable spatial frequencies) and stability to get the necessary suppression of diffracted starlight (~1E-10 contrast at an angular separation ~4 lambda/D). TPF Coronagraph’s primary platform for experimentation is the High Contrast Imaging Testbed, which will provide laboratory validation of key technologies as well as demonstration of a flight-traceable approach to implementation. Precision wavefront control in the testbed is provided by a high actuator density deformable mirror. Diffracted light control is achieved through use of occulting or apodizing masks and stops. Contrast measurements will establish the technical feasibility of TPF requirements, while model and error budget validation will demonstrate implementation viability. This paper describes the current testbed design, development approach, and recent experimental results.
Dispersed Fringe Sensing (DFS) is an efficient and robust method for coarse phasing of segmented primary mirrors (from a quarter of a wavelength up to the depth of focus of a single segment, typically several tens of microns). Unlike phasing techniques currently used for ground-based segmented telescopes, DFS does not require the use of edge sensors to sense changes in the relative heights of adjacent segments; this makes it particularly well-suited to the phasing of space-borne segmented telescopes, such as the James Webb Space Telescope (JWST). In this work we validate DFS by using it to measure the pistons of the segments of one of the Keck telescopes; the results agree with those of the Shack-Hartmann based phasing scheme currently in use at Keck to within 2% over a range of initial piston errors of ±16 μm.
The Segmented Telescope Control Software (STCS) uses science camera information to align and phase a deployable segmented optical telescope. It was developed the for the Next Generation Space Telescope (NGST) and has been successfully utilized on the Wavefront Control Testbed (WCT) for NGST and a portable phase retrieval camera (PPRC) system. The software provides an operating environment that will be used for the prime contractor's testbeds for NGST, and will eventually evolve into the Wavefront Sensing and Control (WFS&C) ground support software for NGST. This paper describes the engineering version of the STCS, the algorithms it incorporates, and methods of communicating with the testbed hardware.
The NGST Wavefront Control Testbed (WCT) is a joint technology program managed by the Goddard Space Flight Center (GSFC) and the Jet Propulsion Laboratory (JPL) for the purpose of developing technologies relevant to the NGST optical system. The WCT provides a flexible testing environment that supports the development of wavefront sensing and control algorithms that may be used to align and control a segmented optical system. WCT is a modular system consisting of a Source Module (SM), Telescope Simulator Module (TSM) and an Aft-Optics (AO) bench. The SM incorporates multiple sources, neutral density filters and bandpass filters to provide a customized point source for the TSM. The telescope simulator module contains a flip-in mirror that selects between a small deformable mirror and three actively controlled spherical mirror segments. The TSM is capable of delivering a wide range of aberrated, unaberrated, continuous and segmented wavefronts to the AO optical bench for analysis. The AO bench consists of a series of reflective and transmissive optics that images the exit pupil of the TSM onto a 349 actuator deformable mirror that is used for wavefront correction. A Fast Steering Mirror (FSM) may be inserted into the system (AO bench) to investigate image stability and to compensate for systematic jitter when operated in a closed loop mode. We will describe the optical design and performance of the WCT hardware and discuss the impact of environmental factors on system performance.
A method of coarse phasing segmented mirrors using white light interferometry (WLI) has been developed for the Next Generation Space Telescope (NGST) wavefront sensing and control. Using the broadband point PSF of the segmented mirrors taken during a segment piston scan, the WLI can accurately detect small residual piston errors. WLI does not rely on extra optics and uses only the final imaging camera. With its high sensitivity to small segment piston error WLI can be used as a complementary phasing algorithm to the dispersed fringe sensor (DFS) for NGST. This paper will present the results from modeling and experiment on the NGST's Wavefront Control Testbed (WCT).
The NGST Phase Retrieval Camera (PRC) is a portable wavefront sensor useful for optical testing in high-vibration environments. The PRC uses focus-diverse phase retrieval to measure the wavefront propagating from the optical component or system under test. Phase retrieval from focal plane images is less sensitive to jitter than standard pupil plane interferometric measurements; the PRC's performance is further enhanced by using a high-speed shutter to freeze out seeing and jitter along with a reference camera to maintain the correct boresight in defocused images. The PRC hardware was developed using components similar to those in NGST's Wavefront Control Testbed (WCT), while the PRC software was derived from WCT's extensive software infrastructure. Primary applications of the PRC are testing and experimenting with NGST technology demonstrator mirrors, along with exploring other wavefront sensing and control problems not easily studied using WCT. An overview of the hardware and testing results will be presented.
