Extreme adaptive optics (ExAO) systems are optimized for high-contrast imaging and coronagraphy. ExAO systems are currently limited to wavefront sensing using a bright natural guide star (NGS) due to the need for high precision wavefront control. Recent advances in sodium laser guide star (LGS) technology such as high power, efficient laser sources and pre-compensation of the laser uplink overcome previous limitations in LGS technology, opening up the potential for LGS technology to improve ExAO system performance with dim targets. The LAser guide Star Sensor Integrated Extreme adaptive optics (LASSIE) project at the Starfire Optical Range will explore the trade space in beacon size, brightness, coherence, and wavefront sensor design to preform path-finding research on the potential performance of an uplink-corrected LGS-ExAO system. In this presentation we will discuss the current progress of the LASSIE project.
The next generation of giant ground and space telescopes will have the light-collecting power to detect and characterize potentially habitable terrestrial exoplanets using high-contrast imaging for the first time. This will only be achievable if the performance of the Giant Segment Mirror Telescopes (GSMTs) extreme adaptive optics (ExAO) systems are optimized to their full potential. A key component of an ExAO system is the wavefront sensor (WFS), which measures aberrations from atmospheric turbulence. A common choice in current and next-generation instruments is the pyramid wavefront sensor (PWFS). ExAO systems require high spatial and temporal sampling of wavefronts to optimize performance and, as a result, require large detectors for the WFS. We present a closed-loop testbed demonstration of a three-sided pyramid wavefront sensor (3PWFS) as an alternative to the conventional four-sided pyramid wavefront (4PWFS) sensor for GSMT-ExAO applications on the innovative comprehensive adaptive optics and coronagraph test instrument (CACTI). The 3PWFS is less sensitive to read noise than the 4PWFS because it uses fewer detector pixels. The 3PWFS has further benefits: a high-quality three-sided pyramid optic is easier to manufacture than a four-sided pyramid. We describe the design of the two components of the CACTI system, the adaptive optics simulator and the PWFS testbed that includes both a 3PWFS and 4PWFS. We detail the error budget of the CACTI system, review its operation and calibration procedures, and discuss its current status. A preliminary experiment was performed on CACTI to study the performance of the 3PWFS to the 4PWFS in varying strengths of turbulence using both the raw intensity and slopes map signal processing methods. This experiment was repeated for a modulation radius of 1.6 and 3.25 λ / D. We found that the performance of the two wavefront sensors is comparable if modal loop gains are tuned.
Extreme adaptive optics (ExAO) systems require high spatial and temporal sampling of wavefronts to optimize performance, and as a result, require large detectors for the wavefront sensor (WFS). We present a testbed demonstration of a three-sided pyramid wavefront sensor (3PWFS) as an alternative to the conventional four-sided pyramid wavefront sensor (4PWFS) for Giant Segmented Mirror Telescope ExAO applications. The 3PWFS is less sensitive to read noise than the 4PWFS because it uses fewer detector pixels. We describe the design of the Comprehensive Adaptive Optics and Coronagraph Test Instrument (CACTI). An experiment was performed on CACTI to determine the relative performance of the 3PWFS to the 4PWFS in varying strengths of turbulence using both the Raw Intensity and Slopes Map signal processing methods. We found that the performance of the two WFS is comparable if modal loop gains are tuned.
The Giant Segmented Mirror Telescopes (GSMTs) including the Giant Magellan Telescope (GMT), the Thirty Meter Telescope (TMT), and the European Extremely Large Telescope (E-ELT), all have extreme adaptive optics (ExAO) instruments planned that will use pyramid wavefront sensors (PWFS). The ExAO instruments all have common features: a high-actuator-count deformable mirror running at extreme speeds (>1 kHz); a high-performance wavefront sensor (WFS); and a high-contrast coronagraph. ExAO WFS performance is currently limited by the need for high spatial sampling of the wavefront which requires large detectors. For ExAO instruments for the next generation of telescopes, alternative architectures of WFS are under consideration because there is a trade-off between detector size, speed, and noise that reduces the performance of GSMT-ExAO wavefront control. One option under consideration for a GSMT-ExAO wavefront sensor is a three-sided PWFS (3PWFS). The 3PWFS creates three copies of the telescope pupil for wavefront sensing, compared to the conventional four-sided PWFS (4PWFS), which uses four pupils. The 3PWFS uses fewer detector pixels than the 4PWFS and should therefore be less sensitive to read noise. Here we develop a mathematical formalism based on the diffraction theory description of the Foucault knife-edge test that predicts the intensity pattern after the PWFS. Our formalism allows us to calculate the intensity in the pupil images formed by the PWFS in the presence of phase errors corresponding to arbitrary Fourier modes. We use these results to motivate how we process signals from a 3PWFS. We compare the raw intensity (RI) method, and derive the Slopes Maps (SM) calculation for the 3PWFS, which combines the three pupil images of the 3PWFS to obtain the X and Y slopes of the wavefront. We then use the Object Oriented MATLAB Adaptive Optics toolbox (OOMAO) to simulate an end-to-end model of an AO system using a PWFS with modulation and compare the performance of the 3PWFS to the 4PWFS. In the case of a low read noise detector, the Strehl ratios of the 3PWFS and 4PWFS are within 0.01. When we included higher read noise in the simulation, we found a Strehl ratio gain of 0.036 for the 3PWFS using RI over the 4PWFS using SM at a stellar magnitude of 10. At the same magnitude, the 4PWFS RI also outperformed the 4PWFS SM, but the gain was only 0.012 Strehl. This is significant because 4PWFS using SM is how the PWFS is conventionally used for AO wavefront sensing. We have found that the 3PWFS is a viable WFS that can fully reconstruct a wavefront and produce a stable closed-loop with correction comparable to that of a 4PWFS, with modestly better performance for high read-noise detectors.
