Segment Tip Research Articles

Need for Y-stenting in stent-assisted coiling of wide-neck bifurcation aneurysms

Background and PurposeStent-assisted coiling of wide neck bifurcation aneurysms in the anterior communicating segment and basilar tip region can be performed with varying stent configurations, including single stenting or Y-stenting. Y-stenting requires two stents and thus incurs greater cost and procedural complexity than single-stent constructs. The influence of first stent type on the need for Y-stenting remains unknown. Materials and MethodsClinical and angiographic data were retrospectively obtained for patients that underwent stent-assisted coiling for basilar tip or anterior communicating aneurysms at a high-volume center. Patients were included in this study if stent-assisted coiling was performed using Neuroform Atlas or LVIS Jr stents. A multivariate binary logistic regression was performed to measure the influence of first stent type on the need for Y-stenting. ResultsStent-assisted coiling was used to treat 82 aneurysms in 81 patients during the study period, and Y-stenting was performed in 18.3% (15/82) of cases. In multivariate logistic regression analysis, use of LVIS Jr. as the first stent did not significantly influence the need for subsequent Y-stenting after controlling for aneurysm morphology (OR 0.65, 95% CI 0.18–2.43). ConclusionControlling for aneurysm morphology and location, the use of Y-stenting for stent-assisted coiling was not independently influenced by the choice of LVIS Jr or Neuroform Atlas as the first stent. A larger cohort may reveal differences between these two stents, particularly for aneurysms with large neck sizes.

Comparative Transcriptome Analysis of Gleditsia sinensis Thorns at Different Stages of Development

G. sinensis thorn (called "zào jiǎo cì", ZJC) has important medicinal and economic value, however, little is known about the molecular mechanisms behind the development of ZJC. In this study, we measured the content of soluble sugar and starch during the growth and development of the thorn, and performed transcriptome sequencing of the thorn segment, non-thorn segment, apex, and root tip at five distinct stages of thorn formation. The results showed that, with the growth of ZJC, the soluble sugar content of the roots, hypocotyls, thorn stems, thornless stems, leaves, and the starch content of the roots and leaves all firstly increased and then decreased after the basic structure of thorns was formed; the soluble sugar content and starch content of ZJC showed an overall downward trend (decreased by 59.26% and 84.56%, respectively). Myb-like, YABBY2, Growth-regulating factor 3, TCP2, Zinc transporter 8, and another 25 genes may be related to the maintenance and growth of thorns. Gene Ontology (GO) enrichment analysis of differentially expressed genes (DEGs) between stems with thorn and thorn-free stems found that a significant number of DEGs were annotated with terms related to the positive regulation of development, heterochronic (GO:0045962), the positive regulation of photomorphogenesis (GO:2000306), and other biological process (BP) terms. The developmental initiation regulation of ZJC may be regulated by TCP transcription factors (TFs). Eight genes were selected randomly to validate the RNA-seq results using real-time quantitative PCR (RT-qPCR) and they indicated that the transcriptome data were reliable. Our work provided a comprehensive review of the thorn development of G. sinensis.

Effects of Bionic Pectoral Fin Rays’ Spanwise Flexibility on Forwarding Propulsion Performance

Oscillating pectoral fins’ spanwise flexibility is a key factor influencing the forwarding propulsion performance of bionic cownose rays, including thrust and heave-pitch stability. This study explores the effects of the bionic pectoral fin ray’s spanwise flexibility on its propulsion performance via experiments. Inspired by the cownose ray, a pair of bionic pectoral fins with fin rays and fabric skin was designed, and two motors drive the controllable flapping motion. The bionic pectoral fins’ flexibility can be quantified by using fin rays’ bending stiffness. The experiments were carried out in a water tank to measure the thrust, the lift force, and the pitch moment of the bionic cownose ray. The fin rays are divided into plastic sheets of five thicknesses and three fin rays with more obvious stiffness variations. The movement parameters included the following: the flapping frequency of 0.3–0.7 Hz, the flapping amplitude of 20–40°, and the phase difference of 20–40°. The experimental results show that the stiffness of the bionic pectoral fin rays plays an important role in the thrust, lift force, and pitch moment. The fin rays with high stiffness root segment and low stiffness tip segment have lower lift and pitch moment while maintaining a high thrust. This shows that the pectoral fins’ flexible characteristics of the cownose ray are of great significance to the design of the bionic prototype.