KEYWORDS: Point spread functions, Image segmentation, Mirrors, Wavefronts, Data modeling, Received signal strength, Space telescopes, Image processing, Computer simulations, Wavefront sensors
A technique for measuring the low-order wavefront aberrations in segmented-mirror telescopes using in-focus point-spread functions -"PSF Monitoring" - has been developed for the Next Generation Space Telescope (NGST). PSF Monitoring will enable the continuous monitoring of the mirror segment alignment using the PSFs readily available in science data. An extension of PSF Monitoring - in-focus wavefront control, or the explicit determination and correction of wavefront errors from in-focus images - may allow for the nominal maintenance of the NGST mirror alignment without detracting from valuable science observing time. PSF Monitoring and in-focus wavefront control have been rigorously tested on the segmented aperture system of the NGST Wavefront Control Testbed (WCT). This paper presents the results of our experiments and simulations to characterize the capture range and accuracy on WCT, as well as a two-wavelength algorithm that has been used to extend the piston capture range. A real-time PSF Monitoring and control experiment on WCT will also be presented. Finally, we show preliminary simulation results of PSF Monitoring on the two candidate NGST systems.
Experience and infrastructure from NGST's Wavefront Control Testbed (WCT) were utilized to develop a portable wavefront sensor, the Phase Retrieval Camera (PRC). The PRC is useful for the testing of optics in high-jitter environments. The principal uses of the PRC will be testing and experimenting with NGST technology demonstration mirrors as well as exploring other issues of wavefront sensing and control not easily studied using the WCT. This presentation will detail the packaging and hardware chosen for the PRC, the PRC software, and calibration of the instrument.
A piston sensing and control algorithm for segmented mirror coarse phasing using a dispersed fringe sensor (DFS) has been developed for the Next Generation Space Telescope (NGST) wavefront sensing and control. The DFS can detect residual piston errors as large as the order of a depth-of-focus and can phase the segment mirrors with accuracy better than 0.1 microns, which is well within the capture range of fine phasing for NGST. A series of experiments have been carried out on the NGST's Wavefront Control Testbed (WCT) to validate the modeling results, evaluate the DFS performance, and systematically explore the factors that affect the DFS performance. This paper reports the testbed results for several critical issues of DFS performance, including DFS dynamic range, accuracy, fringe visibility, and the effects of segment mirror aberrations.
Adaptive optics systems with Shack-Hartmann wavefront sensors require reconstruction of the atmospheric phase error from subaperture slope measurements, with every sensor in the array being used in the computation of each actuator command. This fully populated reconstruction matrix can result in a significant computational burden for adaptive optics systems with large numbers of actuators. A method for generating sparse wavefront reconstruction matrices for adaptive optics is proposed. The method exploits the relevance of nearby subaperture slope measurements for control of an individual actuator, and relies upon the limited extent of the influence function for a zonal deformable mirror. Relying only on nearby sensor information can significantly reduce the calculation time for wavefront reconstruction. In addition, a hierarchic controller is proposed to recover some of the global wavefront information. The performance of these sparse wavefront reconstruction matrices was evaluated in simulation, and tested on the Palomar Adaptive Optics System. This paper presents some initial results from the simulations and experiments.
Adaptive optics (AO) systems currently under investigation will require at least two orders of magitude increase in the number of actuators, which in turn translates to effectively a 104 increase in compute latency. Since the performance of an AO system invariably improves as the compute latency decreases, it is important to study how today's computer systems will scale to address this expected increase in actuator utilization. This paper answers this question by characterizing the performance of a single deformable mirror (DM) Shack-Hartmann natural guide star AO system implemented on the present-generation digital signal processor (DSP) TMS320C6701 from Texas Instruments. We derive the compute latency of such a system in terms of a few basic parameters, such as the number of DM actuators, the number of data channels used to read out the camera pixels, the number of DSPs, the available memory bandwidth, as well as the inter-processor communication (IPC) bandwidth and the pixel transfer rate. We show how the results would scale for future systems that utilizes multiple DMs and guide stars. We demonstrate that the principal performance bottleneck of such a system is the available memory bandwidth of the processors and to lesser extent the IPC bandwidth. This paper concludes with suggestions for mitigating this bottleneck.
