Final Image Plane Research Articles

Aberration correction of aperture-divided off-axis simultaneous polarization super-resolution imaging optical system

According to the requirements for broadband simultaneous polarization high-resolution imaging, a divided-aperture simultaneous polarization super-resolution imaging system based on reflective free-form surface optical system and digital micro-mirror device is proposed. It has the advantages of wide working wavelength band, simultaneous imaging of multiple polarization states, single detector, high resolution and lightweight. The aberration correction principle and design optimization method of the optical structure for this imaging system are given. The Wassermann-Wolf theory is further developed, and the reflective Wassermann-Wolf differential equations that eliminate a variety of aberrations are derived. At the same time, combined with Seidel aberration theory and using iterative method, the distortion elimination boundary condition is added when solving the Wassermann-Wolf equation. Through the iterative method, the optical initial structure is obtained, which can correct spherical aberration, coma, astigmatism and distortion at the same time. The initial structure is subjected to off-axis treatment and further optimized, and the user-defined optimization evaluation function is written to strictly control the position of the light falling point of each sub aperture and each field of view on the middle image plane and the final image plane, so as to effectively suppress the distortion in the final system and avoid the mismatch error between the mirror element and the pixel in the process of super-resolution reconstruction. The reconstruction quality can be improved. Finally, the design of the four-sub-aperture free-form surface off-axis reflective super-resolution imaging optical system is completed, which possesses a large relative aperture (<i>F</i># = 2.5) and compact structure. The imaging quality of each polarization channel is close to the diffraction limit. The above aberration correction principle and the image quality optimization method can effectively guide the design of the wide band simultaneous polarization super-resolution imaging optical system.

Active Correction of Aperture Discontinuities-Optimized Stroke Minimization. I. A New Adaptive Interaction Matrix Algorithm

Abstract Future searches for bio-markers on habitable exoplanets will rely on telescope instruments that achieve extremely high contrast at small planet-to-star angular separations. Coronagraphy is a promising starlight suppression technique, providing excellent contrast and throughput for off-axis sources on clear apertures. However, the complexity of space- and ground-based telescope apertures goes on increasing over time, owing to the combination of primary mirror segmentation, the secondary mirror, and its support structures. These discontinuities in the telescope aperture limit the coronagraph performance. In this paper, we present ACAD-OSM, a novel active method to correct for the diffractive effects of aperture discontinuities in the final image plane of a coronagraph. Active methods use one or several deformable mirrors that are controlled with an interaction matrix to correct for the aberrations in the pupil. However, they are often limited by the amount of aberrations introduced by aperture discontinuities. This algorithm relies on the recalibration of the interaction matrix during the correction process to overcome this limitation. We first describe the ACAD-OSM technique and compare it to the previous active methods for the correction of aperture discontinuities. We then show its performance in terms of contrast and off-axis throughput for static aperture discontinuities (segmentation, struts) and for some aberrations evolving over the life of the instrument (residual phase aberrations, artifacts in the aperture, misalignments in the coronagraph design). This technique can now obtain the Earth-like planet detection threshold of contrast on any given aperture over at least a 10% spectral bandwidth, with several coronagraph designs.

Active Correction of Aperture Discontinuities-Optimized Stroke Minimization. II. Optimization for Future Missions

Abstract High-contrast imaging and spectroscopy provide unique constraints for exoplanet formation models as well as for planetary atmosphere models. Instrumentation techniques in this field have greatly improved over the last two decades, with the development of stellar coronagraphy, in parallel with specific methods of wavefront sensing and control. Next generation space- and ground-based telescopes will enable the characterization of cold solar-system-like planets for the first time and maybe even in situ detection of bio-markers. However, the growth of primary mirror diameters, necessary for these detections, comes with an increase of their complexity (segmentation, secondary mirror features). These discontinuities in the aperture can greatly limit the performance of coronagraphic instruments. In this context, we introduced a new technique, Active Correction of Aperture Discontinuities-Optimized Stroke Minimization (ACAD-OSM), to correct for the diffractive effects of aperture discontinuities in the final image plane of a coronagraph, using deformable mirrors. In this paper, we present several tools that can be used to optimize the performance of this technique for its application to future large missions. In particular, we analyzed the influence of the deformable setup (size and separating distance) and found that there is an optimal point for this setup, optimizing the performance of the instrument in contrast and throughput while minimizing the strokes applied to the deformable mirrors. These results will help us design future coronagraphic instruments to obtain the best performance.

