Cassegrain Focus Research Articles

The Colorado Ultraviolet Transit Experiment Mission Overview

Atmospheric escape is a fundamental process that affects the structure, composition, and evolution of many planets. The signatures of escape are detectable on close-in, gaseous exoplanets orbiting bright stars, owing to the high levels of extreme-ultraviolet irradiation from their parent stars. The Colorado Ultraviolet Transit Experiment (CUTE) is a CubeSat mission designed to take advantage of the near-ultraviolet stellar brightness distribution to conduct a survey of the extended atmospheres of nearby close-in planets. The CUTE payload is a magnifying near-ultraviolet (2479–3306 Å) spectrograph fed by a rectangular Cassegrain telescope (206 mm × 84 mm); the spectrogram is recorded on a back-illuminated, UV-enhanced CCD. The science payload is integrated into a 6U Blue Canyon Technology XB1 bus. CUTE was launched into a polar, low-Earth orbit on 2021 September 27 and has been conducting this transit spectroscopy survey following an on-orbit commissioning period. This paper presents the mission motivation, development path, and demonstrates the potential for small satellites to conduct this type of science by presenting initial on-orbit science observations. The primary science mission is being conducted in 2022–2023, with a publicly available data archive coming online in 2023.

The On-orbit Performance of the Colorado Ultraviolet Transit Experiment Mission

We present the on-orbit performance of the Colorado Ultraviolet Transit Experiment (CUTE). CUTE is a 6U CubeSat that launched on 2021 September 27 and is obtaining near-ultraviolet (NUV; 2480 Å-3306 Å) transit spectroscopy of short-period exoplanets. The instrument comprises a 20 cm × 8 cm rectangular Cassegrain telescope, an NUV spectrograph with a holographically ruled aberration-correcting diffraction grating, and an NUV-optimized CCD detector. The telescope feeds the spectrograph through an 18′ × 60″ slit. The detector is a passively cooled, back-illuminated NUV-enhanced CCD. The spacecraft bus is a Blue Canyon Technologies XB1, which has demonstrated ≤ 6″ jitter in 56% of CUTE science exposures. Following spacecraft commissioning, an on-orbit calibration program was executed to characterize the CUTE instrument’s on-orbit performance. The results of this calibration indicate that the effective area of CUTE is ≈19.0–27.5 cm2 and that the average intrinsic resolution element is 2.9 Å across the bandpass. This paper describes the measurement of the science instrument performance parameters as well as the thermal and pointing characteristics of the observatory.

Reliability, availability, and maintainability analysis of the Enhanced Resolution Imager and Spectrograph adaptive optics module and lesson learned before commissioning

Enhanced Resolution Imager and Spectrograph is an instrument currently under commissioning at the Cassegrain focus of the Very Large Telescope UT4. Its mission is to replace the suite of instruments NAOS-CONICA and SINFONI and push to the edge the capabilities of this 8-meter class telescope, by leveraging the adaptive optics module. The instrument has been designed for maximum lifetime and reliability and minimum downtime. We will present the instrument constraints and our approach to the reliability, availability, and maintainability (RAM) analysis. We identified the main actors in the system, then for each of them, we compiled a database of reliability parameters in order to build-up the reliability diagram, describing the failure sources. Starting from this information, we computed the system-wide reliability parameters and compared them with the requirements by the customer. Such a scheme is very general and may be taken as an example of RAM analysis for astronomical instrumentation; it may be also customized for the needs of other projects. In the end, we summarize the lessons learned.

UWB full polarization single-plane monopulse reflector antenna

ABSTRACT UWB dual polarization azimuth plane monopulse feed-reflector antenna is presented. Quadridge ‎open ended waveguide working in frequency range from 6 to 12 GHz is proposed as feed for Cassegrain ‎reflector. The feed phase center distance is 0.65 and 1.3 at lowest and highest operating ‎frequency respectively. Sum (Σ) and Diff (Δ) radiation pattern of the feed array are measured in ‎anechoic chamber using same electrical length coaxial cable and 180°Coupler. The results are in good ‎agreement (vertical and horizontal polarizations) with numerical simulation especially for the main ‎beam. To enhance gain value, 50 cm reflector antenna has been used. A proper crossover is achieved ‎by controlling F/D ratio of the equivalent reflector. A compact Cassegrain reflector compatible to feed ‎array is designed and fabricated. The radiation characteristics measurements of the feed-reflector ‎system have been performed using outdoor measurement setup. By ignoring coupler insertion loss, ‎Sum (Diff) gain range is between 24.8 (17) dBi at 6 GHz and 31.7 (29.5) dBi at 12 GHz. Cross pol and null ‎depth better than −31.5 and −24 have been obtained. ‎

