Dither Patterns Research Articles

Basic Survey Scheduling for the Wide Field Survey Telescope (WFST)

Aiming at improving the survey efficiency of the Wide Field Survey Telescope, we have developed a basic scheduling strategy that takes into account the telescope characteristics, observing conditions, and weather conditions at the Lenghu site. The sky area is divided into rectangular regions, referred to as “tiles,” with a size of 2.°577 × 2.°634 slightly smaller than the focal area of the mosaic CCDs. These tiles are continuously filled in annulars parallel to the equator. The brightness of the sky background, which varies with the moon phase and distance from the moon, plays a significant role in determining the accessible survey fields. Approximately 50 connected tiles are grouped into one block for observation. To optimize the survey schedule, we perform simulations by taking into account the length of exposures, data readout, telescope slewing, and all relevant observing conditions. We utilize the Greedy Algorithm for scheduling optimization. Additionally, we propose a dedicated dithering pattern to cover the gaps between CCDs and the four corners of the mosaic CCD array, which are located outside of the 3° field of view. This dithering pattern helps to achieve relatively uniform exposure maps for the final survey outputs.

Euclidpreparation

Euclidis a mission of the European Space Agency that is designed to constrain the properties of dark energy and gravity via weak gravitational lensing and galaxy clustering. It will carry out a wide area imaging and spectroscopy survey (theEuclidWide Survey: EWS) in visible and near-infrared bands, covering approximately 15 000 deg2of extragalactic sky in six years. The wide-field telescope and instruments are optimised for pristine point spread function and reduced stray light, producing very crisp images. This paper presents the building of theEuclidreference survey: the sequence of pointings of EWS, deep fields, and calibration fields, as well as spacecraft movements followed byEuclidas it operates in a step-and-stare mode from its orbit around the Lagrange point L2. Each EWS pointing has four dithered frames; we simulated the dither pattern at the pixel level to analyse the effective coverage. We used up-to-date models for the sky background to define theEuclidregion-of-interest (RoI). The building of the reference survey is highly constrained from calibration cadences, spacecraft constraints, and background levels; synergies with ground-based coverage were also considered. Via purposely built software, we first generated a schedule for the calibrations and deep fields observations. On a second stage, the RoI was tiled and scheduled with EWS observations, using an algorithm optimised to prioritise the best sky areas, produce a compact coverage, and ensure thermal stability. The result is the optimised reference survey RSD_2021A, which fulfils all constraints and is a good proxy for the final solution. The current EWS covers ≈14 500 deg2. The limiting AB magnitudes (5σpoint-like source) achieved in its footprint are estimated to be 26.2 (visible bandIE) and 24.5 (for near infrared bandsYE,JE,HE); for spectroscopy, the Hαline flux limit is 2 × 10−16erg−1cm−2s−1at 1600 nm; and for diffuse emission, the surface brightness limits are 29.8 (visible band) and 28.4 (near infrared bands) mag arcsec−2.

A Random Modulation Spread-Spectrum Digital PWM for a Low System Clock Digital Buck Converter

The need for a high-efficiency power system has emerged since many IoT devices are mainly operated by batteries. A power system with analog control can have a high control bandwidth, but it is relatively vulnerable to noise disturbances and may suffer from control problems. On the other hand, a digitally controlled power system has high reliability and great controllability at the expense of switching noise and EMI issues. To implement a digitally controlled PWM with high resolution and low EMI, this study proposes a random carrier frequency modulation with dithered duty cycle method, where wide spectrum dithering and random modulation of spread spectrum PWM are tightly integrated. The proposed system was not only able to suppress voltage ripples, but also possible to minimize the occurrence of dependent frequencies due to the fixed dithering pattern. The spectrum of the output voltage showed that the switching frequency component was reduced by about 6.1dB and the output voltage ripples was also reduced compared to the existing PWM techniques.

High-quality 3D shape measurement by kernel-optimized high sinusoidal similarity dither patterns

The error-diffusion dithering technique is widely used in three-dimensional (3D) measurement. The sinusoid similarity of dithering fringe patterns greatly affects measurement accuracy, while the traditional dithering methods lack this quality. In this paper, we present a kernel-optimized dithering technique based on the Floyd-Steinberg dithering method and establish an optimization strategy considering both intensity and phase aspects to generate high-quality sinusoidal fringes for 3D measurement by utilizing the binary defocusing technique. Both simulations and experiments demonstrate that the proposed method could improve the performance of 3D measurement effectively under different defocusing amounts, especially in narrow strips.

Exhaustive dithering algorithm for 3D shape reconstruction by fringe projection profilometry.

