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All sourcesNASA NTRSDOE OSTI
NASA NTRS2022

Defining Pupil Knowledge Requirements for Roman Space Telescope Integrated Payload Assembly Testing

This thesis examines the use of phase retrieval to characterize an optical system under testing conditions with pupil knowledge error. The purpose of this research is to define pupil knowledge requirements for the integrated payload testing of the Roman Space Telescope (RST). I have conducted a Monte Carlo study to determine how uncertainty in the location of the ground support equipment relative to the payload will affect the ability to use image-based wavefront sensing during the test to characterize system alignment. This test will be conducted in the Space Environment Simulator (SES) with the presence of Wavefront Error (WFE) due to gravity sag on the primary mirror. In this study, I incrementally decentered a piece of ground support equipment, the SubAperture Metrology System (SAMS), and evaluated how the phase retrieval algorithm attempted to fit the wavefront with the pupil mismatch. The results of this study are a factor in the overall Integrated Payload Assembly (IPA) testing WFE budget.

Roman Space Telescope↗
NASA NTRS2001

Adjustable Focus Optical Correction Lens (AFOCL)

This report describes a metrology plan that was developed for the characterization of PLZT-based devices, such as the Adjustable Focus Optical Correction Lens (AFOCL) in support of and as part of the deliverables for NASA contract NAS8-00118. The areas to be investigated include intensiometric effects (those that limit or alter the intensity of the light transmitted through the optic); interferometric effects (the phase change induced through the optic); and polarimetric effects (evaluating the differential lag between two polarization states propagating through the optic). These distinct phenomena are often coupled together in real applications consequently, there is a need to develop different standardized testing apparatus to: (1) isolate one effect from another; (2) gather information for understanding the physical effects; (3) anchor wavefront corrector modeling efforts; (4) develop the ability to decouple different effects; (5) demonstrate the suitability of PLZT technology to perform wavefront correction. The Center for Applied Optics (CAO) at the University of Alabama in Huntsville (UAH) is skilled in the characterization of transmission wavefront shaping devices using traditional interferometers available within the CAO Optical Metrology Laboratory and their Advanced Polarization Test Facility. Besides the imaging and interferometers available, the polarimetry facility has at its disposal, a Mueller Matrix Imaging Polarimeter (MMIP) which is well suited to the characterization of SLMs, polarizers, and thin film coatings within the visible and near-IR spectrums. In addition, the phase-shifting interferometry facilities at NASA-MSFC and the unique interferometers they processes are some of the most advanced available and may be of value especially for performing real-time optical performance evaluation of AFOCL test components.

Peters, Bruce R.↗
NASA NTRS2017

Wavefront sensing for a future HabEx space telescope.

HabEx is a concept study for a next generation space telescope having a 4 to 6.5 m primary mirror diameter. Primarily designed for exoplanet science, but also carrying general astrophysics instruments, it will have stability requirements orders of magnitude more stringent than any previously flown space-based observatory. Wavefront control, including active sensing/control of wavefront jitter and drift, will be essential to achieving and maintaining coronagraph performance. There are currently two complementary techniques that can be used to sense the wavefront drift. An attractive method is to use starlight rejected from the coronagraph to sense changes in the wavefront incident on the coronagraph mask. This method suffers from photon noise especially when imaging dim stars. It is also unavailable during slews between target stars. Laser metrology is a complementary system in which a laser “truss” enables rigid body displacements of the optics to be measured from changes in the separation of pairs of optical fiducials which can be separated by many meters. In closed loop with actuated optics, laser metrology actively maintains rigid body alignment of the front-end optics, thereby eliminating the dominant source of wavefront drift. This method can maintain rigid body stability of the telescope optics and thereby wavefront control, even during slews between targets.

Martin, Stefan↗
NASA NTRS2012

Stitching of near-nulled subaperture measurements

A metrology system for measuring aspheric test objects by subaperture stitching. A wavefront-measuring gauge having a limited capture range of wavefront shapes collects partially overlapping subaperture measurements over the test object. A variable optical aberrator reshapes the measurement wavefront with between a limited number of the measurements to maintain the measurement wavefront within the capture range of the wavefront-measuring gauge. Various error compensators are incorporated into a stitching operation to manage residual errors associated with the use of the variable optical aberrator.

