High resolution satellite imaging 0.5U-size freeform telescope for CubeSat

For recent years, with the development of manufacture, free-form surfaces are widely used in optical systems because of the degrees of freedom and the capacity of correcting optical aberrations. Nowadays, commercial optical design software such as Optics Studio are commonly applied to optimize an optical system from a suitable initial structure to obtain high-quality imaging characteristics. The initial structure optimized by software must include the surface types and its parameters. Notice that the free-form surface type is unknown, let alone the parameters. Therefore, the initial structure of a free-form surface cannot be designed by commercial optical design software directly. The software cannot optimize or analyze the system with free-form surfaces either. This paper proposed an integral design method of optical system with free-form surfaces based on theoretical calculation and optimization of optical design software. Firstly, Fermat’s principle, equal optical path length between object and image, is applied to calculate the sampling data points of the free-form surface. Data points are fitted using least-square fitting algorithm to obtain a the surface type and its parameters, which are used as the initial structure. Secondly, a commercial optical design software further optimize the optical system containing the free-form surface. Not only the free-form surface but also other parameters can be optimized by the software. With the powerful functions of image quality and tolerance analysis of the software, we can obtain a high-quality imaging system with free-form surfaces in a short time. This method solves the problem that commercial software cannot design optical systems with free-form surfaces directly. Meanwhile initial structure with free-form surface can be acquired directly by this method. In addition,the powerful functions of the software such as optimization, image quality and tolerance analysis are fully utilized in the process of integral optimization. Since the system will be further optimized by the software, a small amount of sampling data points is enough to calculate the initial structure which speeds up the calculation. A passive ranging and three-dimensional imaging system with chiral phase mask is designed as an example. Different from conventional imaging system, two image points will be obtained by the system when one point is used as the object. Apparently, it is difficult for any commercial optical design software to design. The novel integral method proposed in this paper is applied to this system.Firstly, the system is designed in reverse sequence for the sake of simplification. A doublet with effective focal length 200mm and f-number 8 is used as the initial structure. Two points at (0,0.3,256.1) and (0,-0.3,256.1) are used as Please verify that (1) all pages are present, (2) all figures are correct, (3) all fonts and special characters are correct, and (4) all text and figures fit within the red margin lines shown on this review document. Complete formatting information is available at http://SPIE.org/manuscripts Return to the Manage Active Submissions page at http://spie.org/submissions/tasks.aspx and approve or disapprove this submission. Your manuscript will not be published without this approval. Please contact author_help@spie.org with any questions or concerns. the “object” which will image at (0,0,-63.6) as the “image” in the reverse sequence. A free-form surface is added in front of the doublet, thus the whole system turns to a triplet system. 6X12 sampling data points are calculated to fit an XY polynomial surface. Secondly a commercial optical design software further optimizes the optical system with the free-form surface. Finally, we obtain a high-quality imaging system with a free-form surface. The MTF (Modulation Transfer Function) performance of the system is closed to diffraction limit.

We discuss the conceptual and practical guidelines of a method to calculate the cross-polarization of a telescope, including its relay optics, using a commercial optical design software, without the need to use complex, slow and expensive Physical Optics programs. These effects are usually negligible at visible and infrared wavelengths but may be of considerable importance at radio wavelengths. Offset reflector antenna configurations, common in the telecommunication industry, and antenna relay optics consisting of offset mirrors, common in millimeter and submillimeter-wave telescopes, result in an increased contribution to the cross-polarization. Polarization measurements are also becoming very important in Radio Astronomy. In fact, dust emission polarimetry and the study of linearly polarized, nonmasing, rotational lines (e.g., CO) with submm telescopes are both powerful diagnostic of magnetic fields in molecular clouds. However, the low average source polarization requires a careful optimization of the optical design to keep any instrumental polarization contribution from both telescope and relay optics as low as possible in astronomical polarimetry experiments. Likewise, in telecommunications applications polarization separation can be used to effectively double the available bandwidth provided the isolation between the two orthogonal polarization states is sufficient.

Optical designers are encouraged to adopt aspheric and free-form surfaces into an increasing number of design spaces because of their improved performance. However, residual tooling marks from advanced aspheric fabrication techniques are difficult to remove. These marks, typically in the mid-spatial frequency (MSF) regime, give rise to structured image artifacts. Using a theory developed in previous publications, this paper applies the fundamentals of MSF modeling to demonstrate how MSF errors are evaluated and toleranced in an optical system. Examples of as-built components with MSF errors are analyzed using commercial optical design software.

Clocking of lens elements is frequently used as an effective method of compensating for two-dimensional tolerances such as material inhomogeneity and surface figure errors. Typically, the lens designer has to determine the optimum angles of rotation by manually modeling lens element clocking in the commercial optical design software because the nature of errors resolved by lens clocking does not lead to good convergences for clocking optimization. In this paper, a method of automatic clocking optimization is developed. The method is implemented using a combination of particle swarm optimization algorithm and commercial optical design software. The optimum angles of rotation and predicted imaging performance are automatically calculated using this method. Methods of implementation and optimization examples are also given.

