Entrance Aperture Research Articles

Extraordinary suppressed transmission of inverse polarizing gratings with tapered slits in deep ultraviolet band

This paper has investigated the transmission characteristics of inverse polarizing gratings with tapered slits in the deep ultraviolet band. It is demonstrated that the extraordinary suppressed transmission (EOST) of TM-polarized light can be boosted up by tapered slits. The exit and entrance aperture widths of the tapered slits play a crucial role in polarization transmission, not the taper angle. The TM transmission valley and extinction ratio peak appear about the same exit aperture width for different grating heights. The phenomenon can be partly explained as the exit aperture width satisfying the charge movement is in a low-frequency mode to suppress TM transmission further. The presented results provide the basis to quickly focus the structural parameters of grating with excellent polarization performance.

Improving Angular Response of Crossed Compound Parabolic Concentrators Using Rectangular Entry Aperture

Improving Angular Response of Crossed Compound Parabolic Concentrators Using Rectangular Entry Aperture

The Daniel K. Inouye Solar Telescope (DKIST)/Visible Broadband Imager (VBI)

The Daniel K. Inouye Solar Telescope (DKIST) is a ground-based observatory for observations of the solar atmosphere featuring an unprecedented entrance aperture of four meters. To address its demanding scientific goals, DKIST features innovative and state-of-the-art instrument subsystems that are fully integrated with the facility and designed to be capable of operating mostly simultaneously. An important component of DKIST’s first-light instrument suite is the Visible Broadband Imager (VBI). The VBI is an imaging instrument that aims to acquire images of the solar photosphere and chromosphere with high spatial resolution and high temporal cadence to investigate the to-date smallest detectable features and their dynamics in the solar atmosphere. VBI observations of unprecedented spatial resolution ultimately will be able to inform modern numerical models and thereby allow new insights into the physics of the plasma motion at the smallest scales measurable by DKIST. The VBI was designed to deliver images at various wavelengths and at the diffraction limit of DKIST. The diffraction limit is achieved by using adaptive optics in conjunction with post-facto image-reconstruction techniques to remove residual effects of the terrestrial atmosphere. The first images of the VBI demonstrate that DKIST’s optical system enables diffraction-limited imaging across a large field of view of various layers in the solar atmosphere. These images allow a first glimpse at the exciting scientific discoveries that will be possible with DKIST’s VBI.

Increased detector response in optical overfilled measurements due to gas lens formation by nitrogen flow through the entrance aperture

According to our experimental results, a nitrogen flow used to prevent dust and moisture entering a detector may influence measurements performed with trap detectors in overfilled conditions. A stable light source was measured with a wedged trap detector with 4 mm aperture, and the nitrogen flow rate was varied. The nitrogen flow was found to have the largest effect of up to 0.8% on the responsivity of the detector at around 1.0 l min−1 flow rate. The effect of nitrogen flow can be removed down to 0.02% by an added crossflow which removes the nitrogen out of the optical axis. In another experiment, the effect was removed almost completely by changing the flowing gas from nitrogen to synthetic dry air. We also present measurement results that indicate the responsivity changes with nitrogen to be smaller than 0.05% with underfilled beam geometry, even without the added crossflow. Based on simulations, the nitrogen flow through the detector forms a gradient-index type gas lens in front of the detector increasing the effective aperture area and thus the responsivity. In the underfilled measurement geometry there is no light close to the aperture edge which could be refracted inside the detector. Finally, we consider methods to ensure that the responsivity changes due to the gas flow remain below 0.05% in overfilled measurement geometry, without compromising the cleanliness of the detector with too small gas flow rate.

Medium energy charged particle detector with a deflecting electrostatic field for measurement of suprathermal electrons, protons, and neutrals aboard next-generation small satellite-1

This paper presents the development, testing, and early operation of a space-borne instrument that can measure and identify charged particles and neutrals in the energy range of 20–400 keV with a 6.25 keV energy resolution. The instrument generates electric fields perpendicular to its entrance aperture, which allows it to identify electrons, ions, and neutrals by deflecting the trajectories of charged particles along the direction of the electric fields. Four identical detector pixels with thin windows, relatively positioned along the direction of the electric fields, independently measure each energy distribution of particles with a total geometric factor of approximately 0.01 cm2·sr. In addition, to measure higher particle fluxes, up to 109/(cm2∙sr∙s), a reduction in particle fluxes by a factor of ∼100 is possible with a mechanical attenuator. Two identical telescopes, each with a field-of-view of 15° × 70°, are orthogonally placed to measure particles with different pitch angles relative to local magnetic fields. The test results of the flight model instrument against laboratory radioisotopes, 241Am, 133Ba, and 14C, are provided, together with results from a numerical simulation to estimate the instrument’s performance. The instrument capabilities are successfully demonstrated with energy spectra of particle distributions acquired from in-orbit operations.

