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
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.
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.
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
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
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".