In this work, an optical design approach is presented to design an ultrashort throw distance projection system by combination of an off-the-shelf refractive lens and two off-axis freeform mirrors. These two freeform mirrors are used to greatly shorten the projection distance by more than three times compared to conventional (rotationally symmetric) systems, while still maintaining a good imaging quality. Firstly, a direct design method that enables the simultaneous calculation of two off-axis freeform-profile mirrors by partially coupling more than three fields is introduced. The specifications of the conventional refractive lens are taken into account during this procedure. The pupil matching principle is applied to ensure good performance between the two sub-systems. The calculated mirrors then serve as a good starting point for optimization using commercial optical design software. To step from freeform profiles to freeform surfaces, the calculated two profiles are fitted into odd polynomials to evaluate the image quality and then re-fitted into XY polynomials for further optimization. Finally, the polynomial coefficients of the two freeform mirrors are imported into the optical design program. The merit function is built from RMS spot radii over the full field, and additional constraints are made for correcting distortion. After optimization, the calculated initial design quickly converges to a well performing imaging system. As an example, an ultrashort throw distance projection lens with a large 80-inch diagonal image at 400mm throw distance is designed, analyzed and compared with literature data. The values of MTF are over 0.6 at 0.5 lp/mm and the distortion is less than 1.5%: showing a very good and well balanced imaging performance over the entire field of view.

The information extracted from the high spatial resolution remote sensing images has become one of the important data sources of the GIS large scale spatial database updating. The realization of the building information monitoring using the high resolution remote sensing, building small scale information extracting and its quality analyzing has become an important precondition for the applying of the high-resolution satellite image information, because of the large amount of regional high spatial resolution satellite image data. In this paper, a clustering segmentation classification evaluation method for the high resolution satellite images of the typical rural buildings is proposed based on the traditional KMeans clustering algorithm. The factors of separability and building density were used for describing image classification characteristics of clustering window. The sensitivity of the factors influenced the clustering result was studied from the perspective of the separability between high image itself target and background spectrum. This study showed that the number of the sample contents is the important influencing factor to the clustering accuracy and performance, the pixel ratio of the objects in images and the separation factor can be used to determine the specific impact of cluster-window subsets on the clustering accuracy, and the count of window target pixels (Nw) does not alone affect clustering accuracy. The result can provide effective research reference for the quality assessment of the segmentation and classification of high spatial resolution remote sensing images.

Five search algorithms from the literature of black-box optimization were implemented and applied to optical design problems. These algorithms are the Particle Swarm Optimization, Gravity Search Algorithm, Cuckoo Search, Covariance Matrix Adaptation Evolution Strategy and Nelder&Mead Simplex search. The performance of these search algorithms' implementations was assessed using the BBOB2009 (Black Box Optimization Benchmark) benchmark suite. These algorithms were compared in the context of two optical case studies, one with conventional rotationally symmetric optics and one with freeform optics. A comparison was performed against a commercial optical design software. Finally we provided a simple restart scheme applicable in the workflow of an optical designer. To the best of our knowledge, this is the first in-depth quantitative comparison of optimization algorithms for optical design.

Novel freeform optical design methods can be classified in two categories, depending on whether they focus on the generation of a starting point or the development of new optimization tools. In this paper, we design a freeform three-mirror anastigmat (TMA) and compare different surface representations using either a differential ray tracer as a new optimization tool or a commercial ray tracer (ANSYS-ZEMAX OpticStudio). For differential ray tracing, we used FORMIDABLE (Freeform Optics Raytracer with Manufacturable Imaging Design cApaBiLitiEs), an optical design library with differential ray tracing and Non-Uniform Rational B-Splines (NURBS) optimization capabilities, available under the European Software Community License (ESCL). NURBS allow a freeform surface to be represented without needing any prior knowledge of the surface, such as the polynomial degree in polynomial descriptions. OpticStudio and other commercial optical design software are designed to optimize polynomial surfaces but are not well-suited to optimize NURBS surfaces, requiring a custom optical design library. In order to demonstrate the interest in using NURBS representation, we designed and independently optimized two freeform telescopes over different iteration cycles; with NURBS using FORMIDABLE or with XY polynomials using OpticStudio. We then compared the resulting systems using their root mean square field maps to assess the optimization quality of each surface representation. We also provided a full-system comparison, including mirror freeform departures. This study shows that NURBS can be a relevant alternative to XY polynomials for the freeform optimization of reflective three-mirror telescopes as it achieves more a uniform imaging quality in the field of view.

Like classical optical design, joint digital-optical design of complex lenses requires a skilled optical designer helped by powerful optical design software. Consequently, if optimization criteria have to be modified to take into account digital post-processing, the convenient optimization environment provided by commercial optical design software needs to be preserved. For that purpose, we define a joint-design criterion based on a merit function that contains terms classically implemented in optical design software but used in a non-standard way. After validation on a simple design problem, the proposed method is applied to the design of a very fast (f/0.75) complex lens. The obtained joint-designed lens is shown to be superior to a classically designed one in terms of weight and image quality in the field.

