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All sourcesNASA NTRSDOE OSTI
NASA NTRS2012

Method to Enhance the Operation of an Optical Inspection Instrument Using Spatial Light Modulators

For many aspheric and freeform optical components, existing interferometric solutions require a custom computer-generated hologram (CGH) to characterize the part. The overall objective of this research is to develop hardware and a procedure to produce a combined, dynamic, Hartmann/ Digital Holographic interferometry inspection system for a wide range of advanced optical components, including aspheric and freeform optics. This new instrument would have greater versatility and dynamic range than currently available measurement systems. The method uses a spatial light modulator to pre-condition wavefronts for imaging, interferometry, and data processing to improve the resolution and versatility of an optical inspection instrument. Existing interferometers and Hartmann inspection systems have either too small a dynamic range or insufficient resolution to characterize conveniently unusual optical surfaces like aspherical and freeform optics. For interferometers, a specially produced, computer-generated holographic optical element is needed to transform the wavefront to within the range of the interferometer. A new hybrid wavefront sensor employs newly available spatial light modulators (SLMs) as programmable holographic optical elements (HOEs). The HOE is programmed to enable the same instrument to inspect an optical element in stages, first by a Hartmann measurement, which has a very large dynamic range but less resolution. The first measurement provides the information required to precondition a reference wave that avails the measurement process to the more precise phase shifting interferometry. The SLM preconditions a wavefront before it is used to inspect an optical component. This adds important features to an optical inspection system, enabling not just wavefront conditioning for null testing and dynamic range extension, but also the creation of hybrid measurement procedures. This, for example, allows the combination of dynamic digital holography and Hartmann sensing procedures to cover a virtually unlimited dynamic range with high resolution. Digital holography technology brings all of the power and benefits of digital holographic interferometry to the requirement, while Hartmann-type wavefront sensors bring deflectometry technologies to the solution. The SLM can be used to generate arbitrary wavefronts in one leg of the interferometer, thereby greatly simplifying its use and extending its range. The SLM can also be used to modify the system into a dynamic Shack-Hartmann system, which is useful for optical components with large amounts of slope. By integrating these capabilities into a single instrument, the system will have tremendous flexibility to measure a variety of optical shapes accurately.

Trolinger, James↗
NASA NTRS2009

Investigation of Primary Mirror Segment's Residual Errors for the Thirty Meter Telescope

The primary mirror segment aberrations after shape corrections with warping harness have been identified as the single largest error term in the Thirty Meter Telescope (TMT) image quality error budget. In order to better understand the likely errors and how they will impact the telescope performance we have performed detailed simulations. We first generated unwarped primary mirror segment surface shapes that met TMT specifications. Then we used the predicted warping harness influence functions and a Shack-Hartmann wavefront sensor model to determine estimates for the 492 corrected segment surfaces that make up the TMT primary mirror. Surface and control parameters, as well as the number of subapertures were varied to explore the parameter space. The corrected segment shapes were then passed to an optical TMT model built using the Jet Propulsion Laboratory (JPL) developed Modeling and Analysis for Controlled Optical Systems (MACOS) ray-trace simulator. The generated exit pupil wavefront error maps provided RMS wavefront error and image-plane characteristics like the Normalized Point Source Sensitivity (PSSN). The results have been used to optimize the segment shape correction and wavefront sensor designs as well as provide input to the TMT systems engineering error budgets.

warping harness↗
NASA NTRS2020

Design and Performance of the PALM-3000 3.5 kHz Upgrade

PALM-3000 (P3K), the second-generation adaptive optics (AO) instrument for the 5.1 meter Hale telescope atPalomar Observatory, was released as a facility class instrument in October 2011 and has since been used on-skyfor over 600 nights as a workhorse science instrument and testbed for coronagraph and detector development.In late 2019 P3K underwent a significant upgrade to its wavefront sensor (WFS) arm and real-time control(RTC) system to reinforce its position as a state-of-the-art AO facility and extend its faint-end capability forhigh-resolution imaging and precision radial velocity follow-up of Kepler and TESS targets. The main featuresof this upgrade include an EM-CCD WFS camera capable of 3.5 kHz framerates, and an advanced Digital SignalProcessor (DSP) based RTC system to replace the aging GPU based system. Similar to the pre-upgrade system,the Shack-Hartmann wavefront sensor supports multiple pupil sampling modes using a motorized lenslet stage.The default sampling mode with 64×64 subapertures has been re-commissioned on-sky in late 2019, witha successful return to science observations in November 2019. In 64×mode the upgraded system is alreadyachieving K-band Strehl ratios up to 85% on-sky and can lock on natural guide stars as faint as mV=16. A16×16 subaperture mode is scheduled for on-sky commissioning in Fall 2020 and will extend the system’s faintlimit even further. Here we present the design and on-sky re-commissioning results of the upgraded system,dubbed P3K-II.