Performance of the hybrid externally occultedLyotsolar coronagraph

Context. High-contrast hybrid coronagraphs, which combine an external occulter and a Lyot-style coronagraph became a reality in recent years, despite the lack of analytic and numerical end-to-end performance studies. The solar coronagraph ASPIICS which will fly on the future ESA Formation Flying mission Proba-3 is a good example of such a hybrid coronograph. Aims. We aim to provide a numerical model to compute theoretical performance of the hybrid externally occulted Lyot-style coronagraph, which we then aim to compare to the performance of the classical Lyot coronagraph and the externally occulted solar coronagraph. We will provide the level and intensity distribution of the stray light, when the Sun is considered as an extended source. We also investigate the effect of different sizes for the internal occulter and Lyot stop. Methods. First, we have built on a recently published approach, to express the diffracted wave front from Fresnel diffraction produced by an external occulter at the entrance aperture of the coronagraph. Second, we computed the coherent propagation of the wave front coming from a given point of the Sun through the instrument. This is performed in three steps: from the aperture to the image of the external occulter, where the internal occulter is set, from this plane to the image of the entrance aperture, where the Lyot stop is set, and from there to the final image plane. Making use of the axis-symmetry, we considered wave fronts originating from one radius of the Sun and we circularly average the intensities. Our numerical computation used the parameters of ASPIICS. Results. The hybrid externally occulted Lyot coronagraph rejects sunlight below 10E-8Mean Solar Brightness from 1,3 solar radius - in the particular configuration of ASPIICS. The Lyot coronagraph effectively complements the external occultation. We show that reducing the Lyot stop allows a clear gain in rejection, being even better than oversizing the internal occulter, that tends to exclude observations very close to the solar limb. As an illustration, we provide a graph that allows us to estimate performance as a function of the internal occulter and Lyot stop sizes. Conclusions. Our work consists of a methodological approach to compute the end-to-end performance for solar coronagraph.

The research on the illumination distribution law of the first-order scattered light in the focal plane of transmission optical system

With the advantages of simple structure, low-cost, large field of view, and high image quality, the transmission optical system is widely used in detection system, microscope, telescope, etc. However, the research on the illumination distribution law in the focal plane of transmission optical system is rarely reported. In this paper, this issue is studied. During the study on the first-order scattered light distribution law in the focal plane of the transmission optical system, the limitations of the two-parameter Harvey bi-directional scatter distribution function (BSDF) scattering theory are found, namely in the condition of small scattering angle, the two-parameter Harvey BSDF theory cannot accurately describe the scattering properties of the optical surface material. So the scattering model of the transmission optical system under small scattering angle is established by introducing parameter l, and the accuracy of the new theoretical model is verified experimentally. This model complements the two-parameter Harvey BSDF scattering model and broadens the application scope of the Harvey BSDF scattering model so that it can better explain the imaging law of scattered light spot in the focal plane of transmission optical system. At a small scattering angle, the conclusions can be drawn from the new theoretical model as follows. 1) The irradiance of the final image plane increases linearly with the increase of the incident optical power. 2) In the transmission optical system, the contribution of each optical surface with the same scattering properties to scattered spot irradiance in the final image plane can be expressed by a Gaussian function. 3) The irradiance of scattered spot in the final image plane can be expressed as the superposition of n Gaussian functions, where n is the number of optical surfaces with different scattering properties in the transmission optical system.

Optical properties of a multibeam column with a single-electron source

A novel single-column multi-electron-beam system, called a beam-split array, has been developed for a high-resolution, high-throughput lithography tool. In this system, a single electron beam is divided into 1024 beams by a multisource module (MSM) composed of an aperture array (a beam-dividing aperture), a static lens array (Einzel lenses for each divided beam), and a blanker array (BLA, blanking electrode pairs for each focused beam). The MSM is used to form multiple intermediate images of the electron source at the BLA. These images are demagnified to form final images through a projection optics consisting of a double lens doublet with a blanking aperture and deflector. To align the multiple beam paths in the MSM, aligners between these arrays are used, and the aligner conditions are determined by monitoring the blanking-aperture image. Moreover, because each beam current is about 0.1% of the total beam current on the specimen, a high-contrast transmission detection method is used to detect the electrons at the final image plane. As a result, 1024 point beams are successfully formed. In the final image, the measured beam size is less than 55nm, and the displacement due to distortion is less than 56nm, even on the off-axial beams. In addition, individual beam blanking by BLA is verified, and cross-talk at the BLA is confirmed to be negligible at present accuracy. Moreover, 65nm patterns can be simultaneously delineated by near- and off-axial beams. These results verify the concept of their single-column multibeam formation and indicate that this optics can be applied to lithography for manufacturing on semiconductor devices.