First mirror erosion–deposition studies in JET using an ITER-like mirror test assembly

Mirror tests for ITER have been carried out in JET for over 15 years. During the third JET campaign with the ITER-like wall (2015–2016), comprising a total tokamak plasma exposure duration of 23.4 h and 1027 h of glow discharge cleaning, a new experiment was performed with a specially designed ITER-like test assembly housing six polycrystalline molybdenum mirror samples and featuring trapezoidal entrance apertures simulating the geometry of cut-outs in the diagnostic first wall of the ITER shielding blanket. The assembly was installed on the vacuum vessel wall at the outer midplane, set back radially behind the JET poloidal outer limiters such that the contact with thermal plasma should be largely avoided. The total and diffuse reflectivity of all mirrors was measured in the range 300–2500 nm before and after exposure. Post-exposure studies of mirror surface composition and of surfaces outside and inside the assembly were performed using microscopy, x-ray spectroscopy and ion beam analysis methods. The main results are: (i) no measured degradation of total reflectivity; (ii) diffuse reflectivity increased especially at short wavelengths (below 500 nm) from 1.1 to 2.7% and from 0.8%–1.3% above 1000 nm; (iii) mirrors were coated with a thin co-deposited layer (∼20–30 nm) containing carbon, oxygen and traces of nitrogen, beryllium and metals (Ni, Cr, Fe); (iv) no deuterium was detected; (v) surface composition of the mirror box inner walls was similar to that of the mirrors; (vi) ≲100 nm thick beryllium was the main component on external surfaces of the assembly. These results provide new input to ITER both for the modelling of FM erosion/deposition and for the consideration of requirements for mirror cleaning methods.

Comparison of the geometric accuracy of holes made in CFRP/Ti laminate by drilling and helical milling

Laminates of carbon fibre reinforced polymer (CFRP) and titanium (Ti) alloy are used in aerospace applications and require holes for joining to other components, which can weaken them. In this study, to determine the influences of drilling and helical milling on the geometric accuracy of holes, holes were made in a CFRP/Ti-laminated structure by these two techniques, and variations in aperture and roundness at different positions were investigated. In two processes, apertures are larger in the middle and smaller at the ends, and most are larger than the apertures of standard holes (6 mm). With drilling, roundness is relatively high at hole entrances and exits. In helical milling, the maximum roundness occurs just underneath the hole entrance, while the minimum occurs at the exit. In the drilling of Ti alloy, the maximum aperture occurs at the entrance; however, with helical milling, the hole entrance aperture is smaller. The aperture at other positions is relatively small. During drilling, the maximum roundness occurs at hole entrances, while the minimum occurs just underneath and then increases with depth. Helical milling produces a relatively large roundness at hole entrances, with consistent variation in other positions. Generally, helical milling of a laminated structure produces better geometric accuracy in holes in the Ti alloy layer, better hole roundness in the CFRP layer, and smaller roundness in the transition zone. However, drilling achieves better aperture accuracy in CFRP and smaller aperture errors in the transition zone. These results will help to produce stronger CFRP/Ti-laminated structures.

Comparative study of single-layer, bilayer, and trilayer mirrors with enhanced x-ray reflectance in 0.5- to 8-keV energy region

Effective area is one of the most important parameters of x-ray telescopes. It can be increased by enlarging the entrance aperture or maximizing the reflectivity through the proper designing and optimization of the reflecting coating. A method to increase the reflectivity of grazing incidence x-ray mirrors in the 0.5- to 8-keV energy region is analyzed. The idea consists in the use of a trilayer reflecting coating instead of single-layer one (e.g., C/Ni/Pt mirror instead of Pt one). Deposition of low-absorbing medium-Z and low-Z layers onto the top of strongly absorbing high-Z material results in essential increase in the reflectivity while keeping the same width of the reflectivity plateau. In particular, C/Ni/Pt trilayer mirror demonstrates enhancement of the double reflection coefficient by a factor achieving 1.5 to 3.5 compared to that of Pt-coated mirror. The effective area of a telescope is also considerably increased. The experimental results are in a very good agreement with the theoretical predictions. In addition, the C/Ni/Pt trilayer mirror exhibits a reasonable thermal stability and a relatively low compressive stress of about −550 MPa.