Murphy, Paul↗
NASA NTRSDate not supplied

Picometer-Level Metrology on the Road to Habitable Worlds Observatory

The Habitable Worlds Observatory will revolutionize our understanding of the universe by directly detecting biosignatures on extrasolar planets and allow us to answer the question if we are alone in the universe. To accomplish the tight science goals associated with this mission, the development of an ultrastable observatory with a coronagraphic instrument is necessary. The observatory itself may need to stay stable on the order of 10 picometers over a wavefront control cycle, orders of magnitude more stable than what is required on current space missions. The metrology to verify stability requirements must be roughly a factor of ten more stable. The ultrastable laboratory at NASA’s Goddard Space Flight Center has further stabilized its testbed to allow for dynamic measurements on diffuse and specular objects on the order of single picometers, and we are currently measuring drifts on the orders of tens of picometers over different temporal bands. This paper will discuss the mechanical updates to the testbed setup, the analysis performed on several test articles, and the path forward on the road to measuring achieving the required stability for Habitable Worlds Observatory.

Inteferometry↗
NASA NTRSDate not supplied

Picometer-level metrology of test articles on the road to Habitable Worlds Observatory

The Habitable Worlds Observatory will revolutionize our understanding of the universe by directly detecting biosignatures on extrasolar planets and allow us to answer the question if we are alone in the universe. To accomplish the tight science goals associated with this mission, the development of an ultrastable observatory with a coronagraphic instrument is necessary. The observatory itself may need to stay stable on the order of 10 picometers over a wavefront control cycle, orders of magnitude more stable than what is required on current space missions. The metrology to verify stability requirements must be roughly a factor of ten more stable. The ultrastable laboratory at NASA’s Goddard Space Flight Center has further stabilized its testbed to allow for dynamic measurements on diffuse and specular objects on the order of single picometers, and we are currently measuring drifts on the orders of tens of picometers over different temporal bands. This paper will discuss the mechanical updates to the testbed setup, the analysis performed on several test articles, and the path forward on the road to measuring achieving the required stability for Habitable Worlds Observatory.

Inteferometry↗
NASA NTRSDate not supplied

Picometer-Level Metrology of Test Articles on the Road to Habitable Worlds Observatory

The Habitable Worlds Observatory will revolutionize our understanding of the universe by directly detecting biosignatures on extrasolar planets and allow us to answer the question if we are alone in the universe. To accomplish the tight science goals associated with this mission, the development of an ultrastable observatory with a coronagraphic instrument is necessary. The observatory itself may need to stay stable on the order of 10 picometers over a wavefront control cycle, orders of magnitude more stable than what is required on current space missions. The metrology to verify stability requirements must be roughly a factor of ten more stable. The ultrastable laboratory at NASA’s Goddard Space Flight Center has further stabilized its testbed to allow for dynamic measurements on diffuse and specular objects on the order of single picometers, and we are currently measuring drifts on the orders of tens of picometers over different temporal bands. This paper will discuss the mechanical updates to the testbed setup, the analysis performed on several test articles, and the path forward on the road to measuring achieving the required stability for Habitable Worlds Observatory.

Inteferometry↗
NASA NTRS2002

Wavefront tilt feedforward for the formation interferometer testbad (FIT)

Separated spacecraft interferometry is a candidate architecture for several future NASA missions. The Formation Interferometer Testbed (FIT) is a ground based testbed dedicated to the validation of this key technology for a formation of two spacecraft. In separated spacecraft interferometry, the residual relative motion of the component spacecraft must be compensated for by articulation of the optical components. In this paper, the design of the FIT interferometer pointing control system is described. This control system is composed of a metrology pointing loop that maintains an optical link between the two spacecraft and two stellar pointing loops for stabilizing the stellar wavefront at both the right and left apertures of the instrument. A novel feedforward algorithm is used to decouple the metrology loop from the left side stellar loop. Experimental results from the testbed are presented that verify this approach and that fully demonstrate the performance of the algorithm.