Conventional two-mirror optical telescope designs are well known. An attempt to improve the performance of a two-mirror telescopic system using freeform surface is reported. Four variants of the optical design that use symmetric and off-axis freeform surfaces for achieving superior performances in the spectral range from 400 to 900 nm are proposed. These designs are compared with the conventional Ritchey–Chretien and equivalent two-mirror off-axis telescope designs with rotationally symmetric surfaces. The optical design with freeform surfaces shows marked improvements compared with its counterpart comprising of conics and higher order aspherics. The incorporation of freeform surfaces is obtained by an overlay of fringe Zernike polynomial either on the base sphere or on the conic itself, which is used as a surface descriptor in the envisaged designs. This approach aids in correction of asymmetrical aberrations and also extends the performances to a wider field, which is quite advantageous in the case of off-axis (de-centered and tilted) optical systems.

This paper introduces a novel endoscope design based on the panoramic annular staring imaging technology. This design utilizes a single optical system to realize both panoramic observation and local high resolution on a single sensor. The freeform surface is employed to improve the image quality and reduce system volume. The design results based on the commercial optical design software package ZEMAX, indicate that this optical system is able to acquire an excellent image quality with a modulation transfer function above 0.6. Compared with the traditional ones, this novel endoscope design with wide FOV is likely to decrease the diagnostic time dramatically and improve the lesion detect rate considerably.

To accurately model the specific detection characteristics of spectral sensors based on linear variable filters (LVFs) within an optical design tool, it is essential to consider crucial position-variable spectral properties, such as peak transmittance, central wavelength, half width, or slope steepness. In this context, we propose a straightforward approach, integrating a dynamic link library (DLL) containing all position-dependent spectral properties of the LVF into a commercial optical design software. Exemplary investigations are conducted for an LVF with a detection range of 450-850 nm. For ease of use, the measured position-, wavelength-, and angle-dependent transmission properties of the LVF have been described through a simple yet highly accurate model system. Moreover, to highlight the essential value of this simulation for specific applications, an efficiency-enhancing spectral module is simulated, which is an LVF-mirror arrangement characterized by a multiple-reflected beam path. The introduced optical design tool demonstrates its particular strength by enabling the optimization of the highest detection efficiency for either the short- or long-wavelength range.

Hyperspectral (HS) images obtained from space are useful for monitoring different natural phenomena on regional to global scales. The Environmental Mapping and Analysis Program (EnMAP) is a satellite recently launched by Germany to monitor the environment and explore the capabilities of hyperspectral sensors in the 420 and 2450 nm range of the spectrum. However, the data captured by the EnMAP mission have a ground sampling distance (GSD) of 30 m. This limits the use of the data for some applications that require higher spatial resolution (<10 m). This study examines the potential for improving the resolution of hyperspectral data using high resolution multispectral (MS) data obtained by Cubesats. Specifically, this work uses the data captured by the PlanetScope constellation, which has more than 150 CubeSats in low Earth orbit, with a high spatial and temporal resolution. The approach adopted leverages (1) the spectral capability of the hyperspectral EnMAP sensor, with a bandwidth of 6.5 nm in the visible and near infrared (VNIR) range (420–1000 nm) and 10 nm in the SWIR range (900–2450 nm), and (2) the spatial capability of the multispectral PlanetScope data, with a GSD of 3 meters, to enable significant spatial improvements due to its high spatial resolution. The main components of this work include: (i) area of interest clipping (ii) data co-registration, (iii) HS-MS data fusion, and (iv) quality assessments using the Jointly Spectral and Spatial Quality Index (QNR). In this study, a 2 km x 2 km area of interest was selected in the Malaucene region of France, where six state-of-the-art HS-MS fusion methods were evaluated: (1) fast multi-band image fusion algorithm (FUSE), (2) coupled nonnegative matrix factorization (CNMF), (3) smoothing filtered-based intensity modulation (SFIMHS), (4) maximum a posteriori stochastic mixing model (MAPSMM), (5) Hyperspectral Superresolution (HySure), and (6) generalized laplacian pyramid hypersharpening (GLPHS). Quality assessments of the enhanced data showed that high spectral and spatial fidelity are maintained, with the best performing fusion method being FUSE with a QNR of 0.625 followed by the MAPSMM method with a QNR of 0.604. Overall, this study advocates the benefits associated with the fusion of hyperspectral and multispectral data to obtain enhanced EnMAP data at 3 m GSD. 

Freeform optics design method for illumination and laser beam shaping enabled by least squares and surface optimization

We present an automated method of finding different freeform dioptric starting systems, working in the infrared region, for further optimization in commercial optical design software. Our developed method couples the simultaneous multiple surface (SMS) method, introduced by Benítez and Miñano, with automatic optimization in Zemax OpticStudio. The method allows an optical designer to explore the merit function (MF) landscape of freeform optical problems. In this article, we apply our method to a size, weight, and power (SWaP) problem, and we compare our designed system with a system found in the literature that has the same aperture of F/1.2. Then, we increase the aperture of the system up to F/0.9, taking advantage of the use of freeform surfaces.