Test and Characterization of a New Post-column Imaging Energy Filter

Abstract Spectrometers and energy filters for electron energy-loss spectroscopy and electron spectroscopic imaging in transmission electron microscopy have a long history. The first commercial post-column imaging energy filter became available already in the early 1990s. Since then this technique has become an important tool for analytic electron microscopy. Nevertheless, the imaging performance of post-column energy-filters is undoubtedly inferior compared to in-column energy filters and their principal design is clearly restricted by practical constraints we found that significant improvements with respect to optical performance and alignment flexibility are still possible compared to the presently available instrumentation. By applying calculation methods originally developed for the design of aberration correctors and spending much care on the precise design of the optical elements and the alignment strategies we could develop a new post-column imaging energy filter with a minimalistic and robust optical design but remarkable optical properties. Compared to other state-of-the-art devices in our design the aberrations determining the energy resolution are reduced by one order of magnitude for the same entrance aperture. Therefore, if desirable, the field of view can be enlarged by more than a factor of two in area while the optical aberrations stay comparable or even better. We describe the principal optical design of the CEOS Energy-Filtering and Imaging Device (CEFID), explain methods for aberration measurement and optical characterization, and finally demonstrate its optical performance by measured imaging and spectroscopy data from two prototype systems.

Effect of deposition angle on fabrication of plasmonic gold nanocones and nanodiscs

Metal nanocones can exhibit several strong plasmonic resonances, which are associated with intense and accessible electromagnetic hot spots. They can thus be used to enhance light–matter interactions or to facilitate location-specific sensing while enabling separation of some non-specific contributions towards the sensing signal. Nanocones and similar 3D structures are often fabricated with the use of the so-called self-shading effect, which occurs during the evaporation of a metal film into circular nanowells. Unfortunately, a full description of a successful deposition process with all the essential details is currently missing in literature. Here we present a detailed view of the fabrication of ordered arrays of conical gold nanostructures using electron beam lithography and gold electron beam evaporation. We show that the symmetry of the fabricated nanostructures is influenced by the lateral position of the substrate on the sample holder during the deposition. Off-axis deposition or tilt of the sample leads to asymmetric nanostructures. When the deposited film is thick enough, or the nanowells narrow enough, the entrance aperture is clogged, and nanocones with sharp tips are formed. In contrast, flat-top truncated cones are produced for thinner films or wider nanowells. All these findings help to identify inherent limits for the production of wafer-scale arrays of such non-planar nanostructures. On the other hand, they also suggest new fabrication possibilities for more complicated structures such as mutually connected nanocones for electrically addressable chips.

Stray light analysis of the Xinglong 2.16-m telescope

An effort towards understanding the problems associated with stray light related to the Xinglong 2.16-m telescope is presented to estimate the stray light performance of the telescope itself and provide a method for improving stray light suppression. Stray light analysis for the 2.16-m telescope model, which consists of an onion shaped dome, telescope structure, equatorial mount and telescope optics, has been developed in two cases (1) pointing to 60° and (2) pointing to zenith, in both azimuth and elevation directions. The Point Source Normalized Irradiance Transmittance (PSNIT), which is generally used for assessing stray light and is uncorrelated to entrance aperture, is calculated with a series of off-axis angles. It shows that the PSNIT values are less than 10−7 when off-axis angles are larger than ± 20°. The dominant contributors to stray light (primary and secondary mirror, telescope structure and dome) are identified to guide performance improvement. The analyses indicate that significant benefit can be realized by adding only five vanes inside the bottom portion of the secondary baffle. In the case of pointing to zenith, the PSNIT values will decrease about 40% on average.