Multi-objective optimization of cyclone separators using mathematical modelling and large-eddy simulation for a fixed total height condition

The present study aims at enhancing the cyclone performance by optimizing its geometry while keeping the total height constant. We consider three different cases of optimization with five geometrical entities as common to all cases viz. main body of cyclone, vortex finder diameter and its insertion length, the height of conical segment and cone tip diameter. The way the inlet area is treated classifies the present study in three cases – in the first case (referred to as Case A) the inlet dimensions are not subjected to optimization; in the second case (referred to as Case B), the inlet dimensions are subjected to optimization while retaining the original area magnitude; and in the third case (referred to as Case C), the inlet is subjected to optimization without restrictions on area magnitude. Owing to several geometrical parameters considered for optimization, we use the Muschelknautz method of modelling (MM) – a popular mathematical model – as a metamodel to genetic algorithms (GAs) for optimization. Few selected Pareto front points have been numerically investigated and the performance comparison elucidates a significant improvement over the baseline model. Conclusive results indicate more than a 40% reduction in the pressure drop values over the standard model followed by an improvement in the collection efficiency. Hence, the optimized datasets would provide the new guidelines to designers and would facilitate making selections based upon the relevance.

OCT-based biomarkers for predicting treatment response in eyes with centre-involved diabetic macular oedema treated with anti-VEGF injections: a real-life retina clinic-based study

Background/aimsTo determine whether a combination of baseline and change in spectral domain-optical coherence tomography (SD-OCT)-based biomarkers can predict visual outcomes in eyes with diabetic macular oedema (DMO) treated with antivascular...

Segment tip geometry of sheet intrusions, II: Field observations of tip geometries and a model for evolving emplacement mechanisms

Igneous sheet intrusions are segmented across several orders of magnitude, with segment tip geometry commonly considered indicative of the propagation mechanism (brittle or non-brittle). Proposed propagation mechanisms are inferred to represent host rock mechanical properties during initial magma emplacement; typically, these models do not account for segment sets that show a range of tip geometries within the same lithology. We present a detailed structural characterization of basaltic sill segments and their associated host rock deformation from the Little Minch Sill Complex, Isle of Skye, UK, and a broader comparison with segment geometries in three additional intrusive suites (Utah, USA; and Mull and Orkney, UK). Each separate host lithology shows multiple tip geometries and styles of host rock deformation, from elastic-brittle fracture, to viscous indentation and fluidisation. We attribute this range of host rock deformations to evolving conditions that occur at the tips both during sheet growth and arrest.

Segment tip geometry of sheet intrusions, I: Theory and numerical models for the role of tip shape in controlling propagation pathways

Inferences about sheet intrusion emplacement mechanisms have been built largely on field observations of intrusion tip zones: magmatic systems that did not grow beyond their observed state. Here we use finite element simulation of elliptical to superelliptical crack tips, representing observed natural sill segments, to show the effect of sill tip shape in controlling local stress concentrations, and the potential propagation pathways. Stress concentration magnitude and distribution is strongly affected by the position and magnitude of maximum tip curvature κmax. Elliptical tips concentrate stress in-plane with the sill, promoting coplanar growth. Superelliptical tips concentrate maximum tensile stress (σmax) and shear stress out-of-plane of the sill, which may promote non-coplanar growth, vertical thickening, or coplanar viscous indentation. We find that σmax = Pe(1+ 2(√[aκmax]), where Pe is magma excess pressure and a is sill half length. At short length-scales, blunted tips locally generate large tensile stresses; at longer length-scales, elliptical-tipped sills become more efficient at concentrating stress than blunt sills.

