Cassegrain Focus Research Articles

Atmospheric coherence time measurement by four-aperture DIMM defocus velocity technique.

In this work, we estimate the atmospheric Fried parameter $ {r_0} $r0, average wind speed $ \bar v $v¯, and subsequently, atmospheric coherence time $ {\tau _0} $τ0 by experimental measurement via a four-aperture differential image motion monitor (DIMM) instrument at the Iranian National Observatory (INO) site. The experimental approach is based on the four-aperture DIMM defocus velocity theory, which uses the angle-of-arrival fluctuation measurement of starlight propagation through atmospheric turbulence in the form of a four-spot configuration provided by the four-aperture DIMM telescope. Here, we measure the defocus variance $ \sigma _{{C_4}}^2 $σC42 and its velocity variance $ \sigma _{\partial {C_4}/\partial t}^2 $σ∂C4/∂t2 and use the preceding theory to estimate the atmospheric turbulence parameters. We have implemented the data sampling at the INO site at an altitude of 3600 m above sea level with a 12-inch Meade Cassegrain telescope consisting of a four-aperture mask at its entrance pupil and a fast CCD camera recording short-exposure images with frame rates in the range of 480 fps to 620 fps from the Capella star. The experimental recorded data sets are analyzed and the results compared to those of our simulation and other methods and demonstrate good agreement.

The Experimental Teaching of the Conics Using the Reflection of the Light: The Conceptual Study of the Cassegrain Telescope

The study of conics has to get applications in science, particularly in Astronomy. So the objective of this work is to demonstrate the reflective properties of the hyperbola and the parabola and its relationship with the conceptual study of a Cassegrain telescope from the perspective of Geometry Optics. For this purpose, an optical prototype was designed using low-cost materials. An instructional guide will also be provided to prove that the reflective proprieties of these conics are fulfilled by light rays.

Height variation of magnetic field and plasma flows in isolated bright points

Aims. The expansion with height of the solar photospheric magnetic field and the plasma flows is investigated for three isolated bright points (BPs). Methods. The BPs were observed simultaneously with three different instruments attached to the 1.5 m GREGOR telescope: (1) filtergrams of Ca II H and blue continuum (4505 Å) with the HiFI, (2) imaging spectroscopy of the Na I D2 line at 5890 Å with the GFPI, and (3) slit spectropolarimetry in the 1 μm spectral range with the GRIS. Spectral-line inversions were carried out for the Si I 10827 Å Stokes profiles. Results. Bright points are identified in the Ca II H and blue continuum filtergrams. Moreover, they are also detected in the blue wing of the Na I D2 and Si I 10827 Å lines, as well as in the Ca I 10839 Å line-core images. We carried out two studies to validate the expansion of the magnetic field with height. On the one hand, we compare the photospheric Stokes V signals of two different spectral lines that are sensitive to different optical depths (Ca I vs. Si I). The area at which the Stokes V signal is significantly large is almost three times larger for the Si I line – sensitive to higher layers – than for the Ca I one. On the other hand, the inferred line-of-sight (LOS) magnetic fields at two optical depths (log τ = −1.0 vs. −2.5) from the Si I line reveal spatially broader fields in the higher layer, up to 51% more extensive in one of the BPs. The dynamics of BPs are tracked along the Na I D2 and Si I lines. The inferred flows from Na I D2 Doppler shifts are rather slow in BPs (≲1 km s−1). However, the Si I line shows intriguing Stokes profiles with important asymmetries. The analysis of these profiles unveils the presence of two components, a fast and a slow one, within the same resolution element. The faster one, with a smaller filling factor of ∼0.3, exhibits LOS velocities of about 6 km s−1. The slower component is slightly blueshifted. Conclusions. The present work provides observational evidence for the expansion of the magnetic field with height. Moreover, fast flows are likely present in BPs but are sometimes hidden because of observational limitations.

Terahertz Near-Field Focusing Using a 3-D Printed Cassegrain Configuration for Backscattered Side-Channel Detection

This paper presents the use of terahertz near-field focusing for backscatter side-channel detection. Near-field focusing is done by using a Cassegrain reflector configuration. The focuser is designed to produce the focused beam 28 cm away from the antenna aperture. The focusing is done in the near-field region by axially moving the subreflector from the focal point. It is observed that the subreflector position has to shift approximately 11 wavelengths along the axis to create the focus at the required location. The focused antenna gain is 46 dBi, while the 3 dB focus width and depth of the designed antenna is ~4 mm and 10 cm, respectively. It is found that the focal plane position is sensitive to the subreflector shifts and it is observed that 1 mm change in the subreflector position can shift the focal plane by ~2 cm. The simulations are compared with the measurement results of a fabricated prototype and good agreement is observed. The antenna is fabricated by using 3-D printing technology, which allows rapid prototyping. Finally, we have demonstrated the detection of backscatter side channel from the board placed at 28 cm away from the designed antenna. The received power level of the backscatter signal increases by 6 dB as compared to horn antenna.