Control algorithms developed for coarse phasing the segmented mirrors of the Next Generation Space Telescope (NGST) are being tested in realistic modeling and on the NGST wavefront control testbed, also known as DCATT. A dispersed fringe sensor (DFS) is used to detect piston errors between mirror segments during the initial coarse phasing. Both experiments and modeling have shown that the DFS provides an accurate measurement of piston errors over a range from just under a millimeter to well under a micron.
A telescope simulator was built as part of the Nexus wavefront control testbed, an NGST technology experiment at NASA's Goddard Space Flight Center. This testbed was designed to demonstrate complete control of a segmented telescope, from initial capture of light, through coarse alignment and phasing, to fine phasing and wavefront control. The existing telescope simulator allows testing of the fine phasing and wavefront control steps. A small deformable mirror in the simulator allows generation of an unobscured aberrated wavefront, for use in exploring the range of measurement and correction using the testbed's image-based wavefront sensor and larger deformable mirror. An alternate path under development for the simulator will create a segmented wavefront using three spherical mirrors; three-degree-of-freedom mounts under each mirror enable alignment and phasing experiments that will cover most of the operation sequence. Details of the hardware design and performance will be presented.
This paper describes the results of a few of the initial series of tests being conducted with the first configuration of the Next Generation Space Telescope Wavefront sensing and Control Testbed (WCT1). WCT1 is a 1:1, f/16.6 re-imaging system, incorporating two deformable mirrors located at pupil conjugate positions with 6 actuators/diameter (SM/DM) and 20 actuators/diameter (AO/DM). A CCD on a precision stage is used for obtaining defocused images providing phase diversity for wavefront determination using phase retrieval. In a typical experiment, wavefront error is injected into the optical path with the SM/DM and then corrected using the more densely actuated AO/DM. Wavefront analysis is provided via a phase retrieval algorithm, and control software is used to reshape the AO/DM and correct the wavefront. A summary of the results of some initial tests are presented.
By segmenting and folding the primary mirror, quite large telescopes can be packed into the nose cone of a rocket. Deployed after launch, initial optical performance can be quite poor, due to deployment errors, thermal deformation, fabrication errors and other causes. We describe an automatic control system for capturing, aligning, phasing, and deforming the optics of such a telescope, going from initial cm-level wavefront errors to diffraction-limited observatory operations. This system was developed for the Next Generation Space Telescope and is being tested on the NGST Wavefront Control Testbed.
During its first year of shared-risk observations, the PALAO/PHARO adaptive optics system has been employed to obtain near-infrared R approximately 1000 spectra of solar system targets at spectroscopic slit widths of 0.5 and 0.1 arcsec, and corresponding spatial resolution along the slit as fine as 0.08 arcsec. Phenomena undergoing initial investigation include condensate formation in the atmospheres of Neptune, and the Saturnian moon, Titan. We present the results of this AO spectroscopy campaign and discuss AO specific considerations in the reduction and interpretation of this data.
We describe the current performance of the Palomar 200 inch (5 m) adaptive optics system, which in December of 1998 achieved its first high order (241 actuators) lock on a natural guide star. In the K band (2.2 micrometer), the system has achieved Strehl ratios as high as 50% in the presence of 1.0 arcsecond seeing (0.5 micrometer). Predictions of the system's performance based on the analysis of real-time wavefront sensor telemetry data and an analysis based on a fitted Kolmogorov atmospheric model are shown to both agree with the observed science image performance. Performance predictions for various seeing conditions are presented and an analysis of the error budget is used to show which subsystems limit the performance of the AO system under various atmospheric conditions.
The size of the corrected field of view of an adaptive optics system is though to be a severe limitation on the usefulness of an adaptive optics system. We show that under good seeing conditions the useful field of view is an order of magnitude larger than the field predicted by the classical isoplanatic angle. The corrected field of view is experimentally shown to increase when using spatially degraded wavefront corrections. Two methods of spatially degrading the wavefront were tested: low-order modal wavefront reconstructions and the use of a low-altitude Rayleigh wavefront reference beacon. In both cases the on- axis performance is sacrificed for an increased field of view.