We report on a test bed to compare the performance of three different wavefront sensors, the Shack-Hartmann Wavefront Sensor (SHWFS), the Pyramid Wavefront Sensor (PWFS), and the non-linear Curvature Wavefront Sensor (nlCWFS). No single wavefront sensor easily allows for sensing all aspects of atmospheric turbulence. For instance the SHWFS has a large dynamic range and a linear response to input phase aberrations but is not sensitive to low order modes. The PWFS uses the full spatial resolution of the pupil which gives it increased sensitivity to low order modes, however it still treads the line between achieving high dynamic range and high sensitivity. The nlCWFS is the only wavefront sensor designed to sense low and high, spatial frequencies, however this leads to a complex algorithm. We discuss the reconstruction algorithm for each WFS along with simulated comparisons, we present the optical design for the WFS comparison tes tbed, and outline the adaptive optics controls system.
This paper provides a status update on the Natural Guidestar (NGS) Adaptive Optics (AO)
system being built for Castor, the meter class telescope at the Starfire Optical Range. We present a radiometric case study for a range of variable parameters such as source brightness, number of Shack-Hartmann sub-apertures, AO and Track loop frame rate and bandwidth. We gauge system performance by contrast and adapt the error budget
to allow detection of a dim object near a bright star. We present wave band splits between the different AO components such as track sensor, wavefront sensor, scoring camera, and science camera. We show the different configurations that allow to switch between dim object and bright object tracking. The opto-mechanical design of the AO system is
also presented.
Within the next decade the Extremely Large Telescopes [ELTs] with diameters up to 40m will see first light. To optimize a high contrast pyramid wavefront sensor for an ELT extreme adaptive optics system, we are developing the theoretical framework of a three-sided pyramid wavefront sensor (3PWFS). The 3PWFS should have a higher photon efficiency and therefore be more sensitive to wavefront aberrations than the traditional four-sided pyramid wavefront sensor (4PWFS) in the presence of noise. In this paper we present results from end-to-end simulations, and from test benches at the Laboratoire d’Astrophysique de Marseille, and the University of Arizona.
The 3.5 m telescope, located in Albuquerque, New Mexico, at the Starfire Optical Range (SOR) routinely images asteroids and moons orbiting around the asteroid. However point-spread-functions (PSFs) with trefoil-like structure make it difficult to detect moons at small angular separations from the parent asteroid. This work investigates whether the low wind effect, such as that reported by VLT/SPHERE, produces trefoil-like PSFs (Sauvage et. al 2016).
The Nonlinear Curvature Wavefront Sensor (nlCWFS), first proposed by Guyon, determines wavefront shape from images of a reference beacon in a number of planes between the pupil and focal plane of a telescope. We describe a new algorithm that rapidly recovers the low-order aberrations accurately enough to allow practical use of the nlCWFS in an adaptive optics (AO) system. The algorithm was inspired by refractive strong scintillation in the interstellar medium, which behaves similarly to near-pupil linear curvature focusing, but over larger scales. The refractive component is extracted from the speckled images by binning with the lowest-order aberrations being additionally estimated through the use of first and second distribution moments. The linearity of the refractive scintillation process allows us to use a reconstructor matrix to compute an estimate of the pupil wavefront. The resulting wavefront estimate is then applied in reverse to a deformable mirror (DM), reducing the nonlinearity to the point that a single update phase retrieval algorithm such as a multi-plane version of Gerchberg-Saxton (GS) can be used to estimate the remaining wavefront error (WFE). An AO simulation of a 1.5 m telescope, a 16x16 actuator DM, and four image planes show that the scintillation algorithm works, reducing ~800 nm rms WFE to ~ 40 nm, well below the fitting error (~90 nm) in closed loop. Once corrected to this level, the image planes still show a great deal of information that can then be used with a single-update wavefront retrieval algorithm. A couple simple variants of GS are suggested, including one that can be parallelized for each camera and run in parallel with the scintillation algorithm. A Monte Carlo study will be required to determine the best approach.