Three-dimensional (3D) shape reconstruction by projection of defocused binary patterns overcomes the nonlinearity introduced by the projector. Current patch-based procedures that generate dithered patterns are time consuming and are affected by the harmonics introduced through the tiling process. To overcome this problem, we propose a novel idea, to the best of our knowledge, to generate dithering patterns using the composition of two-dimensional patches as a stack of one-dimensional arrays obtained through an efficient deterministic approach. This procedure is a one-dimension optimization problem in the intensity domain, employing only a quarter of the fringe pitch. Furthermore, the unwanted distorting harmonics are eliminated using a Hilbert transform method. Both numerical simulations and experimental results verify the effectiveness of the proposal.

Extracting Text from Telugu Color Documents by Removing Dither Patterns

Extracting Text from Telugu Color Documents by Removing Dither Patterns

Three-dimensional shape measurement of complex surfaces based on optimized dithering fringe patterns

BackgroundOptimized dithering fringe pattern is a promising method for high-speed, high-accuracy three-dimensional shape measurement. The recently proposed dithering optimization technology optimizes the fringe quality in either phase domain or intensity domain according to their objective functions. Phase-based optimization is direct and effective, but it is sensitive to projector defocusing levels. Intensity-based optimization is robust to projector defocusing levels, but it does not fully improve the phase quality. In practice, it is difficult to control defocusing levels so it is still a challenge to get high quality fringe patterns which affects the measurement quality under different defocusing levels.MethodsIn order to get high-quality binary dithered patterns which are robust to defocusing levels, this paper proposes a weight object function. This function combines two parts: a global intensity part and a local structure part. The global intensity measurement is based on the normalized mean squared error. The local structure measurement is based on residual error of intensity. To generate high quality fringe patterns, the weight object function is applied to the best patch framework.Results and discussionBoth simulation and experimental results demonstrate that the phase-based optimization method and the proposed method perform better than the intensity-based optimization method under nearly focused. However the quality of measurement results from phase-based optimization will decrease with the defoucsing levels increasing. The proposed method is robust to the defocusing levels and it can still reduce the phase error when the projector is strongly defocused.ConclusionsThe proposed method can get high-quality binary dithered patterns under different defocusing levels by combining global similarity and local residual error of intensity. It inherits the merits of binary fringe pattern so that gamma calibration of projector is not required. In practice, the proposed method can be used to generate high quality fringe patterns. The experiment results verify that the proposed method can get better measurement results without considering the projector defocusing levels.

Searching for Short-timescale Variability in the Ultraviolet with the GALEX gPhoton Archive. I. Artifacts and Spurious Periodicities

Abstract In order to develop and test a methodology to search for UV variability over the entire Galaxy Evolution Explorer (GALEX) database down to the shortest timescales, we analyzed time-domain photometry of ∼5000 light curves of ∼300 bright (m FUV, m NUV ≤ 14) and blue (m FUV–m NUV < 0) GALEX sources. Using the gPhoton database tool, we discovered and characterized instrumentally induced variabilities in time-resolved GALEX photometry that may severely impact automated searches for short-period variations. The most notable artifact is a quasi-sinusoidal variation mimicking light curves typical of pulsators, seen occasionally in either one or both detectors, with amplitudes of up to 0.3 mag and periods corresponding to the periodicity of the spiral dithering pattern used during the observation (P ∼ 120 s). Therefore, the artifact may arise from small-scale response variations. Other artifacts include visit-long “sagging” or “hump” in flux, occurring when the dithering pattern is not a spiral, or a one-time change in flux level during the exposure. These instrumentally caused variations were not reported before, and are not due to known (and flagged) artifacts such as hotspots, which can be easily eliminated. To characterize the frequency and causality of such artifacts, we apply Fourier transform analysis to both light curves and dithering patterns, and examine whether artificial brightness variations correlate with visit or instrumental parameters. Artifacts do not correlate with source position on the detector. We suggest methods to identify artifact variations and to correct them when possible.

Impact of material concentration and distribution on composite parts manufactured via multi-material jetting

PurposeMaterial jetting approximates composite material properties through deposition of base materials in a dithered pattern. This microscale, voxel-based patterning leads to macroscale property changes, which must be understood to appropriately design for this additive manufacturing (AM) process. This paper aims to identify impacts on these composites’ viscoelastic properties due to changes in base material composition and distribution caused by incomplete dithering in small features.Design/methodology/approachDynamic mechanical analysis (DMA) is used to measure viscoelastic properties of two base PolyJet materials and seven “digital materials”. This establishes the material design space enabled by voxel-by-voxel control. Specimens of decreasing width are tested to explore effects of feature width on dithering’s ability to approximate macroscale material properties; observed changes are correlated to multi-material distribution via an analysis of ingoing layers.FindingsDMA shows storage and loss moduli of preset composites trending toward the iso-strain boundary as composition changes. An added iso-stress boundary defines the property space achievable with voxel-by-voxel control. Digital materials exhibit statistically significant changes in material properties when specimen width is under 2 mm. A quantified change in same-material droplet groupings in each composite’s voxel pattern shows that dithering requires a certain geometric size to accurately approximate macroscale properties.Originality/valueThis paper offers the first quantification of viscoelastic properties for digital materials with respect to material composition and identification of the composite design space enabled through voxel-by-voxel control. Additionally, it identifies a significant shift in material properties with respect to feature width due to dithering pattern changes. This establishes critical design for AM guidelines for engineers designing with digital materials.