The design and characteristics of an 8 watt diode laser pumped version of the sum frequency laser designed for astronomy is described together with an outline of field test undertaken on the NOAO Vacuum Telescope Tower at Sac Peak. Optical pumping effects were shown to increase the return by a factor of 2.5 subarcsecond beacons were produces with a brightness equal to a 9.2 magnitude star with 1.2 watts incident at the sodium layer under sodium abundance column density of 5 X 109 atoms/cm2.
The Chicago Adaptive Optics System has been installed on the 3.5 meter ARC telescope at Apache Point for two years. Strehl improvements of 12 are obtained at 0.85 microns in 1- 2 arcsecond seeing. The major limitation in performance is set by the telescope vibrations.
Horizontal adaptive optics research has been conducted at Yerkes Observatory since the first tests in 1994. This includes the study of wavefront reconstruction techniques, isoplanatic angle measurements, branch points in the phase function, and combined adaptive optics and phase diversity experiments.
The Wavefront Control Experiment (WCE) is a zonal adaptive optics system that has been used to test many different matrix wavefront reconstructors. This has influenced the choice of reconstructors for experiments conducted using the Wavefront Control Experiment at Yerkes Observatory, at the Starfire Optical Range 1.5 m telescope, and the Chicago Adaptive Optics System (ChAOS) at the Apache Point 3.5 m telescope. The matrices are generated from a Macintosh software package, called A+, with a graphics user interface. The mathematical basis of many of the reconstructors is outlined, along with our experience using them, and thoughts about future reconstructor concepts.
Planetary detection using doppler shifts requires very high resolution spectrographs which are stable over periods of years. Single or low order mode optical fibers have advantages in this application since they produce spatially scrambled Gaussian input illumination and diffraction limited size input image for spectrograph slit. Very high resolution spectroscopy is thus possible with modest dimensions of the spectrograph. This results in greatly lowering the cost of spectrograph and increasing the stability. However, adaptive optics system is needed to improve the light coupling efficiency. This paper reviews the potential of this technique and presents some preliminary laboratory experiments with the Wavefront Control Experiment (WCE). Plans of future experiment after installing the WCE in a Coude room of a 40 degree(s) reflector telescope at Yerkes Observatory are also presented.
The bandwidth of a servo controlling a deformable mirror should be determined by the time delays introduced by integration, read out, and analysis of the signals recorded by the wavefront detector. However, many types of piezoelectric actuators used in the construction of deformable mirrors exhibit considerable hysteresis which destabilizes the servo and requires a reduction in the stable system bandwidth. The effect of various degrees of hysteresis on the overall servo performance will be discussed together with methods of its compensation. These methods include `passive correction,' in which the effect of hysteresis is modeled by the servo system and `active correction,' in which the shape of the mirror is continuously measured with a wide bandwidth servo and is used as the error signal. We discuss both types of correction and present experimental results.
This paper describes some of the history and the current design of the Chicago Adaptive Optics System (ChAOS). ChAOS is a facility AO system using a sodium layer laser beacon which is due to be permanently mounted on a 3.5 meter ARC telescope located at Apache Point New Mexico. First light is expected in 1995.
The Wavefront Control Experiment is a high temporal bandwidth visible wavelength adaptive optics system on loan at the University of Chicago by the Ballistic Missile Defense Organization for astronomical research. In the last year numerous laboratory tests have been conducted with the WCE. The system will soon be installed in a coude laboratory associated with a Ritchey-Cretien 40' telescope at Yerkes Observatory. A brief overview is given of the WCE system, our recent laboratory tests with different matrix reconstructors, the 40' telescope and coude beam optics, and future plans for the instrument. Our scientific plans for the WCE include monitoring the atmosphere of Jupiter prior to and after the projected impact by comet Shoemaker-Levy in July 1994, and to implement a long term program to do high resolution single-mode fiber feed spectroscopy for asteroseismology and extra-solar planetary detection, via radial velocity determinations, of bright stars.
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