Mutual coherence function of a wave front corrected by zonal adaptive optics

Greenwood, Darryl P.

doi:10.1364/josa.69.000549

Cited by 21 publications

(6 citation statements)

References 8 publications

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“…The various errors in a single LGS AO system have already been expressed by analytical formulas in a number of papers. [15][16][17][18] Such analytical expressions allow the AO performance to be studied as a function of wavelength and of NGS magnitude; the study consumes a small amount computation time. The entire error budget of an AO system takes into account errors related to the geometry of the system such as sensor noise ͑ ron 2 ͒, fitting and aliasing errors ͑ fit 2 and alias 2 ͒, and external noise such as sky background and photon noise ͑ ph 2 ͒ that are directly related to the number of photons emitted by the source.…”

Section: A Adaptive-optics Systemmentioning

confidence: 99%

Adaptive optics with four laser guide stars: correction of the cone effect in large telescopes

2002

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We study the performance of an adaptive optics (AO) system with four laser guide stars (LGSs) and a natural guide star (NGS). The residual cone effect with four LGSs is obtained by a numerical simulation. This method allows the adaptive optics system to be extended toward the visible part of the spectrum without tomographic reconstruction of three-dimensional atmospheric perturbations, resolving the cone effect in the visible. Diffraction-limited images are obtained with 17-arc ms precision in median atmospheric conditions at wavelengths longer than 600 nm. The gain achievable with such a system operated on an existing AO system is studied. For comparison, performance in terms of achievable Strehl ratio is also computed for a reasonable system composed of a 40 x 40 Shack-Hartmann wave-front sensor optimized for the I band. Typical errors of a NGS wave front are computed by use of analytical formulas. With the NGS errors and the cone effect, the Strehl ratio can reach 0.45 at 1.25 microm under good-seeing conditions with the Nasmyth Adaptive Optics System (NAOS; a 14 x 14 subpupil wave-front sensor) at the Very Large Telescope and 0.8 with a 40 x 40 Shack-Hartmann wave-front sensor.

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Section: A Adaptive-optics Systemmentioning

confidence: 99%

Adaptive optics with four laser guide stars: correction of the cone effect in large telescopes

2002

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“…An AO system will reduce the spatial power spectrum at low spatial frequencies, the effect of which can be modelled by a high‐pass filter, H (κ d /2) (Greenwood ),

H (κ d / 2) = 1 - {((), \frac{2 J_{1} (κ d / 2)}{κ d / 2})}^{2} - 16 {(() 2 / κ d)}^{2} J_{2}^{2} (κ d / 2),

where d is the diameter of the subapertures and J n is a Bessel function of the first kind of the order of n . The given equation is for a segmented mirror with tip/tilt and piston correction.…”

Section: Simulationmentioning

confidence: 99%

“…An AO system will reduce the spatial power spectrum at low spatial frequencies, the effect of which can be modelled by a high-pass filter, H(κd/2) (Greenwood 1978),…”

Section: Adaptive Opticsmentioning

confidence: 99%

Adaptive pupil masking for quasi-static speckle suppression

Osborn

2012

Monthly Notices of the Royal Astronomical Society

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Quasi-static speckles are a current limitation to faint companion imaging of bright stars. Here we show through simulation and theory that an adaptive pupil mask can be used to reduce these speckles and increase the visibility of faint companions. This is achieved by placing an adaptive mask in the conjugate pupil plane of the telescope. The mask consists of a number of independently controllable elements which can either allow the light in the subaperture to pass or block it. This actively changes the shape of the telescope pupil and hence the diffraction pattern in the focal plane. By randomly blocking subapertures we force the quasi-static speckles to become dynamic. The long exposure PSF is then smooth, absent of quasi-static speckles. However, as the PSF will now contain a larger halo due to the blocking, the signal to noise ratio (SNR) is reduced requiring longer exposure times to detect the companion. For example, in the specific case of a faint companion at 5xlambda/D the exposure time to achieve the same SNR will be increased by a factor of 1.35. In addition, we show that the visibility of companions can be greatly enhanced in comparison to long-exposures, when the dark speckle method is applied to short exposure images taken with the adaptive pupil mask. We show that the contrast ratio between PSF peak and the halo is then increased by a factor of approximately 100 (5 magnitudes), and we detect companions 11 magnitudes fainter than the star at 5xlambda/D and up to 18 magnitudes fainter at 22.5xlambda/D.Comment: 8 pages, 9 figures, MNRAS, accepted 25th May 201

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“…It appears because the actuator spacing is not infinitely small, so all spatial frequency aberrations cannot be compensated for. We used Greenwood (1979) for j 2 fit , the fitting error variance:…”

Section: The Ngs-ao Simulationmentioning

confidence: 99%

Laser Guide Star for 3.6- and 8-m telescopes: Performance and astrophysical implications

Louarn

Foy

Hubin

et al. 1998

Monthly Notices of the Royal Astronomical Society

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We have constructed an analytical model to simulate the behaviour of an adaptive optics system coupled with a sodium laser guide star. The code is applied to 3.6‐ and 8‐m class telescopes. The results are given in terms of Strehl ratio and full width at half‐maximum of the point spread function. Two atmospheric models are used, one representing good atmospheric conditions (20 per cent of the time), the other median conditions. Sky coverage is computed for natural guide star and laser guide star systems, with two different methods. The first one is a statistical approach, using stellar densities to compute the probability of finding a nearby reference. The second is a cross‐correlation of a science‐object catalogue and the USNO catalogue. Results are given in terms of percentage of the sky that can be accessed with given performances, and in terms of the number of science objects that can be observed, with Strehls greater than 0.2 and 0.1 in the K and J bands.

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Mutual coherence function of a wave front corrected by zonal adaptive optics

Cited by 21 publications

References 8 publications

Adaptive optics with four laser guide stars: correction of the cone effect in large telescopes

Adaptive optics with four laser guide stars: correction of the cone effect in large telescopes

Adaptive pupil masking for quasi-static speckle suppression

Laser Guide Star for 3.6- and 8-m telescopes: Performance and astrophysical implications

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