Analyses of the effects of persistent subretinal fluid on visual/anatomic outcomes according to the type of macular neovascularization during the relaxed treat-and-extend protocol in age-related macular degeneration patients

BackgroundTo analyze the long-term effects of persistent subretinal fluid (SRF) on visual/anatomic outcomes according to the type of macular neovascularization (MNV) during relaxed treat-and-extend regimen with anti-vascular endothelial growth factor (anti-VEGF) agents in age-related macular degeneration (AMD) patients.MethodsPatients with fovea-involving type 1 or type 2 MNV, treated with a relaxed treat-and-extend regimen for 2 years were retrospectively reviewed. Eyes with SRF observed more than three times per year were defined as the ‘persistent SRF (+) group’. To exclude the effects of IRF as much as possible, the eyes with persistent IRF were excluded. The effects of persistent SRF on the best-corrected visual acuity (BCVA), central subfield retinal thickness (CST), and changes in the photoreceptor layer (PRL) thickness and outer retinal bands (external limiting membrane, ellipsoid zone, and cone outer segment tip line) after anti-VEGF injection were analyzed for each MNV type.ResultsSeventy-seven eyes with type 1 MNV (44 eyes with persistent SRF) and 53 eyes with type 2 MNV (18 eyes with persistent SRF) were enrolled. Following a relaxed treat-and-extend regimen with anti-VEGF agents, BCVA and CST improved for each MNV type. In comparison between persistent SRF (+) and persistent SRF (−) group, there were no differences in the amount of change in BCVA and CST between the two groups for each MNV type during 2-year follow-up periods. In addition, there were no differences in the amount of reduction in PRL thickness and state of the outer retinal bands between the two groups for each MNV type.ConclusionsUsing a relaxed treat-and-extend regimen with anti-VEGF agents, persistent SRF did not have additional effects on visual and anatomic outcomes by 2 years, regardless of the MNV type.

Photoreceptor Disc Enclosure Is Tightly Controlled by Peripherin-2 Oligomerization.

Mutations in the PRPH2 gene encoding the photoreceptor-specific protein PRPH2 (also known as peripherin-2 or rds) cause a broad range of autosomal dominant retinal diseases. Most of these mutations affect the structure of the light-sensitive photoreceptor outer segment, which is composed of a stack of flattened "disc" membranes surrounded by the plasma membrane. The outer segment is renewed on a daily basis in a process whereby new discs are added at the outer segment base and old discs are shed at the outer segment tip. New discs are formed as serial membrane evaginations, which eventually enclose through a complex process of membrane remodeling (completely in rods and partially in cones). As disc enclosure proceeds, PRPH2 localizes to the rims of enclosed discs where it forms oligomers which fortify the highly curved membrane structure of these rims. In this study, we analyzed the outer segment phenotypes of mice of both sexes bearing a single copy of either the C150S or the Y141C PRPH2 mutation known to prevent or increase the degree of PRPH2 oligomerization, respectively. Strikingly, both mutations increased the number of newly forming, not-yet-enclosed discs, indicating that the precision of disc enclosure is regulated by PRPH2 oligomerization. Without tightly controlled enclosure, discs occasionally over-elongate and form large membranous "whorls" instead of disc stacks. These data show that the defects in outer segment structure arising from abnormal PRPH2 oligomerization are manifested at the stage of disc enclosure.SIGNIFICANCE STATEMENT The light-sensitive photoreceptor outer segment contains a stack of flattened "disc" membranes that are surrounded, or "enclosed," by the outer segment membrane. Disc enclosure is an adaptation increasing photoreceptor light sensitivity by facilitating the diffusion of the second messenger along the outer segment axes. However, the molecular mechanisms by which photoreceptor discs enclose within the outer segment membrane remain poorly understood. We now demonstrate that oligomers of the photoreceptor-specific protein peripherin-2, or PRPH2, play an active role in this process. We further propose that defects in disc enclosure because of abnormal PRPH2 oligomerization result in major structural abnormalities of the outer segment, ultimately leading to loss of visual function and cell degeneration in PRPH2 mutant models and human patients.

Comparison of SDOCT Scan Types for Grading Disorganization of Retinal Inner Layers and Other Morphologic Features of Diabetic Macular Edema.