Design and Fabrication of Multi‐Notch Filter with High Transmittance and Optical Density for GREGOR Telescope System

Design and Fabrication of Multi‐Notch Filter with High Transmittance and Optical Density for GREGOR Telescope System

SPECTRAL OBSERVATIONS OF THE HERBIG Be STAR HD 53367

The unusual spectroscopic behavior of star Herbig Be HD53367 is described based on the spectra obtained on the Cassegrain focus of the 2-meter telescope at the Shamakhy Astrophysical Observatory, using the Fiber Echelle Spectrograph (ShAFES), with a spectral resolution of R=28000. The results of comparative behavior analysis of profiles of selected lines (Hα, Hβ, HeI λ5876A, NaID, H and K CaII) are presented with analogical data published in the li- terature.

The Autonomous Data Reduction Pipeline for the Cute Mission

The Colorado Ultraviolet Transit Experiment (CUTE) is a 6U NASA CubeSat carrying on-board a low-resolution, near-ultraviolet (2479–3306 Å) spectrograph. It has a Cassegrain telescope with a rectangular primary to maximize the collecting area, given the shape of the satellite bus, and an aberration correcting grating to improve the image quality, and thus spectral resolution. CUTE, launched on the of 2021 September 27th to a Low Earth Orbit, is designed to monitor transiting extra-solar planets orbiting bright, nearby stars to improve our understanding of planet atmospheric escape and star-planet interaction processes. We present here the CUTE autONomous daTa ReductiOn pipeLine, developed for reducing CUTE data. The pipeline has been structured with a modular approach, which also considers scalability and adaptability to other missions carrying on-board a long-slit spectrograph. The CUTE data simulator has been used to generate synthetic observations used for developing and testing the pipeline functionalities. The pipeline has been tested and updated employing flight data obtained during commissioning and initial science operations of the mission.

Design of the VLT-CUBES image slicers:

CUBES is a high efficiency spectrograph designed for a Cassegrain focus of the Very Large Telescope and is expected to be in operation in 2028. It is designed to observe point or compact sources in a spectral range from 300 to 405nm. CUBES will provide two spectral resolving powers: Rge20,000 for high resolution (HR) and Rge5,000 for low resolution (LR). This is achieved by using an image slicer for each resolution mode. The image slicers re-format a rectangular on-sky field of view of either 1.5arcsec by 10arcsec (HR) or 6arcsec by 10arcsec (LR) into six side-by-side slitlets which form the spectrograph slit. The slit dimensions are 0.19mm times 88mm for HR and 0.77mm times 88mm for LR. The on-sky and physical widths of the slicer mirrors are 0.25arcsec/0.5mm (HR) and 1arcsec/2mm (LR). The image slicers reduce the spectrograph entrance slit etendue and hence the size of the spectrograph optics without associated slit losses. Each of the proposed image slicers consists of two arrays of six spherical mirrors (slicer mirror and camera mirror arrays) which provide a straight entrance slit to the spectrograph with almost diffraction-limited optical quality. This paper presents the description of the image slicers at the end of the Phase A conceptual design, including their optical design and expected performance.