Purely numerical compensation for microscope objective phase curvature in digital holographic microscopy: influence of digital phase mask position

Introducing a microscope objective in an interferometric setup induces a phase curvature on the resulting wavefront. In digital holography, the compensation of this curvature is often done by introducing an identical curvature in the reference arm and the hologram is then processed using a plane wave in the reconstruction. This physical compensation can be avoided, and several numerical methods exist to retrieve phase contrast images in which the microscope curvature is compensated. Usually, a digital array of complex numbers is introduced in the reconstruction process to perform this curvature correction. Different corrections are discussed in terms of their influence on the reconstructed image size and location in space. The results are presented according to two different expressions of the Fresnel transform, the single Fourier transform and convolution approaches, used to propagate the reconstructed wavefront from the hologram plane to the final image plane.

Advanced technology solar telescope multiple Fabry-Pérot interferometer telecentric optical design

We present four preliminary designs for a telecentric optical train supporting the Advanced Technology Solar Telescope (ATST) multiple Fabry-Perot interferometer (MFPI), which is to be used as an imaging spectrometer and imaging spectropolarimeter. The point of departure for all three designs is the F/40 telecentric image at the Coude focus of the ATST. The first design, representing the high-spectral-resolution mode of operation, produces an intermediate F/300 telecentric image within the triple etalon system and a 34-arcsec field of view (FOV). The second design, intermediate between high- and low-spectral-resolution modes of operation, produces an intermediate F/150 telecentric image at the etalons and a 1.1-arcmin FOV. The third and fourth designs each represent a low-resolution mode of operation, producing an F/82 telecentric image at the etalons and a 2-arcmin FOV. Each design results in good telecentricity and image quality. Departures from telecentricity at the intermediate image plane cause field-dependent shifts of the bandpass peak, which are negligible compared to the bandpass FWHM. The root mean square (rms) geometric spot sizes at the final image plane fit well within the area of a camera pixel, which is itself in accordance with the Nyquist criterion, half the width of the 28-µm-wide resolution element (as determined from the diffraction limit of the ATST). For each configuration, we also examine the impact that the Beckers effect (the pupil apodization caused by the angle-dependent amplitude transmittance of the MFPI) has on the image quality of the MFPI instrument.

Influence of quadratic phase factors on optical imaging and Fourier transforming in anamorphic systems

Propagation of quadratic phase factors in coherent optical anamorphic processors is considered. They can be introduced in a first imaging two-lens system, in order to have an amplitude distribution affected by quadratic phase factors, which serves as the object for a second system. In imaging, the quadratic phase factors do not determine the position of the final image plane. On the contrary, in anamorphic Fourier transformers the quadratic phase factors influence the position of Fourier planes. Therefore, in the design of anamorphic processors their influence on Fourier transformer performance should be taken into account.

Application of acousto-optic cells and video processing to achieve signal-to-noise improvements in variable resolution moire profilometry

Moire techniques can be a powerful tool to determine surface shape or deviation of a shape in progress from a final or desired shape. The presence of the high-contrast viewing grating and the distorted grating in the final image plane makes the moire pattern hard to see. Moving grating techniques have been developed to improve the visibility of the moire pattern, but at the expense of complex moving parts. Several variable resolution projection moire techniques have been developed that either move the grating or eliminate its presence electronically, and have neither mechanical moving parts nor any physical gratings. One system uses an acousto-optics cell to generate, project, and move the gratings, while the moire is viewed through a second synchronized A-O cell. The second system uses an interferometer to generate and project variable spacing gratings that are made to move across the target and across a reference surface by an A-O beam deflector. Video processing of the reference image generates the transmissive filter that produces the moire pattern. A third system removes the grating presence electronically but retains high-contrast moire contours. Noise reduction is shown in a series of moire images of targets.

Direct imaging of nonsolar planets with infrared telescopes using apodized coronagraphs

This research examines the use of modified Lyot coronagraphs with monolithic and segmented infrared telescopic systems for imaging nonsolar planets. These systems are investigated with the aim of reducing the effects of stellar diffracted energy on the planet image in the final image plane. A square telescope objective is best for this purpose. The associated coronagraph is composed of a cross-shaped apodizer in the first focal plane and either a square Lyot stop or circular corner Lyot stops in the corners of the pupil plane. We examine the consequences of segmenting the aperture and the effects of various segment spacings and random piston and tilt errors of the individual segments. A system to correct for the misalignments is proposed.

Tolerances for Focus Stability in Zoom Lenses

Formulae are obtained for the image shifts arising from longitudinal and transverse positional errors and tilting of components during the operation of a zoom system. A technique is described for adjusting the positions of the zoom components (which arise because of residual production errors) to optimise the stability of the final image plane during zooming. A rapid method of determining the tolerances of the zoom system is introduced, which is particularly appropriate for a desk computer.