PurposeTo assess grading reproducibility of disorganization of the retinal inner layers (DRIL) and other morphologic features of diabetic macular edema (DME) across spectral domain optical coherence tomography (SDOCT) instruments and scan types.MethodsA cross-sectional study enrolled participants with current or recent center-involved DME. In group A (27 eyes), we obtained two Cirrus scans (512 × 128 macular cube [Cube] and high-definition five-line raster [HD 5-Line]) and two Spectralis scans (high-resolution [HR] and high-speed [HS]). In group B, 26 eyes underwent HR scans and Optovue AngioVue (OP) 3 × 3-mm scans. All scans were graded for type and extent of DRIL, intraretinal cysts, cone outer segment tip visibility, and subretinal fluid (SRF).ResultsIn the total cohort, mean central subfield thickness was 342.9 ± 83.4 µm. Intraclass correlations were high for DRIL extent across the four different imaging settings (HR vs. HS, r = 0.93; HR vs. Cube, r = 0.84, HR vs. HD 5-Line, r = 0.76, HR vs. OP, r = 0.87) and ranged from good to excellent for intraretinal cyst and SRF area. There were significantly smaller mean normalized differences between HR/HS scans versus HR and all other scan modalities (HR/HS vs. HR/Cube, P = 0.02; HR/HD 5-Line, P = 0.0005; HR/OP, P < 0.0001).ConclusionsOur data suggest that the reproducibility for SDOCT parameters of DRIL and intraretinal cysts was high across all five SDOCT scan types; thus, evaluation of DRIL is feasible using multiple SDOCT models in eyes with DME.Translational RelevanceDME morphological changes can be evaluated on multiple SDOCT devices with good reproducibility, allowing clinicians and researchers flexibility in DME assessment for clinical care and research.

A case of multiple Vertebrobasilar artery fenestration misdiagnosed as vertebral artery dissection

BackgroundFenestration of vertebra-basilar artery is an uncommon congenital vascular anomaly, which is identified by incidental findings on imaging in patients presented without related symptoms or in patients with intracerebral hemorrhage secondary to concomitant artery aneurysm or arteriovenous malformations. Yet, cases of fenestration being misdiagnosed as cerebral artery dissection have never been reported.Case presentationWe present a patient of 66-year-old female with acute onset of dizziness after chiropractic manipulation of the neck. Neck computed tomography angiography (CTA) showed ‘double lumen’ sign and ‘intimal flap’ of the V1 segment of the vertebral artery, which led to the initial diagnosis of vertebral artery dissection (VAD). However, vertebral artery fenestration at V1 segment was eventually identified by multi-directional digital subtraction angiography (DSA). Interestingly, concomitant vertebral fenestration at V3 segment, basilar fenestration and basilar artery tip aneurysm was also revealed by DSA.ConclusionThe triple fenestration at vertebrobasilar artery with basilar tip artery aneurysm is extremely rare, and the fenestration at the V1 segment of vertebral artery was easily misdiagnosed as VAD due to the similar imaging morphology.

Surgical removal and visual outcomes and complications of ERM (Tina Xirou)

AbstractPurposeTo present the newest developments in ERM surgery and assess the visual outcomes and complications.Setting/venueHellenic Red Cross Hospital ‘Korgialenio‐Benakio’.MethodsLiterature review.ResultsThe outcomes after macular surgery for ERM vary widely with regard to function and morphology. Considering functional parameters, better pre‐operative vision is associated with better postoperative vision, but less change in visual acuity (VA). Parameters associated with outer retinal damage are consistently associated with poor visual outcomes after ERM removal, including disruption of the ellipsoid zone or cone outer segment tip length. Inner retinal layer thickness is also correlated with poor outcome, whereas retinal contraction might be associated with good visual outcome. Results from our department show a similar association indicating that inner and outer retina regularity are reproducible prognostic factors for postoperative VA. ERM surgery is generally a safe operation and complications are few. Intra‐operative complications include vitreous hemorrhage, retinal surface damage, and peripheral iatrogenic retinal breaks. Post‐operative complications include the recurrence of ERM, accelerated cataract development, retinal detachment, and visual field depression.ConclusionsIdentifying patients with good postoperative visual prognosis is important. Surgery has been a safe procedure with only few complications.ThemeVitreoretinal Surgery.