Observations on spatial variations of the Sr I 4607 Å scattering polarization signals at different limb distances with ZIMPOL

Context. The Sr I 4607 Å spectral line shows one of the strongest scattering polarization signals in the visible solar spectrum. The amplitude of this polarization signal is expected to vary at granular spatial scales, due to the combined action of the Hanle effect and the local anisotropy of the radiation field. Observing these variations would be of great interest because it would provide precious information on the small-scale activity of the solar photosphere. At present, few detections of such spatial variations have been reported. This is due to the difficulty of these measurements, which require combining high spatial (∼0.1″), spectral (≤20 mÅ), and temporal resolution (< 1 min) with increased polarimetric sensitivity (∼10−4). Aims. We aim to detect spatial variations at granular scales of the scattering polarization peak of the Sr I 4607 Å line at different limb distances, and to study the correlation with the continuum intensity. Methods. Using the Zurich IMaging POLarimeter (ZIMPOL) system mounted at the GREGOR telescope and spectrograph in Tenerife, Spain, we carried out spectro-polarimetric measurements to obtain the four Stokes parameters in the Sr I line at different limb distances, from μ = 0.2 to μ = 0.8, on the solar disk. Results. Spatial variations of the scattering polarization signal in the Sr I 4607 Å line, with a spatial resolution of about 0.66″, are clearly observed at every μ. The spatial scale of these variations is comparable to the granular size. A statistical analysis reveals that the linear scattering polarization amplitude in this Sr I spectral line is positively correlated with the intensity in the continuum, corresponding to the granules, at every μ.

Triple Range Imager and POLarimeter (TRIPOL)—a compact and economical optical imaging polarimeter for small telescopes

We report the design concept and performance of a compact, lightweight and economical imaging polarimeter, the Triple Range Imager and POLarimeter (TRIPOL), capable of simultaneous optical imagery and polarimetry. TRIPOL splits the beam in wavelengths from 400 to 830 nm into g′-, r′- and i′-bands with two dichroic mirrors, and measures polarization with an achromatic half-waveplate and a wire grid polarizer. The simultaneity makes TRIPOL a useful tool for small telescopes for the photometry and polarimetry of time variable and wavelength dependent phenomena. TRIPOL is designed for a Cassegrain telescope with an aperture of ∼1 m. This paper presents the engineering considerations of TRIPOL and compares the expected with observed performance. Using the Lulin 1-m telescope and 100 seconds of integration, the limiting magnitudes are g′ ∼ 19.0 mag, r′ ∼ 18.5 mag and i′ ∼ 18.0 mag with a signal-to-noise ratio of 10, in agreement with design expectation. The instrumental polarization is measured to be ∼ 0.3% in the three bands. Two applications, one to the star-forming cloud IC 5146 and the other to the young variable GM Cep, are presented as demonstrations.

NICSPol: a near-infrared polarimeter for the 1.2-m telescope at Mount Abu Infrared Observatory

NICSPol is a near infrared imaging polarimeter developed for the Near Infrared Camera and Spectrograph (NICS), one of the back end instruments of the 1.2 m Cassegrain telescope at the Mount Abu Infrared Observatory (MIRO), India. The polarimeter consists of a rotating wire grid polarizer which is mounted between the telescope optics and NICS. The polarimetric observations are carried out by rotating the polarizer using a motorized mechanism to determine the Stokes parameters, which are then converted into the polarization fraction and polarization angle. Here we report the details of the instrument and the results of observations of IR polarimetric standards. A set of polarized and unpolarized standards were observed using NICSPol over J, H and Ks bands covering 0.8 to 2.5 micro m. The observations of polarized standards using NICSPol show that, NICSPol can constrain polarization within ~1% for sources brighter than ~16 magnitude in JHKs bands. NICSPol is a general purpose instrument which could be used to study variety of astrophysical sources such as AGNs, Pulsars, XRBs, Supernovae, star forming regions etc. With few NIR polarimeters available world-wide so far, NICSPol would be the first imaging NIR polarimeter in India.