High-quality binary fringe patterns generation by combining optimization on global and local similarity

BackgroundThe recently proposed optimized dithering techniques can improve measurement quality. However, the objective function in these optimization methods just qualifies the global similarity of the pattern while the global optimization methods ignore the influences of local structure.MethodIn order to get high quality binary dithered patterns, this paper presents an algorithm which combines optimization on global and local similarity and implements it in the whole-fringe optimization.ResultsBoth the simulation and experimental results demonstrate that the proposed algorithm can achieve binary fringe patterns with higher phase quality and less intensity difference. The fringe patterns based on the proposed method can get high quality measurement results.ConclusionThe proposed method can get high quality binary dithered patterns under different defocusing levels by combining global similarity and local structure similarity. The proposed method can improve the robustness to the amounts of defocusing.

Searching for moving objects in HSC-SSP: Pipeline and preliminary results

Abstract The Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) is currently the deepest wide-field survey in progress. The 8.2 m aperture of the Subaru telescope is very powerful in detecting faint/small moving objects, including near-Earth objects, asteroids, centaurs and Tran-Neptunian objects (TNOs). However, the cadence and dithering pattern of the HSC-SSP are not designed for detecting moving objects, making it difficult to do so systematically. In this paper, we introduce a new pipeline for detecting moving objects (specifically TNOs) in a non-dedicated survey. The HSC-SSP catalogs are sliced into HEALPix partitions. Then, the stationary detections and false positives are removed with a machine-learning algorithm to produce a list of moving object candidates. An orbit linking algorithm and visual inspections are executed to generate the final list of detected TNOs. The preliminary results of a search for TNOs using this new pipeline on data from the first HSC-SSP data release (2014 March to 2015 November) present 231 TNO/Centaurs candidates. The bright candidates with Hr < 7.7 and i > 5 show that the best-fitting slope of a single power law to absolute magnitude distribution is 0.77. The g − r color distribution of hot HSC-SSP TNOs indicates a bluer peak at g − r = 0.9, which is consistent with the bluer peak of the bimodal color distribution in literature.

Data Reduction and Image Reconstruction Techniques for Non-redundant Masking

Abstract The technique of non-redundant masking (NRM) transforms a conventional telescope into an interferometric array. In practice, this provides a much better constrained point-spread function than a filled aperture and thus higher resolution than traditional imaging methods. Here, we describe an NRM data reduction pipeline. We discuss strategies for NRM observations regarding dithering patterns and calibrator selection. We describe relevant image calibrations and use example Large Binocular Telescope data sets to show their effects on the scatter in the Fourier measurements. We also describe the various ways to calculate Fourier quantities, and discuss different calibration strategies. We present the results of image reconstructions from simulated observations where we adjust prior images, weighting schemes, and error bar estimation. We compare two imaging algorithms and discuss implications for reconstructing images from real observations. Finally, we explore how the current state of the art compares to next-generation Extremely Large Telescopes.

A Search for Rapidly Pulsating Hot Subdwarf Stars in the GALEX Survey

Abstract NASA’s Galaxy Evolution Explorer (GALEX) provided near- and far-UV observations for approximately 77% of the sky over a 10-year period; however, the data reduction pipeline initially only released single NUV and FUV images to the community. The recently released Python module gPhoton changes this, allowing calibrated time-series aperture photometry to be extracted easily from the raw GALEX data set. Here we use gPhoton to generate light curves for all hot subdwarf B (sdB) stars that were observed by GALEX, with the intention of identifying short-period, p-mode pulsations. We find that the spacecraft’s short visit durations, uneven gaps between visits, and dither pattern make the detection of hot subdwarf pulsations difficult. Nonetheless, we detect UV variations in four previously known pulsating targets and report their UV pulsation amplitudes and frequencies. Additionally, we find that several other sdB targets not previously known to vary show promising signals in their periodograms. Using optical follow-up photometry with the Skynet Robotic Telescope Network, we confirm p-mode pulsations in one of these targets, LAMOST J082517.99+113106.3, and report it as the most recent addition to the sdBV r class of variable stars.