formation flying↗
NASA NTRS2006

Results from SIM's Thermo-Opto-Mechanical (TOM3) Testbed

Future space-based optical interferometers, such as the Space Interferometer Mission Planet Quest (SIM), require thermal stability of the optical wavefront to the level of picometers in order to produce astrometric data at the micro-arc-second level. In SIM, the internal path of the interferometer will be measured with a small metrology beam whereas the starlight fringe position is estimated from a large concentric annular beam. To achieve the micro-arc-second observation goal for SIM, it is necessary to maintain the optical path difference between the central and the outer annulus portions of the wavefront of the front-end telescope optics to a few tens of picometers. The Thermo-Opto-Mecha nical testbed (TOM3) was developed at the Jet Propulsion Laboratory to measure thermally induced optical deformations of a full-size flight-like beam compressor and siderostat, the two largest optics on SIM, in flight-like thermal environments. A Common Path Heterodyne Interferometer (COPHI) developed at JPL was used for the fine optical path difference measurement as the metrology sensor. The system was integrated inside a large vacuum chamber in order to mitigate the atmospheric and thermal disturbances. The siderostat was installed in a temperature-controlled thermal shroud inside the vacuum chamber, creating a flight-like thermal environment. Detailed thermal and structural models of the test articles (siderostat and compressor) were also developed for model prediction and correlation of the thermal deformations. Experimental data shows SIM required thermal stability of the test articles and good agreement with the model predictions.

interferometry↗
NASA NTRS2010

Optical Testing of Retroreflectors for Cryogenic Applications

A laser tracker (LT) is an important coordinate metrology tool that uses laser interferometry to determine precise distances to objects, points, or surfaces defined by an optical reference, such as a retroreflector. A retroreflector is a precision optic consisting of three orthogonal faces that returns an incident laser beam nearly exactly parallel to the incident beam. Commercial retroreflectors are designed for operation at room temperature and are specified by the divergence, or beam deviation, of the returning laser beam, usually a few arcseconds or less. When a retroreflector goes to extreme cold (.35 K), however, it could be anticipated that the precision alignment between the three faces and the surface figure of each face would be compromised, resulting in wavefront errors and beam divergence, degrading the accuracy of the LT position determination. Controlled tests must be done beforehand to determine survivability and these LT coordinate errors. Since conventional interferometer systems and laser trackers do not operate in vacuum or at cold temperatures, measurements must be done through a vacuum window, and care must be taken to ensure window-induced errors are negligible, or can be subtracted out. Retroreflector holders must be carefully designed to minimize thermally induced stresses. Changes in the path length and refractive index of the retroreflector have to be considered. Cryogenic vacuum testing was done on commercial solid glass retroreflectors for use on cryogenic metrology tasks. The capabilities to measure wavefront errors, measure beam deviations, and acquire laser tracker coordinate data were demonstrated. Measurable but relatively small increases in beam deviation were shown, and further tests are planned to make an accurate determination of coordinate errors.

Ohl, Raymond G.↗
NASA NTRS2011

Micro-Pixel Image Position Sensing Testbed

The search for Earth-mass planets in the habitable zones of nearby Sun-like stars is an important goal of astrophysics. This search is not feasible with the current slate of astronomical instruments. We propose a new concept for microarcsecond astrometry which uses a simplified instrument and hence promises to be low cost. The concept employs a telescope with only a primary, laser metrology applied to the focal plane array, and new algorithms for measuring image position and displacement on the focal plane. The required level of accuracy in both the metrology and image position sensing is at a few micro-pixels. We have begun a detailed investigation of the feasibility of our approach using simulations and a micro-pixel image position sensing testbed called MCT. So far we have been able to demonstrate that the pixel-to-pixel distances in a focal plane can be measured with a precision of 20 micro-pixels and image-to-image distances with a precision of 30 micro-pixels. We have also shown using simulations that our image position algorithm can achieve accuracy of 4 micro-pixels in the presence of lambda/20 wavefront errors.

astrometry↗
NASA NTRS1991

Segmented mirror figure control for a space-based far-IR astronomical telescope

For segmented large mirrors to be used effectively for astronomy they must be actively aligned and controlled to extreme levels of precision. We consider the figure control problem for a spaceborne far-IR telescope, the Precision Segmented Reflector Project Focus Moderate Mission Telescope. We propose a two-stage approach. A figure initialization controller is used to achieve initial phasing and alignment of the telescope using an imaging science detector. A figure maintenance controller keeps the telescope aligned during normal operation using a laser metrology optical truss sensor system. We show that performance of any figure control system is subject to limits on the controllability of the wavefront. Maintenance controllers are additionally limited by considerations of the observability of the wavefront from the maintenance sensors. We show preliminary results for the figure initialization controller. We present a 'Wavefront Compensation' method for figure maintenance control that minimizes wavefront errors due to misalignment errors.