Open field testing of mid IR DIAL for remote detection of thiodiglycol vapor plumes in the topographic target configuration

A tunable mid-infrared differential absorption lidar (DIAL) mounted on a mobile platform has been built for long range remote detection of thiodiglycol (TDG) vapor plumes. The IR spectra of TDG have been analyzed from available spectroscopy database and from the laboratory measurement. Based on the analysis, laser wavelengths at 3.190 and 3.300 μm have been selected as λon (strong absorption) and λoff (weak absorption) in the measurements. The influence of interfering molecules at these wavelengths has been also studied. The lidar return signal and sensitivity of the system have been numerically simulated based on the system parameters. The experimental DIAL system has been built using an optical parametric oscillator (OPO) based tunable laser operating in the wavelength band 3.0–3.45 μm as a transmitter and Cassegrain telescope having 200 mm diameter mirror integrated with thermoelectric cooled mercury cadmium telluride (MCT) detector as a receiver. The trans-receiver system is mounted on a pan-tilt that enables the aerial scanning operations. The lidar subsystems have been assembled and integrated on a mobile platform for conducting field experiments. An initial field testing of the system has been carried out at a remote site using the enclosed chamber containing the TDG vapor. The chamber was erected at a distance of about 950 m acted as a topographic target that reflected the laser radiation. The differential absorption signal in terms of signal ratio has been monitored continuously in real time during the trial period. The varying column content ranging from 10 to 200 ppm × m under dynamic atmospheric conditions has been calculated and discussed in this paper.

Phase error analysis of displaced-axis dual reflector antenna for satellite earth stations

In a classical Cassegrain geometry, the sub-reflector partly blocks reflected rays from the main reflector. Feed antenna of the subreflector also blocks some of the radiated energy towards main reflector. In a properly designed displaced-axis dual-reflector antenna (DADRA) system based on axially-displaced ellipse (ADE) sub-reflector configuration, these blockage losses are minimized. Thus, with its high gain and low return-loss, DADRA system is a strong candidate for large scale earth station antennas. In general, to achieve maximum gain using a single-reflector system, the phase center of the feed antenna should be placed at the focus of the paraboloidal reflector. However, phase center of a feed antenna moves with the change of frequency. Phase error loss is not avoidable, especially in wide band applications. In this work, phase error losses due to axial defocusing of a DADRA system designed for satellite earth station are analysed in Ku-band. The system is fed by a corrugated horn. Phase center stability of the feed over a wide frequency band is exhibited resulting in negligible loss levels. The results are compared with that of a single-paraboloidal reflector fed by the same horn. Measured and simulated patterns of DADRA system are presented.

Wave manipulation with metasurface lens in the cassegrain system

As opposed to the conventional Cassegrain reflector, we introduce the metasurface lens into the design of the Cassegrain system as the primary component with the combination of a secondary metamirror. The electromagnetic fields reflected by the secondary metamirror would directly transmit through the metasurface lens, thus can remove the blockage effects from the secondary metamirror and generate well collimated beams with very low sidelobes compared with the traditional Cassegrain reflector. Especially, such a metasurface Cassegrain system also demonstrates the perfect capabilities of synthesizing the steerable beams and splitting beams when the metasurface lens is equipped with properly prescribed phase distributions. Finally, we fabricate the proposed metasurface lens based Cassegrain system and experimentally demonstrate the functionalities of generating highly directive radiations with beam scanning and beam splitting.

A New High-Performance Monopulse Feed with a Simple Comparator Network

This paper presents a new high-performance five-element monopulse feed with a simple comparator network for Cassegrain reflector applications. The radiating element is a tapered dielectric rod fed by a circular waveguide which is in turn excited by a printed dipole. The center element is used for the sum pattern, while top and bottom elements are used for the elevation difference pattern, and left and right elements for the azimuth difference pattern. Out-of-phase excitation for the difference pattern is obtained by antisymmetrically placing two dipoles and combining their outputs with a power combiner. The optimally designed feed has been fabricated and measured. Measurement shows that the feed has the maximum sum pattern gain of 16.8 dBi, the difference pattern null depth of –21 dBi, and the reflection coefficient of less than –10 dB over 13.9–16.7 GHz.