YFPOL: A Linear Polarimeter of Lijiang 2.4 m Telescope

Polarimetry plays an important role in investigating physical properties for celestial objects. We present a polarimeter named YFPOL for the Cassegrain focus of the Lijiang 2.4 m Telescope (LJT) of Yunnan Observatories, Chinese Academy of Sciences. YFPOL is a traditional single-beam polarimeter with a rotating polarizer. As the focal-reducer instrument Yunnan Faint Object Spectrograph and Camera (YFOSC) is always positioned on the Cassegrain focal plane of LJT, we develop two sets of ultra-thin (thickness <12 mm) polarizer rotation control systems with wireless charging and control functions, which are suitable for mounting on the two front-wheels of YFOSC. One set is used as the polarimetric calibration unit, and the other is for the polarimetric modulation unit. Both of the polarizers have an ultra-high contrast ratio of 1,000,000:1 in the optical band. We investigate the instrumental polarization characteristics (IPCs) in the full field of view that is transferred from YFOSC. Furthermore, we identify that the IPCs change when the Cassegrain axis rotates. The spurious polarization from the IPCs can be effectively minimized by flat-fielding using the unpolarized domeflat, when the Cassegrain rotation angle is the same or nearest to that of the polarization observation. We develop a quasi-automatic pipeline for YFPOL and its effectiveness has been verified by tests of the polarimetric observation with blazar S5 0716+714. The calibration is performed by observing the zero-polarized and highly-polarized standard stars. We successfully reach high precision polarization in the field of view, and the systematic uncertainty is below 0.8% for a V = 11.68 target with a 10 s exposure. The instrument polarization angle offset is 2.°6. YFPOL is not only a simple polarimeter, but also a spectropolarimeter with grisms that can be considered in the future.

Interstellar polarization and extinction toward the Recurrent Nova T CrB

Spectropolarimetry is a powerful tool for diagnostic of interstellar matter and gives information about the geometry of the ejected material after the novae outbursts. In this paper are presented spectropolarimetric observations of the recurrent nova T CrB at quiescence obtained with FoReRo2 attached to the Cassegrain focus of the 2.0 m RCC telescope of the Bulgarian Rozhen National Astronomical Observatory. The interstellar polarization toward T CrB was estimated using the field stars method. The spectropolarimetric observations were obtained from February 2018 to August 2021. In the wavelength range from 4800 Å to 8200 Å the maximum of the degree of linear polarization is Pmax(obs)(%)=0.46±0.01 at λ≈5200 Å. The position angle is P.A.obs=100°.8±0°.9. During the observations, there is no intrinsic polarization in T CrB, and the derived values represent interstellar polarization. The polarization toward T CrB is due to the foreground interstellar dust located at the distance up to ≈ 400 pc. Based on the degree of polarization the interstellar extinction toward T CrB is EB−V≈0.07.

Assessing the feasibility of a low-throughput gated echelle spectrograph for Laser-induced Breakdown spectroscopy (LIBS)-Raman measurements at standoff distances

We have developed a potentially portable tabletop combined remote LIBS-Raman spectroscopy system using a high-resolution gated echelle spectrograph for measuring both atomic and molecular emission spectra at a standoff distance of 6.5 m. The system consists of a frequency-doubled Nd: YAG laser, a 4X beam expander, a Cassegrain telescope, and a gated echelle spectrograph. The unique advantages of simultaneous broadband coverage (250–850 nm) and high resolution of the echelle system have been employed for measuring the non-identical emission spectrums (LIBS and Raman) at remote distances. The viability and practicability of the low throughput gated echelle spectrograph for detecting the remote Raman signals were evaluated critically by recoding a series of organic chemicals and solid samples. Further, the feasibility of measuring Raman signals of organic chemicals through different colored bottles was also demonstrated. Additionally, the combined LIBS-Raman spectra of several medium Raman cross-section solids samples were recorded from a standoff distance of 6.5 m, and the results are presented here. The benefits of merging the LIBS and Raman spectroscopy system and the bidirectional use of these techniques for remote measurements have also been highlighted.