Multislit spectro-polarimeter: a facility instrument for the Multiapplication Solar Telescope

The multislit spectro-polarimeter (MSSP) is a grating-based littrow spectrograph with five slits at the entrance aperture. The polarimeter consists of a nematic liquid crystal variable retarder as the modulator and a Savart plate as an analyzer. It is one of the facility instruments on the Multiapplication Solar Telescope at the Udaipur Solar Observatory, developed to measure the magnetic fields of the Sun in the photosphere and chromosphere. MSSP currently operates only at 630.2 nm (FeI), but will be upgraded to cover CaII at 854.2 nm, HeI at 1083.2 nm, and FeI at 1565.3 nm. The spectrograph has a spectral dispersion of 15.8 mÅ ± 1.2 mÅ at 630.2 nm. The polarimeter has a sensitivity of the order of 10 − 2 and the root mean squared noise in the Stokes spectrum (continuum wavelength points of Stokes Q, U, and V) is 0.015I. To obtain an estimate of the instrument induced polarization, an analytical model is developed to determine the polarization introduced by the telescope. A polarimetric calibration (PolCal) unit is used to calibrate the downstream optical path from the telescope exit pupil up to the detector in MSSP. A residual polarization cross talk of 10% is measured in the data after applying PolCal corrections. The polarimetric data obtained from the engineering run (first-light) are inverted using NICOLE, to extract the magnetic field parameters. The field strength derived from MSSP observations is compared with the data obtained from helioseismic and magnetic imager and is found to lie within ±70 G in the umbral region and ±200 G in the penumbral region.

Flat-fielding of Full-disk Solar Images with a Gaussian-type Diffuser

Full-disk solar chromospheric and photospheric images are widely used to monitor solar activity. Flat-fielding is a basic step during scientific data production. Here, a Gaussian-type diffuser is used to measure the flat-field of full-disk solar chromospheric and photospheric images. The effectiveness of the method is verified by theoretical simulation and practical measurement. First, the uniformity of the light source generated by the Gaussian diffuser and the sunlight, with a field of view the same as the full-disk solar image, is calculated. The full width at half maximum (FWHM) of the scatter angle of the Gaussian diffuser is one degree, such that the light source presents a Gaussian distribution. After the correction of the Gaussian background, the uniformity of the area light source is 99.6%, which can be regarded as a uniform area light source. Second, experiments are conducted on full-disk solar telescopes at Huairou solar observing station and Yangbajing station. The diffuser covers the entire entrance aperture when taking flat-field frames. Through qualitative and quantitative analysis of experimental results, it is confirmed that a revised flat-field frame can modify uniformity caused by various aspects of the telescope, including vignetting, non-uniformities of filters, and contamination of optical components.

Gradient magnet design for simultaneous detection of electrons and positrons in the intermediate MeV range.

We report the design and development of a compact electron and positron spectrometer based on tapered neodymium iron boron magnets to characterize the pairs generated in laser-matter experiments. The tapered design forms a gradient magnetic field component allowing energy dependent focusing of the dispersed charged particles along a chosen detector plane. The mirror symmetric design allows for simultaneous detection of pairs with energies from 2 MeV to 500 MeV with an accuracy of ≤10% in the wide energy range from 5 to 110 MeV for a parallel beam incident on a circular aperture of 20 mm. The energy resolution drops to ≤20% for 4-90 MeV range for a divergent beam originating from a point source at 20 cm away (i.e., a solid angle of ∼8 milli steradians), with ≤10% accuracy still maintained in the narrower energy range from 10 to 55 MeV. It offers higher solid angle acceptance, even for the divergent beam, compared to the conventional pinhole aperture-based spectrometers. The proposed gradient magnet is suitable for the detection of low flux and/or monoenergetic type electron/positron beams with finite transverse sizes and offers unparalleled advantages for gamma-ray spectroscopy in the intermediate MeV range.

Computation of Astronomical Spectrograph Based on a Flat Reflective Diffraction Grating

A technique for computing the spectrograph for astrophysical studies based on the basic input and output parameters of the optical system. In the framework of the proposed technique, it is possible to determine the main properties of the spectrograph components: the focal length of the collimator, the focal length of the projection camera, the blaze angle of the grating, the frequency of the grating. The following parameters are used as the basic ones: the required spectral resolution, the spectral range studied, the spectrograph entrance aperture, the height of the detector in the direction of dispersion, and the available length of the diffraction grating. Recommendations are proposed on the choice of the schematic of the spectrograph based on the computed magnification in the optical system of the device inferred from the basic parameters.