A stroboscopic online three-dimensional measurement for fast rotating object with binary dithered patterns

In the three-dimensional (3D) measurement, phase measuring profilometry (PMP) requires no less than three deformed sinusoidal patterns, limiting its applications to static scenarios only. In this paper, a stroboscopic online PMP for fast rotating object with binary dithered patterns is proposed. N frames of novel circular binary grating patterns are designed to measure a rotating object. A defocused digital light projector projects the designed patterns onto the object with stroboscopic method, the corresponding deformed patterns can be “frozen” by charge-coupled device (CCD) camera. Owing to the repeatability and periodicity of rotating movement, a synchronization control unit is used to keep the object at the fixed position in each “frozen” deformed pattern. Every projected pattern is shifted from the previous pattern by a factor of 2π/N. This method makes the images of the object appear stationary in the deformed patterns. The experimental results show the accurate 3D shape of fast rotating object can be reconstructed with the proposed method.

Large-scale retrospective relative spectrophotometric self-calibration in space

We consider the application of relative self-calibration using overlap regions to spectroscopic galaxy surveys that use slit-less spectroscopy. This method is based on that developed for the SDSS by Padmanabhan at al. (2008) in that we consider jointly fitting and marginalising over calibrator brightness, rather than treating these as free parameters. However, we separate the calibration of the detector-to-detector from the full-focal-plane exposure-to-exposure calibration. To demonstrate how the calibration procedure will work, we simulate the procedure for a potential implementation of the spectroscopic component of the wide Euclid survey. We study the change of coverage and the determination of relative multiplicative errors in flux measurements for different dithering configurations. We use the new method to study the case where the flat-field across each exposure or detector is measured precisely and only exposure-to-exposure or detector-to-detector variation in the flux error remains. We consider several base dither patterns and find that they strongly influence the ability to calibrate, using this methodology. To enable self-calibration, it is important that the survey strategy connects different observations with at least a minimum amount of overlap, and we propose an "S"-pattern for dithering that fulfills this requirement. The final survey strategy adopted by Euclid will have to optimise for a number of different science goals and requirements. The large-scale calibration of the spectroscopic galaxy survey is clearly cosmologically crucial, but is not the only one.

Superfast 3D absolute shape measurement using five binary patterns

This paper presents a method that recovers high-quality 3D absolute coordinates point by point with only five binary patterns. Specifically, three dense binary dithered patterns are used to compute the wrapped phase; and the average intensity is combined with two additional binary patterns to determine fringe order pixel by pixel in phase domain. The wrapped phase is temporarily unwrapped point by point by referring to the fringe order. We further developed a computational framework to reduce random noise impact due to dithering, defocusing and random noise. Since only five binary fringe patterns are required to recover one 3D frame, extremely high speed 3D shape measurement can be achieved. For example, we developed a system that captures 2D images at 3333Hz, and thus performs 3D shape measurement at 667Hz.

Hardware‐Based Non‐Photorealistic Rendering Using a Painting Robot

AbstractWe describe a painting machine and associated algorithms. Our modified industrial robot works with visual feedback and applies acrylic paint from a repository to a canvas until the created painting resembles a given input image or scene. The color differences between canvas and input are used to direct the application of new strokes. We present two optimization‐based algorithms that place such strokes in relation to already existing ones. Using these methods we are able to create different painting styles, one that tries to match the input colors with almost transparent strokes and another one that creates dithering patterns of opaque strokes that approximate the input color. The machine produces paintings that mimic those created by human painters and allows us to study the painting process as well as the creation of artworks.

色の数や組合せを制限しない明度差ディザ近似を用いたカラー・ユニバーサルデザイン（ディザパターン識別性および照明ロバスト性の分析）

色の数や組合せを制限しない明度差ディザ近似を用いたカラー・ユニバーサルデザイン（ディザパターン識別性および照明ロバスト性の分析）

High-speed absolute three-dimensional shape measurement using three binary dithered patterns.

This paper describes a method to reconstruct high-speed absolute three-dimensional (3D) geometry using only three encoded 1-bit binary dithered patterns. Because of the use of 1-bit binary patterns, high-speed 3D shape measurement could also be achieved. By matching the right camera image pixel to the left camera pixel in the object space rather than image space, robust correspondence can be established. Experiments demonstrate the robustness of the proposed algorithm and the potential to achieve high-speed 3D shape measurements.

Three-dimensional shape measurement with binary dithered patterns

The previously proposed binary defocusing technique and its variations have proven successful for high-quality three-dimensional (3D) shape measurement when fringe stripes are relatively narrow, but they suffer if fringe stripes are wide. This paper proposes to utilize the binary dithering technique to conquer this challenge. Both simulation and experimental results show the phase error is always less than 0.6% even when the fringe stripes are wide and the projector is nearly focused.