Vasisht, Gautam↗
DOE OSTI2025

A renewable double plasma mirror for Petawatt-class lasers

Exceptional pulse contrast can be critical for ultraintense laser experiments, particularly when using solid density targets, and their use is becoming widespread. However, current plasma mirror technology is becoming inadequate for the new generation of high repetition rate, high power lasers now available. We describe a novel double plasma mirror configuration based on renewable, free standing, ultrathin liquid crystal films tested at the BELLA Petawatt Laser Center. Although operating at a repetition rate of several shots per minute, this system can be scaled to a high repetition rate exceeding 1 Hz and represents an important step towards enabling sustained, continuous operation of plasma mirrors. We demonstrate an improvement of two to three orders of magnitude in contrast and a total throughput of 80%. We present the first measurements of a beam reflected from a single or double plasma mirror system using a wavefront sensor, showing a well preserved wavefront and spatial mode. Finally, we introduce a model that predicts the total throughput through this double plasma mirror. This is the first model that accurately predicts the peak reflectivity of a plasma mirror when given the laser temporal profile.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1998

Developmental Cryogenic Active Telescope Testbed, a Wavefront Sensing and Control Testbed for the Next Generation Space Telescope

As part of the technology validation strategy of the next generation space telescope (NGST), a system testbed is being developed at GSFC, in partnership with JPL and Marshall Space Flight Center (MSFC), which will include all of the component functions envisioned in an NGST active optical system. The system will include an actively controlled, segmented primary mirror, actively controlled secondary, deformable, and fast steering mirrors, wavefront sensing optics, wavefront control algorithms, a telescope simulator module, and an interferometric wavefront sensor for use in comparing final obtained wavefronts from different tests. The developmental. cryogenic active telescope testbed (DCATT) will be implemented in three phases. Phase 1 will focus on operating the testbed at ambient temperature. During Phase 2, a cryocapable segmented telescope will be developed and cooled to cryogenic temperature to investigate the impact on the ability to correct the wavefront and stabilize the image. In Phase 3, it is planned to incorporate industry developed flight-like components, such as figure controlled mirror segments, cryogenic, low hold power actuators, or different wavefront sensing and control hardware or software. A very important element of the program is the development and subsequent validation of the integrated multidisciplinary models. The Phase 1 testbed objectives, plans, configuration, and design will be discussed.

Leboeuf, Claudia M.↗
NASA NTRS2011

Accelerated Adaptive MGS Phase Retrieval

The Modified Gerchberg-Saxton (MGS) algorithm is an image-based wavefront-sensing method that can turn any science instrument focal plane into a wavefront sensor. MGS characterizes optical systems by estimating the wavefront errors in the exit pupil using only intensity images of a star or other point source of light. This innovative implementation of MGS significantly accelerates the MGS phase retrieval algorithm by using stream-processing hardware on conventional graphics cards. Stream processing is a relatively new, yet powerful, paradigm to allow parallel processing of certain applications that apply single instructions to multiple data (SIMD). These stream processors are designed specifically to support large-scale parallel computing on a single graphics chip. Computationally intensive algorithms, such as the Fast Fourier Transform (FFT), are particularly well suited for this computing environment. This high-speed version of MGS exploits commercially available hardware to accomplish the same objective in a fraction of the original time. The exploit involves performing matrix calculations in nVidia graphic cards. The graphical processor unit (GPU) is hardware that is specialized for computationally intensive, highly parallel computation. From the software perspective, a parallel programming model is used, called CUDA, to transparently scale multicore parallelism in hardware. This technology gives computationally intensive applications access to the processing power of the nVidia GPUs through a C/C++ programming interface. The AAMGS (Accelerated Adaptive MGS) software takes advantage of these advanced technologies, to accelerate the optical phase error characterization. With a single PC that contains four nVidia GTX-280 graphic cards, the new implementation can process four images simultaneously to produce a JWST (James Webb Space Telescope) wavefront measurement 60 times faster than the previous code.