“Design and building of a high speed Schmidt-Cassegrain objective for space qualified remote sensing camera”

Remote sensing satellites require very accurate pointing to specific locations of interest with high resolution and small latency. Therefore, the space imaging systems attempted to achieve the possible highest ground resolution with minimum cost as possible. In this paper, design process of an electro-optical imaging payload is described considering the limited weight and small volume in addition to minimum power consumptions. The most promising prospects for high resolution imaging are connected with passive optical sub-systems, especially push broom one. For design a multiband Space remote sensing payload with 4°7′ field of view that operates in multi spectral bands and a panchromatic band, it requires the high speed Schmidt–cassegrain objective. The most promising prospects for high resolution imaging are connected with passive optical sub-systems, especially push broom one. The Schmidt-Cassegrain objective is a catadioptric objective that combines a Cassegrain reflector’s optical path with a Schmidt corrector plate to make a compact objective that uses simple spherical surfaces only. Because of the limited weight of the camera (22.2 kg), the multispectral consists of only one single optical system large F-Number (F5) and provided with spectrum splitter.In addition, the key points in design of such types of cameras were listed as well as clarifying the optical design of such objective.

Analysis on the Isostatic Bipod Mounts for the HERA Mission LIDAR

The Light Detection and Ranging (LIDAR) is a time-of-flight altimeter instrument being developed for the HERA mission, designated as Planetary ALTimeter (PALT). PALT is positioned in the center of the top face of the HERA probe, and therefore, it cannot use radiators to stabilize its internal temperature. The contribution of this paper is the design of isostatic bipod mounts for the LIDAR primary mirror. The performance of PALT must be maintained over a wide operational range, from −60 °C to 80 °C. These temperature requirements imply that a careful isostatic mount structure design is critical to maintaining performance in all operational scenarios. The purpose of the instrument is to perform range measurements from 500 m to 14 km. The instrument will contribute to the detailed characterization of the asteroid’s topography, assist the probe navigation in operations such as fly-bys (including on the dark side of the asteroid) or landing. PALT has an emitter system that generates 2 ns, 100 µJ, 1535 nm laser pulses and a receiver system that collects the backscattered signal from the asteroid. The receiver system is composed of a 70 mm diameter Cassegrain telescope and a refractive system that focuses the signal on the sensor.

Active Optics–Freeform Segment Mirror Replications from a Deformable Matrix

We present a new active optics method for making smooth aspheric, or freeform, mirrors by replication technique from an elastically deformable matrix. The mirror replicas provide an equivalent aberration correction to that of an off-axis segment of a Schmidt plate. The method describes geometry of a chromium stainless steel deformable matrix. Polished flat at rest, the matrix was used as submaster when in a bent state for single replications on a glass substrate. Two plane-aspheric segment mirror replicas are then used as a pair of correctors for a spectrograph telemeter. Located outside the Schmidt plate, in opposite diametric directions, the mirror replicas allow aberration compensation of the singlet convex-plane lens used both as collimator and camera-optics of a spectrograph where the beams are passed twice. The spectrograph design is a “white pupil mounting” for a Cassegrainian telescope. The detector focusing is controlled by fusion imaging from the two mirror replicas. Our results show that the He-Ne beams wave-front error performed by the spectrograph, with each of the two replica mirrors passed twice, compensates at least 93% of the required total aspheric sag. This provides satisfactory results for the telemeter focusing device, which then is quasi-diffraction-limited. A similar replication technique is proposed to obtain a pair of off-axis Schmidt plates for a unit magnification Schmidt-Offner “ideal imager”. Such a system is well suited for Laser Guide Star adaptive optics applications in modern astronomy.

Experimental investigation in nodal aberration theory (NAT): separation of astigmatic figure error from misalignments in a Cassegrain telescope.

We present simulations and experimental validations for separating astigmatic figure error from misalignments in Nodal Aberration Theory (NAT) with a high-precision Cassegrain telescope. Both the primary mirror figure error and the secondary mirror misalignments induce binodal astigmatism for the telescope systems. The separation of these two aberration factors plays a crucial role in the telescope alignment process. In this study, the figure error of the aspheric primary mirror of the Cassegrain telescope induced by the mirror mounts was measured interferometrically utilizing a computer-generated hologram (CGH). According to the primary mirror figure error, the astigmatic node locations in the image plane were simulated using real raytracing. The center of the nodes was located on the field center, and the nodes were placed symmetrically with respect to the field center in the image plane. The telescope's alignment was performed using the simulation results, and the node locations were measured on a grid of field points interferometrically. Thereafter, secondary mirror misalignments around the coma-free pivot point were introduced into the optical system, and the node's center was shifted from the field center in the image plane as predicted by NAT. The simulations and interferometric field measurements were performed and compared on a grid of field points for the misaligned state in the presence of primary mirror figure error. The experimental results confirm the predictions from NAT. Statistical analysis was also performed to confirm the accuracy and stability of the measurements.