Redding, David↗
NASA NTRS2011

Mirror Technology Roadmap for NASA's Exoplanet Exploration Program

There are several possible approaches to designing exoplanet missions: (1) Coronagraphs (2) Interferometers (3) Starshades Wavefront sensing and control is the central concern, not mirror size (1) Starlight suppression with deformable mirrors (2) Thermal and structural stability (3) Metrology for sensing and control Diffraction-limited optical primary mirrors 4-m or larger are needed to detect Earthlike planets (1) Surface figure similar to HST required (2) Smaller primary mirrors can be used with aggressive coronagraph designs, but the stability tolerances become the driving concern (3) Stability tolerances of coronagraphs are greatly reduced when larger primaries are used in conjunction with 8th-order masks Long term vision for large telescope development includes space-based segmented-mirror telescopes using actively-controlled glass segments or silicon carbide hybrid-mirror designs

technology↗
NASA NTRS2007

Zernike-like Orthogonal Basis Functions for Wavefront Characterization over Sampled, Irregular Apertures

For optical systems with circular apertures, wavefronts are often analyzed using Zernike polynomials, and individual Zernike functions are associated with familiar optical aberrations. For systems with noncircular apertures, or in practical situations in which wavefronts are measured at a limited number of points in the aperture, the Zernike polynomials are no longer an orthogonal basis for the measured data. Although there are an endless number of ways to create a basis for such measured data, a "Zernike-like" basis is useful to connect with our experience with the usual optical aberrations. In this paper, the steps required to identify a Zernike-like basis for wavefronts over sampled, irregular apertures are presented, based on the Gram-Schmidt orthogonalization technique. The benefits of analyzing optical wavefronts using an orthogonal basis specific to an optical system's aperture shape and wavefront sampling, instead of using the traditional Zernike polynomials, are detailed in two examples, from image-based wavefront sensing on a segmented-aperture telescope (the James Webb Space Telescope Testbed Telescope at Ball Aerospace) and from interferometer characterization for surface metrology of a hexagonal mirror segment.

Aronstein, David L.↗
NASA NTRS2010

Wavefront Control and Image Restoration with Less Computing

PseudoDiversity is a method of recovering the wavefront in a sparse- or segmented- aperture optical system typified by an interferometer or a telescope equipped with an adaptive primary mirror consisting of controllably slightly moveable segments. (PseudoDiversity should not be confused with a radio-antenna-arraying method called pseudodiversity.) As in the cases of other wavefront- recovery methods, the streams of wavefront data generated by means of PseudoDiversity are used as feedback signals for controlling electromechanical actuators of the various segments so as to correct wavefront errors and thereby, for example, obtain a clearer, steadier image of a distant object in the presence of atmospheric turbulence. There are numerous potential applications in astronomy, remote sensing from aircraft and spacecraft, targeting missiles, sighting military targets, and medical imaging (including microscopy) through such intervening media as cells or water. In comparison with prior wavefront-recovery methods used in adaptive optics, PseudoDiversity involves considerably simpler equipment and procedures and less computation. For PseudoDiversity, there is no need to install separate metrological equipment or to use any optomechanical components beyond those that are already parts of the optical system to which the method is applied. In Pseudo- Diversity, the actuators of a subset of the segments or subapertures are driven to make the segments dither in the piston, tilt, and tip degrees of freedom. Each aperture is dithered at a unique frequency at an amplitude of a half wavelength of light. During the dithering, images on the focal plane are detected and digitized at a rate of at least four samples per dither period. In the processing of the image samples, the use of different dither frequencies makes it possible to determine the separate effects of the various dithered segments or apertures. The digitized image-detector outputs are processed in the spatial-frequency (Fourier-transform) domain to obtain measures of the piston, tip, and tilt errors over each segment or subaperture. Once these measures are known, they are fed back to the actuators to correct the errors. In addition, measures of errors that remain after correction by use of the actuators are further utilized in an algorithm in which the image is phase-corrected in the spatial-frequency domain and then transformed back to the spatial domain at each time step and summed with the images from all previous time steps to obtain a final image having a greater signal-to-noise ratio (and, hence, a visual quality) higher than would otherwise be attainable.

Lyon, Richard G.↗