Lam, Raymond K.↗
NASA NTRS2012

Wavefront Compensation Segmented Mirror Sensing and Control

The primary mirror of very large submillimeter-wave telescopes will necessarily be segmented into many separate mirror panels. These panels must be continuously co-phased to keep the telescope wavefront error less than a small fraction of a wavelength, to ten microns RMS (root mean square) or less. This performance must be maintained continuously across the full aperture of the telescope, in all pointing conditions, and in a variable thermal environment. A wavefront compensation segmented mirror sensing and control system, consisting of optical edge sensors, Wavefront Compensation Estimator/Controller Soft ware, and segment position actuators is proposed. Optical edge sensors are placed two per each segment-to-segment edge to continuously measure changes in segment state. Segment position actuators (three per segment) are used to move the panels. A computer control system uses the edge sensor measurements to estimate the state of all of the segments and to predict the wavefront error; segment actuator commands are computed that minimize the wavefront error. Translational or rotational motions of one segment relative to the other cause lateral displacement of the light beam, which is measured by the imaging sensor. For high accuracy, the collimator uses a shaped mask, such as one or more slits, so that the light beam forms a pattern on the sensor that permits sensing accuracy of better than 0.1 micron in two axes: in the z or local surface normal direction, and in the y direction parallel to the mirror surface and perpendicular to the beam direction. Using a co-aligned pair of sensors, with the location of the detector and collimated light source interchanged, four degrees of freedom can be sensed: transverse x and y displacements, as well as two bending angles (pitch and yaw). In this approach, each optical edge sensor head has a collimator and an imager, placing one sensor head on each side of a segment gap, with two parallel light beams crossing the gap. Two sets of optical edge sensors are used per segment-to-segment edge, separated by a finite distance along the segment edge, for four optical heads, each with an imager and a collimator. By orienting the beam direction of one edge sensor pair to be +45 away from the segment edge direction, and the other sensor pair to be oriented -45 away from the segment edge direction, all six degrees of freedom of relative motion between the segments can be measured with some redundancy. The software resides in a computer that receives each of the optical edge sensor signals, as well as telescope pointing commands. It feeds back the edge sensor signals to keep the primary mirror figure within specification. It uses a feed-forward control to compensate for global effects such as decollimation of the primary and secondary mirrors due to gravity sag as the telescope pointing changes to track science objects. Three segment position actuators will be provided per segment to enable controlled motions in the piston, tip, and tilt degrees of freedom. These actuators are driven by the software, providing the optical changes needed to keep the telescope phased.

Redding, David C.↗
NASA NTRS1991

Surface control techniques for the segmented primary mirror in the large lunar telescope

The large lunar telescope is a proposed moon-based telescope which incorporates a sixteen-meter segmented primary mirror. An error budget is developed for the active control system of the primary mirror. A control methodology for the primary mirror is then described which utilizes piston sensors for measuring the relative piston error between adjacent segments as well as a separate sensor which measures the tilt of each segment with respect to the pointing direction of the telescope. A trade study is conducted in which the following types of tilt sensors are examined to determine their applicability to this program: stellar wavefront sensors, such as a Hartmann-Shack or a shearing interferometer; holographic optical elements; interferometers; scanning systems; and some nonoptical systems which electronically measure the relative tilt between adjacent segments. In addition, two independent methods of quantitatively verifying the performance of the telescope using either a phase retrieval algorithm or an image sharpening technique, both of which are based on the quality of a stellar image, are presented.