High efficiency transmission grating for the ESO CUBES UV spectrograph

CUBES is the Cassegrain U-Band Efficient Spectrograph, a high-efficiency instrument operating in the UV spectral range between 300 nm and 400 nm with a resolution not less than 20,000. CUBES is to be installed at a Cassegrain focus of the Very Large Telescope of the European Southern Observatory. The paper briefly reviews various types of devices used as dispersing elements in astronomical spectrographs to achieve high resolution, before identifying binary transmission gratings produced by microlithography as the best candidate technology for the CUBES instrument. We describe the lithographic fabrication technology in general, two different design considerations to achieve the required high-resolution transmission grating, its prototyping by a direct-write lithographic fabrication technology, and the characterization of the achieved optical performance. An outlook to the realization of the grating for the final instrument, taking the most recent developments of lithographic writing capabilities into consideration is given.

Two-mirror aplanatic telescopes with a flat field

A complete description is given of two-mirror telescopes with a flat medial focal surface, on which the images of stars are circles of least confusion. Particular attention is paid to aplanats, since their field of view is noticeably larger than that of classical systems. Two sets of appropriate solutions correspond to Schwarzschild and Gregorian telescopes. As a result, it becomes possible to use flat light detectors with wide-field two-mirror telescopes. New designs are of particular interest when as few reflective surfaces as possible are required, which is typical for space exploration and non-optical observations.

A Fourier method for the determination of focus for telescopes with stars

ABSTRACT We introduce a Fourier method (Fm) for the determination of best focus for telescopes with stars. Our method fits a power function, that we will derive in this paper, to a set of images taken as a function of focuser position. The best focus position is where the power is maximum. Fm was first tested with small refractor and Schmidt–Cassegrain telescopes. After the successful small telescope tests, we then tested Fm with a 2 m Ritchey–Chrétien–Coudé. Our tests show that Fm is immune to the problems inherent in the popular half-flux diameter method.

ISHELL: a 1–5 micron R = 80,000 Immersion Grating Spectrograph for the NASA Infrared Telescope Facility

iSHELL is a 1.06–5.3 μm high spectral resolution spectrograph built for the 3.2 m NASA Infrared Telescope Facility (IRTF) on Maunakea, Hawaii. Dispersion is accomplished with a silicon immersion grating in order to keep the instrument small enough to be mounted at the Cassegrain focus of the telescope. The white pupil spectrograph produces resolving powers of up to about R ≡ λ/δλ = 80,000 (0.″375 slit). Cross-dispersing gratings mounted in a tiltable mechanism allow observers to select different wavelength ranges and, in combination with a slit wheel and Dekker mechanism, slit widths ranging from 0.″375 to 4.″0 and slit lengths ranging from 5″ to 25″. One Teledyne 2048 × 2048 HAWAII-2RG array is used in the spectrograph, and one Raytheon 512 × 512 Aladdin 2 array is used in a 1–5 μm slit viewer for object acquisition, guiding, and scientific imaging. iSHELL has been in productive regular use on IRTF since first light in 2016 September. In this paper we discuss details of the science case, design, construction and astronomical use of iSHELL.

Расчет двузеркальной антенны по методу физической оптики с учетом многократных переотражений.

Calculation of a double reflector Cassegrain antenna by physical optics method with taking into account multiple reflections between the reflector and the subreflector was performed. It was shown that the convergence of antenna characteristics (gain, antenna efficiency, side-lobe level) with the increase of accounting reflections is observed. To check the method of multiple reflections the same Cassegrain antenna was calculated by the method of electric field integral equation with the unknown surface electric currents on the reflector and the sub-reflector. The comparison showed the coincidence of the result obtained by two different methods. The calculation of gain and efficiency of a Cassegrain antenna in frequency range from 1 to 10 GHz was performed by physical optics method with multiple reflections.