Correction Of Wavefront Errors Research Articles

Development of a period-by-period EUV multilayer milling system for the final nm figure error correction by 0.1 nm per period

A period-by-period ion milling system has been developed for final reflection wavefront error correction of an imaging EUV multilayer mirror by a stepwise 0.1 nm-per-period correction of the residual nm figure errors. For effective, gentle and uniform milling of the surface areas selected by a template, the system is designed with a rotating substrate holder exposed to a 150 mm-wide ion beam with a dose homogenizer mask plate. For demonstration of the wavefront correction principle, local milling of a dielectric multilayer mirror for visible light was carried out by the system. The wavefront as measured by a phase shifting interferometer showed that the reflection phase was "advanced" by the milling, which formed a geometrical depression at the multilayer mirror surface. This confirmed the physical optics principle of our method and proved the procedure of the method being promising and practical for the accurate reflection phase correction of an EUV multilayer mirror.

Wavefront error correction and Earth-like planet detection by a self-coherent camera in space

Context. In the context of exoplanet detection, the performance of coronagraphs is limited by wavefront errors. Aims. To remove efficiently the effects of these aberrations using a deformable mirror, the aberrations themselves must be measured in the science image to extremely high accuracy. Methods. The self-coherent camera which is based on the principle of light incoherence between star and its environment can estimate these wavefront errors. This estimation is derived directly from the encoded speckles in the science image, avoiding differential errors due to beam separation and non common optics. Results. Earth-like planet detection is modeled by numerical simulations with realistic assumptions for a space telescope. Conclusions. The self-coherent camera is an attractive technique for future space telescopes. It is also one of the techniques under investigation for the E-ELT planet finder the so-called EPICS.

On representing and correcting wavefront errors in high-contrast imaging systems

Conventional adaptive-optics systems correct the wavefront by adjusting a deformable mirror (DM) based on measurements of the phase aberration taken in a pupil plane. The ability of this technique, known as phase conjugation, to correct aberrations is normally limited by the maximum spatial frequency of the DM. In this paper we show that conventional phase conjugation is not able to achieve the dark nulls needed for high-contrast imaging. Linear combinations of high frequencies in the aberration at the pupil plane "fold" and appear as low-frequency aberrations at the image plane. After describing the frequency-folding phenomenon, we present an alternative optimized solution for the shape of the deformable mirror based on the Fourier decomposition of the effective phase and amplitude aberrations.

Digital correction of wave-front errors caused by detector nonlinearity of second order in phase-shifting interferometry

Even if the phase error in recording plane is ignorable, the amplitude error may still occur in phase-shifting interferometry (PSI) and it will affect both the amplitude and phase distributions of the reconstructed wave-front in original object plane when inverse Fresnel diffraction is involved. A novel and very simple algorithm of digital data processing for correcting the wave-front reconstruction errors caused by detector nonlinearity of the second order in PSI is introduced and satisfactorily verified by computer simulations. This method can eliminate the amplitude distortion resulting from this error source without the specific knowledge about it, and then greatly depress both the amplitude and phase errors of the reconstructed wave-front of a diffraction object by approximately three orders of magnitude.

Correction of wave-front errors caused by the slight tilt of a reference beam in phase-shifting interferometry.

In standard phase-shifting interferometry the reference beam is supposed to be a plane wave exactly normal to the recording plane. A slight tilt of the reference beam, however, may occur in practice, and it will introduce phase distortion for the reconstructed object wave front. The effects of reference wave tilt on the wave reconstruction are analyzed, and a novel method is proposed to correct the errors caused by this tilt. This method is simple and convenient without the need of any additional optical devices and measurements, and it can be used for both the smooth and the diffuse object surfaces. The effectiveness of this method is verified by a series of computer simulations.

Imaging Extrasolar Planets by Stellar Halo Suppression in Separately Corrected Color Bands

Extrasolar planets have not been imaged directly with existing ground or space telescopes because they are too faint to be seen against the halo of the nearby bright star. Most techniques being explored to suppress the halo are achromatic, with separate correction of diffraction and wave-front errors. Residual speckle structure may be subtracted by differencing images taken through narrowband filters, but photon noise remains and ultimately limits sensitivity. Here we describe two ways to take advantage of narrow bands to reduce speckle photon flux and to obtain better control of systematic errors. Multiple images are formed in separate color bands of 5%-10% bandwidth and recorded by coronagraphic interferometers equipped with active control of wave-front phase and/or amplitude. In one method, a single deformable pupil mirror is used to actively correct both diffraction and wave-front components of the halo. This yields good diffraction suppression for complex pupil obscuration, with high throughput over half the focal plane. In a second method, the coronagraphic interferometer is used as a second stage after conventional apodization. The halo from uncontrollable residual errors in the pupil mask or wave front is removed by destructive interference made directly at the detector focal plane with an antihalo, synthesized by spatial light modulators in the reference arm of the interferometer. In this way very deep suppression may be achieved by control elements with greatly relaxed, and thus achievable, tolerances. In both examples, systematic errors are minimized because the planet imaging cameras themselves also provide the error-sensing data.

Single-pixel demonstration of innovative adaptive optics by use of a charge-transfer membrane light modulator

We have constructed and demonstrated a single-pixel implementation of an all-optical membrane-based spatial light modulator as a compact optical wave-front error correction device. High rates of response of as much as 20 kHz in an open-loop configuration were obtained. The device was then used in an adaptive-optics servo to compensate successfully for a 1-kHz sinusoidal phase error with a peak-to-peak excursion of approximately pi/7 rad. A small-signal servo gain of the order of 10 was inferred from the closed-loop measurements.

15 meter multiple mirror telescope design study

Taking as a starting point the existing Multiple Mirror Telescope (MMT), a concept for a larger and more advanced instrument has been developed. It makes use of four 7.5-m diameter paraboloidal glass primaries of the honeycomb sandwich type being developed by the University of Arizona. These are mounted quite close together in a square configuration, with their axes coaligned. Separate optical configurations are provided, for optical and infrared applications. To minimze telescope emissivity in the thermal infrared at the combined focus, all the beam combining and streering optics that follow the tertiary mirrors are enclosed in a large central dewar and cooled with liquid nitrogen. The diffraction-limited resolution at the combined focus of 0.11 arcsec at 10 micrometers wavelength is equivalent to that of a 20.5 meter filled aperture. Diffraction-limited resolution should be routinely achievable at 10 and 20 micrometers, if active correction of large-scale wavefront errors is implemented.

Requirements for piezoelectric materials for deformable mirrors

Abstract Deformable mirrors to correct wavefront errors in real time have emerged as a key component in adaptive optical systems being developed for a variety of applications. Piezoelectric materials, particularly lead zinconate titanate have proven to be the most desirable actuators for high speed, high element density, electrically controlled mirrors. However, this application has place particularly tight constraints on the properties of the materials employed, some of which remain to be optimized. A general description of deformable mirror types is given, followed by a discussion of the material properties required for this application.