Revisiting the building blocks of solar magnetic fields by GREGOR

AbstractThe Sun is our dynamic host star due to its magnetic fields causing plentiful of activity in its atmosphere. From high energetic flares and coronal mass ejections (CMEs) to lower energetic phenomena such as jets and fibrils. Thus, it is of crucial importance to learn about formation and evolution of solar magnetic fields. These fields cover a wide range of spatial and temporal scales, starting on the larger end with active regions harbouring complex sunspots, via isolated pores, down to the smallest yet resolved elements – so-called magnetic bright points (MBPs). Here, we revisit the various manifestations of solar magnetic fields by the largest European solar telescope in operation, the 1.5-meter GREGOR telescope. We show images from the High-resolution Fast Imager (HiFI) and spectropolarimetric data from the GREGOR Infrared Spectrograph (GRIS). Besides, we outline resolved convective features inside the larger structures – so-called light-bridges occurring on large to mid-sized scales.

Fast downflows in a chromospheric filament

AbstractAn active region filament in the upper chromosphere is studied using spectropolarimetric data in He i 10830 Å from the GREGOR telescope. A Milne-Eddingon based inversion of the Unno-Rachkovsky equations is used to retrieve the velocity and the magnetic field vector of the region. The plasma velocity reaches supersonic values closer to the feet of the filament barbs and coexist with a slow velocity component. Such supersonic velocities result from the acceleration of the plasma as it drains from the filament spine through the barbs. The line-of-sight magnetic fields have strengths below 200 G in the filament spine and in the filament barbs where fast downflows are located, their strengths range between 100 - 700 G.

Spectropolarimetric analysis of an active region filament

Aims. The determination of the magnetic filed vector in solar filaments is made possible by interpreting the Hanle and Zeeman effects in suitable chromospheric spectral lines like those of the He I multiplet at 10 830 Å. We study the vector magnetic field of an active region filament (NOAA 12087). Methods. Spectropolarimetric data of this active region was acquired with the GRIS instrument at the GREGOR telescope and studied simultaneously in the chromosphere with the He I 10 830 Å multiplet and in the photosphere Si I 10 827 Å line. As has been done in previous studies, only a single-component model was used to infer the magnetic properties of the filament. The results are put into a solar context with the help of the Solar Dynamic Observatory images. Results. Some results clearly point out that a more complex inversion had to be performed. First, the Stokes V map of He I does not show a clear signature of the presence of the filament. Second, the local azimuth map follows the same pattern as Stokes V; it appears that polarity of Stokes V is conditioning the inference to very different magnetic fields even with similar linear polarization signals. This indication suggests that the Stokes V could be dominated from below by the magnetic field coming from the active region, and not from the filament itself. This evidence, and others, will be analyzed in depth and a more complex inversion will be attempted in the second part of this series.

Spectropolarimetric analysis of an active region filament.

Aims. Our aim is to demonstrate the limitations of using a single-component model to study the magnetic field of an active region filament. To do this, we analyzed the polarimetric signals of the He I 10830 Å multiplet, which were acquired with the infrared spectrograph GRIS of the GREGOR telescope (Tenerife, Spain). Methods. After a first analysis of the general properties of the filament using HAZEL under the assumption of a single-component model atmosphere, in this second part we focus our attention on the observed Stokes profiles and the signatures that cannot be explained with this model. Results. We have found an optically thick filament whose blue and red components have the same sign in the linear polarization as an indication of radiative transfer effects. Moreover, the circular polarization signals inside the filament show strong magnetic field gradients. We also show that even a filament with such high absorption still shows signatures of the circular polarization that is generated by the magnetic field below the filament. The reason is that the absorption of the spectral line decays very quickly toward the wings, just where the circular polarization has a larger amplitude. In order to separate the two contributions, we explore the possibility of a two-component model, but the inference becomes impossible to overcome because very many solutions are compatible with the observations.

Measurements of atmospheric turbulence parameters at Vainu Bappu Observatory using short-exposure CCD images

We report atmospheric turbulence parameters, namely atmospheric seeing, tilt-anisoplanatic angle (θ0) and coherence time (τ0), measured under various sky conditions, at Vainu Bappu Observatory in Kavalur. Bursts of short exposure images of selected stars were recorded with a high-speed, frame-transfer CCD mounted on the Cassegrain focus of a newly commissioned 1.3 m telescope. The estimated median seeing is ≈ 1.85″ at wavelength of ∼ 600 nm, the image motion correlation between different pairs of stars is ∼ 44% for θ0 ≈ 36″ and mean τ0 is ≈ 2.4 ms. This work was motivated by the design considerations and expected performance of an adaptive optics system that is currently being planned for the telescope.

Analysis and correction of stray thermal radiation in infrared optical systems including an experimental case study.