Gleckler, Anthony D.↗
NASA NTRS2008

Algorithm for Wavefront Sensing Using an Extended Scene

A recently conceived algorithm for processing image data acquired by a Shack-Hartmann (SH) wavefront sensor is not subject to the restriction, previously applicable in SH wavefront sensing, that the image be formed from a distant star or other equivalent of a point light source. That is to say, the image could be of an extended scene. (One still has the option of using a point source.) The algorithm can be implemented in commercially available software on ordinary computers. The steps of the algorithm are the following: 1. Suppose that the image comprises M sub-images. Determine the x,y Cartesian coordinates of the centers of these sub-images and store them in a 2xM matrix. 2. Within each sub-image, choose an NxN-pixel cell centered at the coordinates determined in step 1. For the ith sub-image, let this cell be denoted as si(x,y). Let the cell of another subimage (preferably near the center of the whole extended-scene image) be designated a reference cell, denoted r(x,y). 3. Calculate the fast Fourier transforms of the sub-sub-images in the central NxN portions (where N < N and both are preferably powers of 2) of r(x,y) and si(x,y). 4. Multiply the two transforms to obtain a cross-correlation function Ci(u,v), in the Fourier domain. Then let the phase of Ci(u, v) constitute a phase function, phi(u,v). 5. Fit u and v slopes to phi (u,v) over a small u,v subdomain. 6. Compute the fast Fourier transform, Si(u,v) of the full NxN cell si(x,y). Multiply this transform by the u and phase slopes obtained in step 4. Then compute the inverse fast Fourier transform of the product. 7. Repeat steps 4 through 6 in an iteration loop, cumulating the u and slopes, until a maximum iteration number is reached or the change in image shift becomes smaller than a predetermined tolerance. 8. Repeat steps 4 through 7 for the cells of all other sub-images.

Sidick, Erkin↗
NASA NTRS2013

Estimate Low and High Order Wavefront Using P1640 Calibrator Measurements

P1640 high contrast imaging system on the Palomar 200 inch Telescope consists of an apodized-pupil Lyot coronagraph, the PALM-3000 adaptive optics (P3K-AO), and P1640 Calibrator (CAL). Science images are recorded by an integral field spectrograph covering J-H bands for detecting and characterizing stellar companions. With aberrations from atmosphere corrected by the P3K-AO, instrument performance is limited mainly by the quasi-static speckles due to noncommon path wavefront aberrations for the light to propagate to the P3K-AO wavefront sensor and to the coronagraph mask. The non-common path wavefront aberrations are sensed by CAL, which measures the post-coronagraph E-field using interferometry, and can be effectively corrected by offsetting the P3K-AO deformable mirror target position accordingly. Previously, we have demonstrated using CAL measurements to correct high order wavefront aberrations, which is directly connected to the static speckles in the image plane. Low order wavefront, on the other hand, usually of larger amplitudes, causes light to leak through the coronagraph making the whole image plane brighter. Knowledge error in low order wavefront aberrations can also affect the estimation of the high order wavefront. Even though, CAL is designed to sense efficiently high order wavefront aberrations, the low order wavefront front can be inferred with less sensitivity. Here, we describe our method for estimating both low and high order wavefront aberrations using CAL measurements by propagating the post-coronagraph E-field to a pupil before the coronagraph. We present the results from applying this method to both simulated and experiment data.

aberrations↗
NASA NTRS2008

Adaptive MGS Phase Retrieval

Adaptive MGS Phase Retrieval software uses the Modified Gerchberg-Saxton (MGS) algorithm, an image-based sensing method that can turn any focal plane science instrument into a wavefront sensor, avoiding the need to use external metrology equipment. Knowledge of the wavefront enables intelligent control of active optical systems.

Basinger, Scott A.↗
NASA NTRS2001

Preliminary Investigation of an Active PLZT Lens

The design, analysis and preliminary testing of a prototype Adjustable Focus Optical Correction Lens (AFOCL) is described. The AFOCL is an active optical component composed of solid state lead lanthanum-modified zirconate titanate (PLZT) ferroelectric ceramic with patterned indium tin oxide (ITO) transparent surface electrodes that modulate the refractive index of the PLZT to function as an electro-optic lens. The AFOCL was developed to perform optical re-alignment and wavefront correction to enhance the performance of Ultra-Lightweight Structures and Space Observatories (ULSSO). The AFOCL has potential application as an active optical component within a larger optical system. As such, information from a wavefront sensor would be processed to provide input to the AFOCL to drive the sensed wavefront to the desired shape and location. While offering variable and rapid focussing capability (controlled wavefront manipulation) similar to liquid crystal based spatial light modulators (SLM), the AFOCL offers some potential advantages because it is a solid-state, stationary, low-mass, rugged, and thin optical element that can produce wavefront quality comparable to the solid refractive lens it replaces. The AFOCL acts as a positive or negative lens by producing a parabolic phase-shift in the PLZT material through the application of a controlled voltage potential across the ITO electrodes. To demonstrate the technology, a 4 mm diameter lens was fabricated to produce 5-waves of optical power operating at 2.051 micrometer wavelength. Optical metrology was performed on the device to measure focal length, optical quality, and efficiency for a variety of test configurations. The data was analyzed and compared to theoretical data available from computer-based models of the AFOCL.