Light reflectivity and interference in cone photoreceptors.

In several modes of retinal imaging, the primary means of visualizing cone photoreceptors is from reflected light. Understanding how such images are formed, particularly when adaptive optics techniques are used, will help to guide their interpretation. Toward this end, we used finite difference beam propagation to model reflections from cone photoreceptors. We investigated the formation of cone images in adaptive optics scanning laser ophthalmoscopy (AOSLO) and optical coherence tomography (AOOCT). Three cone models were tested, one made up of three segments of varying refractive index, the other two having additional boundaries at the inner/outer segment junction and outer segment tip. Images formed by the first model did not correspond to AOOCT observations in the literature, while the latter two did. The predicted distributions of reflected light intensity from the latter cone models were compared to the distribution from AOSLO images, both studied with light sources of varied coherence length. The cone model with the most reflections at the inner/outer segment junction best fit the data measured in vivo. These results show that variance in cone reflection can originate from light interfering from reflectors much more closely spaced than the outer segment length. We also show that subtracting images taken with different coherence length sources highlights these changes in interference. Differential coherence images of cones occasionally revealed an annular reflection profile, which modeling showed to be very sensitive to cone size and the gaps bracketing the outer segment, suggesting that such imaging may be useful for probing photoreceptor morphology.

Cervical Ultrasound Utilization in Selective Cervical Nerve Root Injection for the Treatment of Cervical Radicular Pain: a Review

This paper aims to review the available literature addressing the safety, accuracy, and efficacy of ultrasound-guided injections for cervical radicular pain. Ultrasound-guided injections remain a “partially blind technique” due to the inability to visualize relevant structures and the distribution of injectate. There remains a need for adequately powered studies evaluating the safety, accuracy, and effectiveness of ultrasound-guided steroid injections in the cervical spine. Fluoroscopy remains the best evidence-based image-guidance modality for injections into the cervical epidural space to treat cervical radicular pain. If ultrasound guidance is planned for treatment of cervical radicular pain, the authors recommend concomitant use of fluoroscopy guidance. Given the risk of inaccurate needle placement, physicians should retain fluoroscopic images prior to contrast injection to document the proper vertebral segment and final needle tip position. As epidural flow cannot be fully visualized with ultrasound, it is recommended to use fluoroscopy and, if needed, digital subtraction imaging to confirm the absence of intravascular injection. At a minimum, images should be saved post-contrast injection to document epidural flow.

Visual Prognostic Factors of Epiretinal Membrane Surgery in Patients with Pseudophakia

Purpose: To evaluate the visual prognostic factors in patients with pseudophakic epiretinal membrane (ERM) after vitrectomy using spectral domain optical coherence tomography (SD-OCT). Method: A retrospective review of patients with pseudophakic ERM having undergone vitrectomy was conducted. Best corrected visual acuity (BCVA) and SD-OCT were conducted before and 1, 3, and 6 months after vitrectomy. Known visual prognostic factors, such as inner-retina irregularity index, central foveal thickness (CFT), central inner retinal layer thickness (CIRLT), cone outer segment tip defect length, and photoreceptor outer segment length, were reviewed and their correlation with BCVA was analyzed. Results: Forty-three patients (mean age: 64.88 ± 10.46 years) with pseudophakic ERM were included. BCVA significantly improved after vitrectomy (logMAR 0.30 ± 0.24 vs. 0.11 ± 0.14, p < 0.001). The preoperative high inner-retina irregularity index significantly correlated with poor postoperative BCVA in patients with pseudophakic ERM (correlation coefficient 0.583, p < 0.001). Postoperative improvements of inner retinal SD-OCT findings, such as inner-retina irregularity index, CFT, and CIRLT, were significantly associated with the amount of BCVA improvement after ERM surgery (correlation coefficients were as follows: inner-retina irregularity index – 0.711, p < 0.001; CFT – 0.462, p = 0.002; CIRLT – 0.596, p < 0.001). However, preoperative outer retinal SD-OCT findings were not associated with postoperative visual prognosis. Conclusion: From this study, we determined the visual prognostic factors of ERM surgery without confounding factors, such as visual acuity improvement following combined cataract surgery, and inner retinal SD-OCT findings more significantly associated with the visual prognosis of ERM surgery compared to outer retinal SD-OCT findings.