In infrared systems, the stray radiation from optical elements and mechanical structures is an important factor affecting quantitative measurements because the irradiance on detectors due to stray radiation depends on the operating temperature of the optical system. Without correcting for this effect, the accuracy of quantitative measurements made with such systems is degraded. To better understand this phenomenon, we derive herein a mathematical model that describes stray radiation as a function of temperature and use the model to quantitatively analyze the stray radiation of an infrared system at different operating temperatures. To test the theory, we use it to calculate the stray radiation from an experimental infrared system comprising a Cassegrain reflector in the first stage and a transmission mirror in the second stage. The maximum relative error between theory and experiment was 8.72%. At the same time, a corrective measure of stray radiation is provided to account for the effect of stray radiation on quantitative measurements. The relative error of quantitative measurements decreases from 2.16% to 0.31%. The measurement accuracy of the infrared system has been improved effectively.

A Search for High-Frequency Coronal Brightness Variations in the 21 August 2017 Total Solar Eclipse

We report on a search for short-period intensity variations in the green-line (Fe xiv 530.3 nm) emission from the solar corona during the 21 August 2017 total eclipse viewed from Idaho in the United States. Our experiment was performed with a much more sensitive detection system, and with better spatial resolution, than on previous occasions (1999 and 2001 eclipses), allowing fine details of quiet coronal loops and an active-region loop system to be seen. A guided 200-mm-aperture Schmidt–Cassegrain telescope was used with a state-of-the-art CCD camera having 16-bit intensity discrimination and a field-of-view (0.43^{ circ } times 0.43^{circ }) that encompassed approximately one third of the visible corona. The camera pixel size was 1.55 arcseconds, while the seeing during the eclipse enabled features of {approx}, 2 arcseconds (1450 km on the Sun) to be resolved. A total of 429 images were recorded during a 122.9 second portion of the totality at a frame rate of 3.49~mbox{s}^{-1}. In the analysis, we searched particularly for short-period intensity oscillations and travelling waves, since theory predicts fast-mode magneto-hydrodynamic (MHD) waves with short periods may be important in quiet coronal and active-region heating. Allowing first for various instrumental and photometric effects, we used a wavelet technique to search for periodicities in some 404mbox{,}000 pixels in the frequency range 0.5,mbox{--},1.6mbox{ Hz} (periods 2 seconds to 0.6 seconds). We also searched for travelling waves along some 65 coronal structures. However, we found no statistically significant evidence in either. This negative result considerably refines the limit that we obtained from our previous analyses, and it indicates that future searches for short-period coronal waves may be better directed towards Doppler shifts as well as intensity oscillations.

Conformal Cassegrain reflecting systems using meta-surfaces

Meta-surfaces have been applied into various conformal reflectors for the design of a Cassegrain system. In particular, we introduce different primary meta-surface reflectors of concave, planar, and convex profiles into the design of a dual reflecting system together with a secondary flat meta-mirror to generate highly directive beams. All these kinds of conformal Cassegrain reflecting systems are shown to possess aperture efficiencies comparable with conventional Cassegrain reflectors. While the concave meta-surface reflector offers the opportunity to build up a more compact system with a much smaller focal length, the planar meta-surface reflector demonstrates the characteristics of a low profile and easy fabrication, and the convex meta-surface reflector enables the whole system to be planted on a curved platform. Subwavelength square ring patches are finally employed to synthesize these reflectors and demonstrate their functionalities of tuning reflecting phases of electromagnetic beams with greatly enhanced radiation, both numerically and experimentally.

Colorado Ultraviolet Transit Experiment data simulator

The Colorado Ultraviolet Transit Experiment (CUTE) is a 6U NASA CubeSat carrying on-board a low-resolution (R~2000--3000), near-ultraviolet (2500--3300 {\AA}) spectrograph. It has a rectangular primary Cassegrain telescope to maximize the collecting area. CUTE, which is planned for launch in Spring 2020, is designed to monitor transiting extra-solar planets orbiting bright, nearby stars aiming at improving our understanding of planet atmospheric escape and star-planet interaction processes. We present here the CUTE data simulator, which we complemented with a basic data reduction pipeline. This pipeline will be then updated once the final CUTE data reduction pipeline is developed. We show here the application of the simulator to the HD209458 system and a first estimate of the precision on the measurement of the transit depth as a function of temperature and magnitude of the host star. We also present estimates of the effect of spacecraft jitter on the final spectral resolution. The simulator has been developed considering also scalability and adaptability to other missions carrying on-board a long-slit spectrograph. The data simulator will be used to inform the CUTE target selection, choose the spacecraft and instrument settings for each observation, and construct synthetic CUTE wavelength-dependent transit light curves on which to develop the CUTE data reduction pipeline.