Lightsey, W. D.↗
NASA NTRS2000

Interferometric Techniques for Gravitational Wave Detection in Space

The Laser Interferometer Space Antenna (LISA) mission will detect gravitational waves from galactic and extragalactic sources, most importantly those involving supermassive black holes. The primary goal of this project is to investigate stability and robustness issues associated with LISA interferometry. We specifically propose to study systematic errors arising from: optical misalignments, optical surface errors, thermal effects and pointing tolerances. This report covers the first fiscal year of the grant, from January 1st to December 31st 1999. We have employed an optical modeling tool to evaluate the effect of misplaced and misaligned optical components. Preliminary results seem to indicate that positional tolerances of one micron and angular tolerances of 0.6 millirad produce no significant effect on the achievable contrast of the interference pattern. This report also outlines research plans for the second fiscal year of the grant, from January 1st to December 31st 2000. Since the work under NAG5-6880 has gone more rapidly than projected, our test bed interferometer is operational, and can be used for measurements of effects that cause beam motion. Hence, we will design, build and characterize a sensor for measuring beam motion, and then install it. We are also planning a differential wavefront sensor based on a quadrant photodiode as a first generation sensor.

Stebbins, Robin T↗
NASA NTRS2007

Active Correction of Aberrations of Low-Quality Telescope Optics

A system of active optics that includes a wavefront sensor and a deformable mirror has been demonstrated to be an effective means of partly correcting wavefront aberrations introduced by fixed optics (lenses and mirrors) in telescopes. It is envisioned that after further development, active optics would be used to reduce wavefront aberrations of about one wave or less in telescopes having aperture diameters of the order of meters or tens of meters. Although this remaining amount of aberration would be considered excessive in scientific applications in which diffraction-limited performance is required, it would be acceptable for free-space optical- communication applications at wavelengths of the order of 1 m. To prevent misunderstanding, it is important to state the following: The technological discipline of active optics, in which the primary or secondary mirror of a telescope is directly and dynamically tilted, distorted, and/or otherwise varied to reduce wavefront aberrations, has existed for decades. The term active optics does not necessarily mean the same thing as does adaptive optics, even though active optics and adaptive optics are related. The term "adaptive optics" is often used to refer to wavefront correction at speeds characterized by frequencies ranging up to between hundreds of hertz and several kilohertz high enough to enable mitigation of adverse effects of fluctuations in atmospheric refraction upon propagation of light beams. The term active optics usually appears in reference to wavefront correction at significantly lower speeds, characterized by times ranging from about 1 second to as long as minutes. Hence, the novelty of the present development lies, not in the basic concept of active or adaptive optics, but in the envisioned application of active optics in conjunction with a deformable mirror to achieve acceptably small wavefront errors in free-space optical communication systems that include multi-meter-diameter telescope mirrors that are relatively inexpensive because their surface figures are characterized by errors as large as about 10 waves. Figure 1 schematically depicts the apparatus used in an experiment to demonstrate such an application on a reduced scale involving a 30-cm-diameter aperture.

Hemmati, Hamid↗
NASA NTRS2002

Sparse matrix wavefront reconstruction: simulations and experiments

Adaptive optics systems with Shack-Hartmann wavefront sensors require reconstruction of the atmospheric phase error from 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 isproposed. The method exploits the relevance of nearby 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 will present some initial results from the simulations and experiments.

space↗
NASA NTRS2002

Bench top interferometric test bed for LISA

Adaptive optics systems with Shack-Hartmann wavefront sensors require reconstruction of the atmospheric phase error from 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 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 will present some initial results from the simulations and experiments.

LISA↗
NASA NTRS2015

Effect of DM Actuator Errors on the WFIRST/AFTA Coronagraph Contrast Performance

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 errors introduce high-spatial frequency errors to the DM surface and thus worsen the contrast performance of the coronagraph. We have investigated the effects of actuator gain 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.

adaptive optics↗