Density Pseudocolor Encoding By Holographic Techniques With Three Primary Colors

A new optical method to generate a family of color-coded images using holographic techniques is presented. This method involves the introduction of three primary colors. Thus, this technique overcomes an important limitation of previous holographic techniques. An image hologram of a black-and-white transparency is first registered. This hologram is put in contact with a beam splitter, and the sandwich is illuminated by three partially coherent waves. One wave produces a positive image, a second wave pro-duces an image with reversed contrast, the negative, and the third image corresponds to the product of the positive and the negative. Coding these waves in three primary colors and adding them at the final image plane results in a color-coded image. The principle of the method, theory, and some experimental color results are presented.

Generation of a uniform line source using a binary filter

A uniform or nearly uniform 10 mm long line source has been generated from a gaussian laser beam using a binary filter. This is done by processing the binary filter throughout a well corrected optical system, so that the equivalent of a continuous inverse gaussian filter is obtained at the final image plane. This equivalent continuous filter generates a uniform line source with maximum conversion efficiency of 50%.

Scanning mirror microscope with optical sectioning characteristics: applications in ophthalmology

Optical sectioning is the simultaneous illumination and viewing of only a thin region of a specimen. An illuminated slit is imaged at the plane of interest and is swept laterally by the action of an oscillating mirror. The light returning from the specimen reflects from a second facet of the oscillating mirror and forms a stationary image of the illuminated slit. At this stationary image a second slit is placed, which passes light from the desired plane and rejects scattered light from other depths within the specimen. Light passing through the second slit is reflected from the third facet of the oscillating mirror and is focused to the final image plane. The image is reconstructed as the image of the second slit sweeps across the image plane. An important ophthalmological application is the examination of the endothelial cell layer of the cornea, either by contact or noncontact techniques. Optimization for image illuminance and resolution is discussed.

Electron micro-densitometry during in-situ TEM investigations

An electron detector with small detection area for use in the final image plane of a transmission electron microscope has been commercially offered by TEM manufacturers (1). However, little has been reported about the application of such a device. In this report, the construction principle and the operation of a novel microdensitometry device is described which combines precision image point selection with versatility, high sensitivity, ease of operation, and comparatively low cost.The electron microdensitometer system consists of a transparent phosphor screen onto which the electron image is projected, a small “channeltron” electron multiplier (supplied by Galileo/Bendix) underneath, and a closed-circuit TV system (Fig.l). The screen has an effective diameter of 45 mm and contains a 2 mm concentric hole which allows a small beam of electrons to pass unobstructed to the electron multiplier. The multiplier consists of a 3 mm O.D. tube in custom L-shaped configuration and is mounted such that its collector is located outside the screen area to minimize obstruction of the image viewed on the TV monitor.

The Accurate Measurement of the Energy Dependence of Radiation Damage Rates in Organic Materials

In electron microscopy and radiation damage studies in organic materials it is important to be able to make an accurate assessment of the rate at which the damage processes proceed.Approximate values for these radiation damage rates have been calculated using solid state detectors in order to measure the appropriate beam current densities, however the strong intensity dependence of these devices (Fig. 1) and their susceptibility to radiation damage imposes large errors when any consistency in their efficiency is assumed.A Faraday cup has been constructed (Fig. 2) which is capable of measuring currents down to 10-15 amperes cm-2 from the final image plane of an electron microscope, with an error of less than 1%. The cup, being positioned beneath the final viewing screen is therefore capable of distinguishing local incident intensities in the object plane.

Electron Microscope Image Intensification and Specimen Exposure

The efficiency of electron microscope image intensification (EMII) systems is often represented in terms of signal-to-noise ratio as a function of current density in the final microscope image plane. Such a characteristic is one way of evaluating the sensitivity of an EMII-system. It determines the intensification necessary to raise the signals of individual electrons well above the equipment noise level of the intensifier/display chain.This approach to the problem of EMU may lead to misinterpretations since it does not consider the spatial resolution in the specimen. Usually, the spatial resolution of the EMII-chain is considerably inferior to the resolution of conventional photographic material. In internal photography one can, therefore, normally work with considerably lower electron optical magnifications than with an image intensifier system to resolve specimen details of the same dimensions.

Image-distortion due to glass-thickness in lens systems. Part 2

The simplest method of applying the formulae given in part 1* is discussed. The formulae are applied to a Series V Cooke lens of 10-in. equivalent focal length. The effects of the three axial glass-thicknesses in producing fractional distortion in the final image or focal plane are ascertained by the application of two alternative but mutually confirming methods, the older and simpler of which is recommended for adoption.