Binospec: A Wide-field Imaging Spectrograph for the MMT

Binospec is a high-throughput, 370 to 1000 nm, imaging spectrograph that addresses two adjacent 8′ by 15′ fields of view. Binospec was commissioned in late 2017 at the f/5 focus of the 6.5 m MMT and is now available to all MMT observers. Aperture masks cut from stainless steel with a laser cutter are used to define the entrance apertures that range from 15′ long slits to hundreds of 2″ slitlets. System throughputs, including the MMT’s mirrors and the f/5 wide-field corrector peak at ∼30%. Three reflection gratings, duplicated for the two beams, provide resolutions (λ/Δλ) between 1300 and >5000 with a 1″ wide slit. Two through-the-mask guiders are used for target acquisition, mask alignment, guiding, and precision offsets. A full-time Shack-Hartmann wavefront sensor allows continuous adjustment of primary mirror support forces, telescope collimation and focus. Active flexure control maintains spectrograph alignment and focus under varying gravity and thermal conditions.

Design of a compound parabolic concentrator for a multi-source high-flux solar simulator

A three-dimensional compound parabolic concentrator (CPC) is designed for high-temperature solar thermochemical applications driven by radiation from a multi-source high-flux solar simulator. The basic geometrical parameters of the CPC including the acceptance angle and the entry aperture radius are determined using optical simulations. A cooling system for overheating protection is designed using engineering heat transfer correlations. A prototype CPC is manufactured using additive manufacturing and single point diamond turning techniques. The optical simulations show that the CPC increases the concentration ratio by a factor of 4.1 at an optical efficiency of 85.4%, reduces spillage loss from 78.9% to 32.1%, and reduces the flux non-uniformity on the target surface.

Proposal for Measuring Magnetism with Patterned Apertures in a Transmission Electron Microscope.

We propose a magnetic measurement method utilizing a patterned postsample aperture in a transmission electron microscope. While utilizing electron magnetic circular dichroism, the method circumvents previous needs to shape the electron probe to an electron vortex beam or astigmatic beam. The method can be implemented in standard scanning transmission electron microscopes by replacing the spectrometer entrance aperture with a specially shaped aperture, hereafter called a ventilator aperture. The proposed setup is expected to work across the whole range of beam sizes-from wide parallel beams down to atomic resolution magnetic spectrum imaging.

Improving the Performance of Light Pipe System Using Laser Cut Panel

Light pipe systems (LPS) are simple devices that allow the transmission of daylight into interiors of buildings. It is a convenient concept for illumination of windowless parts of buildings. The light performance of this is highly dependent on the solar altitude due to the small inlet aperture. Placing the laser-cut panel (LCP) at the entrance aperture in light pipe system, will increase the light performance during low solar angle. This paper presents the results of light performance a 400mm diameter light pipe integrated with a laser-cut panel at entrance aperture as collector. For that reason, this research adopted an experimental approach to compare the light pipe performance via 1:10 scale model. Two scale models were built. LCP in the form of pyramids was fitted at the entrance aperture of light pipe in different tilted angle 40° and 47.5°. The system has been monitored 30 minutes interval with lux meter to evaluate the improvement light performance on a working plane for 6 days under good sky condition. The results of this experiment shows that LCP with 40° tilted angle performed better than 47.5° tilted angle. The light pipe with LCP as collector the minimum recommended illuminance by Malaysian Standard (MS) 1525:2007 can be achieved as early at 8am.

Machining millimeter-scale deep holes in SiCf/SiC material using femtosecond laser filamentation effect

A 3.5 mm thick SiCf/SiC material was drilled in air environment using a femtosecond laser filament effect. The surface morphology of deep micropores was observed by scanning electron microscopy and the depth and profile of the pores were observed using μm-CT. The variation of entrance diameter, exit diameter and depth variation with laser focus position and processing time was further analyzed. The results showed that as the processing time of femtosecond laser increases, the ablation threshold of the material reached saturation. The exit and entrance diameter also stopped increasing and the aperture tend to saturate. The focus entered the interior of the material, allowing the location of the peak power near the surface of the material. So the entrance aperture was of good quality and the exit aperture was round.