A new technique for phase retrieval in non-linear Curvature Wavefront Sensors is presented. Unlike the iterative Gerchberg-Saxton technique, this technique retrieves pupil phase in a single step. It starts by calculating the optical transfer function from several images each with its own known aberration. It then solves for the pupil phase by inverting the process of autocorrelation, which, in effect, produced the optical transfer functions.
ELTs will offer angular resolution around 10mas in the near-IR and unprecedented sensitivity. While direct imaging of
Earth-like exoplanets around Sun-like stars will stay out of reach of ELTs, we show that habitable planets around nearby
M-type main sequence stars can be directly imaged. For about 300 nearby M dwarfs, the angular separation at maximum
elongation is at or beyond 1 ë/D in the near-IR for an ELT. The planet to star contrast is 1e-7 to 1e-8, similar to what the
upcoming generation of Extreme-AO systems will achieve on 8-m telescopes, and the potential planets are sufficiently
bright for near-IR spectroscopy. We show that the technological solutions required to achieve this goal exist. For
example, the PIAACMC coronagraph can deliver full starlight rejection, 100% throughput and sub-ë/D IWA for the EELT,
GMT and TMT pupils. A closely related coronagraph is part of SCExAO on Subaru. We conclude that large
ground-based telescopes will acquire the first high quality spectra of habitable planets orbiting M-type stars, while future
space mission(s) will later target F-G-K type stars.
In this paper we explain why a non-linear curvature wavefront sensor (nlCWFS) is more sensitive
than conventional wavefront sensors such as the Shack Hartmann wavefront sensor (SHWFS) and
the conventional curvature wavefront sensor (cCWFS) for sensing mV < 14 natural guide stars.
The non-linear approach builds on the successful curvature wavefront sensing concept but uses a
non-linear Gerchberg-Saxton (GS) phase diversity algorithm to reconstruct the wavefront. The nonlinear
reconstruction algorithm is an advantage for sensitivity but a challenge for fast computation.
The current speed is a factor of 10 to 100 times slower than needed for high performance groundbased
AO. We present a two step strategy to increase the speed of the algorithm. In the last
paper3 we presented laboratory results obtained with a monochromatic source, here we extend our
experiment to incorporate a broadband source. The sensitivity of the nlCWFS depends on the
ability to extract wavefront phase from diffraction limited speckles therefore it is essential that
the speckles do not suffer from chromatic aberration when used with a polychromatic source. We
discuss the design for the chromatic re-imaging optics, which through chromatic compensation,
allow us to obtain diffraction limited speckles in Fresnel propagated planes on either side of the
pupil plane.
In this paper we show why a non-linear curvature wavefront sensor (nlCWFS) is superior to both
Shack-Hartmann wavefront sensor (SHWFS) and conventional curvature wavefront sensor (cCWFS)
for sensing mV < 15 natural guide stars. We have developed an experimental setup aimed at
comparing the the rms wavefront error obtained with the nlCWFS and the SHWFS. We describe
our experimental setup and present results from the laboratory demonstration of the nlCWFS. The
non-linear approach builds on the successful curvature wavefront sensing concept. The wavefront
is reconstructed from the defocused pupil images using the
Gerchberg-Saxton (GS) phase diversity
algorithm. We compare results obtained from reconstructing the wavefront using a Shack-Hartmann
wavefront sensor (SHWFS) and a nlCWFS for a monochromatic source. We discuss approaches
to overcome non-linearity issues and discuss the challenge of using two WFSs in the same spatiotemporal
control regime and the implementation of the nlCWFS on the 6.5 m MMT.
This is the first of two papers discussing aspects of placing the deformable mirror in a location
not conjugate to the pupil plane of the telescope.
The Starfire Optical Range, Air Force Research Laboratory's Directed Energy Directorate
is in the process of developing a high efficiency AO system for its 3.5m optical telescope. The
objective is to achieve maximum diffraction limited performance, i.e., largest pupil diameter
possible, and maximum optical throughput. The later can be achieved by placing the deformable
mirror outside the pupil. However placing the DM in a location not conjugate to the pupil results
in a degradation in optical performance. This paper discusses experimental measurements of
the degradation.
In this paper we discuss the DM-not-in-pupil experimental testbed, the difficulties associated
with creating this type of testbed, and how these difficulties were overcome. We also present
results from the successful lab demonstration of closed loop performance with the DM placed out
of pupil. We experimentally measured the degradation in Strehl and implemented a mitigation
technique. Our experimental results indicate the mean degradation in Strehl as a result of placing
the DM out of pupil to be between 7% and 9 %. This result is comparable with wave optics
simulation and theoretical results which will be discussed in a companion paper, "Adaptive
optics with DM not in pupil - Part 2: Mitigation of Degradation".
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