LONG-TERM PROGNOSTIC FACTORS FOR VISUAL IMPROVEMENT AFTER EPIRETINAL MEMBRANE REMOVAL.

To identify prognostic factors that may predict the amount of long-term visual improvement after idiopathic epiretinal membrane removal. A retrospective study of 114 patients who underwent epiretinal membrane removal was performed. The central foveal thickness, inner retinal layer thickness, inner retinal irregularity index, length of the cone outer segment tip line defect, and length of the ellipsoid zone defect were measured. The optical coherence tomography parameters that were associated with the amount of visual improvement were determined at 24 months postoperatively. Central foveal thickness and inner retinal irregularity index were not associated with best-corrected visual acuity at 24 months (P = 0.227 and P = 0.544, respectively), whereas the lengths of cone outer segment tip line defect and ellipsoid zone defect were associated with worse best-corrected visual acuity at 24 months (P = 0.015 and P < 0.001, respectively). Univariate regression analysis indicated that central foveal thickness and inner retinal irregularity index were associated with visual improvement (P = 0.011 and P < 0.001, respectively). Multivariate regression analysis indicated that inner retinal irregularity index, a marker of the inner retinal deformation, was associated with visual improvement after adjusting for age, gender, and other optical coherence tomography findings (P < 0.001). Patients with preoperative inner retinal deformation were found to have significantly improved long-term visual outcomes after epiretinal membrane removal.

Disorganization of Retinal Inner Layers and Ellipsoid Zone Disruption Predict Visual Outcomes in Central Retinal Vein Occlusion

To assess the associations and predictive value of spectral-domain (SD) OCT inner and outer retinal structural parameters and visual acuity (VA) outcomes in macular edema (ME) secondary to central retinal vein occlusion (CRVO). Retrospective, longitudinal cohort study. Eighty-four patients with ME secondary to CRVO receiving pro re nata anti-vascular endothelial growth factor (VEGF) therapy at 3 tertiary-level retina referral centers. In all participants, VA, demographic and clinical parameters, and SD OCT images from baseline, 3 months, and 12 months were reviewed. Spectral-domain OCT-based morphologic features in the 1500-μm foveal zone were analyzed by masked graders for disorganization of the retinal inner layers (DRIL), ellipsoid zone (EZ) and external limiting membrane disruption, cone outer segment tip (COST) visibility, cysts, subretinal and intraretinal fluid, and epiretinal membranes. Spectral-domain OCT-based retinal structural parameters and VA outcomes. In multivariate analyses adjusting for baseline VA, worsening VA over 1 year was associated with 1-year increases in DRIL (point estimate, 0.06 per 100 μm; P < 0.001) and EZ disruption (0.07 per 100 μm; P= 0.023), but decreased COST visibility (-0.09 per 100 μm; P= 0.018). A 3-month increase in DRIL (0.05 per 100 μm; P= 0.003) and EZ disruption (0.10 per 100 μm; P < 0.001) were the only factors predicting VA worsening over 1 year, after controlling for baseline VA. A multivariate model including 3-month evolution in DRIL, EZ disruption, and VA accounted for 86.3% of variability in 1-year VA change. Absolute differences between predicted and actual 1-year VA were within 2 lines in 80.9%. When DRIL increased by 250 μm or more over 3 months, no eyes showed VA improvement of 1 line or more in 1 year. When EZ disruption decreased by 250 μm or more over 3 months, no eyes worsened by 1 line or more over 1 year. Early recovery over 3 months in both DRIL and EZ parameters are key drivers of 1-year VA outcomes. Predictive models incorporating 3-month changes in DRIL and EZ disruption support their usefulness as potential robust determinants of future VA.

Intussusception caused by heterotopic gastric mucosa in small intestine: a case report

BackgroundIntestinal intussusception is the most frequent cause of small bowel obstruction in children between the ages of 2 months and 5 years and often remains idiopathic in etiology, even after surgery. On microscopic examination, in intussusception normal mucosa is noted but in a few cases heterotopic tissue can be seen. Heterotopic gastric mucosa in the small intestine is extremely rare except for its occurrence in remnants of Meckel’s diverticulum. In view of the rarity of this condition, we report a case of ectopic gastric mucosa in the small intestine that was not associated with remnants of vitelline duct.Case presentationA 6-year-old boy of Indo-Aryan ethnicity from India presented with episodes of acute abdominal pain and distension with vomiting and non-passage of stools. On ultrasonography intussusception was suspected. A laparotomy was done and the ileal segment (tip of intussusception) was sent for histopathological examination. On histopathology, sections from the tip of intussusception showed extensive gastric metaplasia of the mucosa.ConclusionsA definitive diagnosis of heterotopic gastric mucosa is established by histopathological examination and it is important to differentiate heterotopia, which is a developmental anomaly, from metaplasia, which is an acquired condition. Heterotopic gastric mucosa is usually clinically silent and surgical intervention can be considered in patients with complications such as gastrointestinal hemorrhage and intestinal obstruction.

Fine cophasing of segmented aperture telescopes with ZELDA, a Zernike wavefront sensor in the diffraction-limited regime

Context. Segmented aperture telescopes require an alignment procedure with successive steps from coarse alignment to monitoring process in order to provide very high optical quality images for stringent science operations such as exoplanet imaging. The final step, referred to as fine phasing, calls for a high sensitivity wavefront sensing and control system in a diffraction-limited regime to achieve segment alignment with nanometric accuracy. In this context, Zernike wavefront sensors represent promising options for such a calibration. A concept called the Zernike unit for segment phasing (ZEUS) was previously developed for ground-based applications to operate under seeing-limited images. Such a concept is, however, not suitable for fine cophasing with diffraction-limited images. Aims. We revisit ZELDA, a Zernike sensor that was developed for the measurement of residual aberrations in exoplanet direct imagers, to measure segment piston, tip, and tilt in the diffraction-limited regime. Methods. We introduce a novel analysis scheme of the sensor signal that relies on piston, tip, and tilt estimators for each segment, and provide probabilistic insights to predict the success of a closed-loop correction as a function of the initial wavefront error. Results. The sensor unambiguously and simultaneously retrieves segment piston and tip-tilt misalignment. Our scheme allows for correction of these errors in closed-loop operation down to nearly zero residuals in a few iterations. This sensor also shows low sensitivity to misalignment of its parts and high ability for operation with a relatively bright natural guide star. Conclusions. Our cophasing sensor relies on existing mask technologies that make the concept already available for segmented apertures in future space missions.

Elimination of the field-dependent aberrations of the JWST-like space telescopes in the multi-field fine-phasing process.

This paper investigates the alignment strategies for eliminating the field-dependent aberrations of the class of large three-mirror anastigmatic (TMA) space telescopes with a segmented primary mirror (PM) like the James Webb Space Telescope (JWST) in the multi-field fine-phasing process based on the framework of nodal aberration theory. During the single-field (on-axis field) fine-phasing process, the individual segment tip, tilt, and piston errors, as well as the de-space of the secondary mirror, are well corrected, and the PM is also adjusted to compensate for those aberrations induced by the misalignments of other mirrors at the center of the science field of view. However, interrogating off-axis field points can reveal the presence of large wavefront errors due to mirror misalignments. Eliminating these field-dependent aberrations is the main goal of the multi-field fine-phasing process. This paper first presents an analytic study on an established alignment strategy used for eliminating the field-dependent aberrations. While it is demonstrated that this alignment strategy has the ability to reduce the field dependency of the wavefront errors, it will, however, also be revealed that this strategy still exhibits some problems, and its alignment efficiency is low. Then, a new alignment strategy with higher alignment efficiency is further proposed. Detailed simulations with a TMA telescope that has similar parameters with the JWST are performed to illustrate the efficiency and rationality of the proposed strategy. This work can not only contribute to an in-depth understanding of the multi-field fine-phasing process, but also present a possibility to improve the efficiency of this process.