Design and Numerical Analysis of an Infrared Cassegrain Telescope Based on Reflective Metasurfaces

Reflective imaging systems such as Cassegrain-type telescopes are widely utilized in astronomical observations. However, curved mirrors in traditional Cassegrain telescopes unavoidably make the imaging system bulky and costly. Recent developments in the field of metasurfaces provide an alternative way to construct optical systems, possessing the potential to make the whole system flat, compact and lightweight. In this work, we propose a design for a miniaturized Cassegrain telescope by replacing the curved primary and secondary mirrors with flat and ultrathin metasurfaces. The meta-atoms, consisting of SiO2 stripes on an Al film, provide high reflectance (>95%) and a complete phase coverage of 0~2π at the operational wavelength of 4 μm. The optical functionality of the metasurface Cassegrain telescope built with these meta-atoms was confirmed and studied with numerical simulations. Moreover, fabrication errors were mimicked by introducing random width errors to each meta-atom; their influence on the optical performance of the metasurface device was studied numerically. The concept of the metasurface Cassegrain telescope operating in the infrared wavelength range can be extended to terahertz (THz), microwave and even radio frequencies for real-world applications, where metasurfaces with a large aperture size are more easily obtained.

PAOS, the Physical Optics Propagation model of the Ariel optical system

<p>Ariel, the Atmospheric Remote-sensing Infrared Exoplanet Large-survey, is a medium-class space mission part of ESA's Cosmic Vision programme, due for launch in 2029. Ariel will survey a diverse sample of about 1000 extrasolar planets in the visible and infrared spectrum to answer questions about their composition, formation and evolution. Ariel mounts an off-axis Cassegrain telescope with a 1100 mm x 730 mm elliptical mirror and has two separate instruments (FGS and AIRS) that cover the 0.5-7.8 micron spectral range. To study the Ariel optical performance and related systematics, we developed PAOS, the Proper Ariel Optical Simulator, an End-to-End physical optics propagation model of the Ariel Telescope and subsystems based on PROPER, an optical propagation library for IDL, Python and Matlab. PAOS is a Python code that consists of a series of calls to PROPER library functions and procedures that reproduces the Ariel optical design, interleaved with additional code that can be specified according to the simulation. Using PAOS, we can investigate how diffraction affects the electromagnetic wavefront as it travels through the Ariel optical systems and the resulting PSFs in the photometric and spectroscopic channels of the mission. This enables to perform a large number of detailed analyses, both on the instrument side and on the optimisation of the Ariel mission. In particular, PAOS can be used to support the requirement on the maximum amplitude of the aberrations for the manufacturing of the Ariel primary mirror, as well as to develop strategies for in-flight calibration, e.g. focussing procedures for the FGS and AIRS focal planes, and to tackle systematics such as pointing jitter and vignetting. With the Ariel mission now in the process of finalizing the instrument design and the data analysis techniques, PAOS will greatly contribute in evaluating the Ariel payload performance with models to be included in the existing Ariel simulators such as ArielRad, the Ariel Radiometric model, and ExoSim, the Exoplanet Observation simulator, for the purpose of studying and optimising the science return from Ariel.</p>

Photometric calibration of 28-cm telecsope of NCAS KFU by jointly modeling equations of transformations and the extinction

This work is devoted to study of transformations equations between Binstr, Ginstr, Rinstr photometric system of 28-cm Schmidt-Cassegrain telescope mounted in NCAS KFU to standard Johnson—Cousins BJ , VJ , RC using modern numerical methods. Observations of Landold Standards at the SA110 region were performed. Absolute photometry of selected stars was obtained with estimatiuon of observational errors. To transform the observational data into the standart system numerical model was built with the use of Markov Chain Monte Carlo sampling. So, we found average parameters of transformations between systems (color reduction coefficients are 0.165, −0.120, −0.378 for B0 J , V 0 J , R0C in dependence of (B − G)0 instr, (G − R)0 instr, (G − R)0 instr respectievely) and medium extinction at the observational period (0.276, 0.205, 0.159 for Binstr, Ginstr, Rinstr respectievely).

Prediction of the amplitude of solar cycle 25 using polar faculae observations

Aims . Based on the monthly number of polar faculae, a forecast of the amplitude of solar cycle 25 (SC25) is provided, as well as a prediction of the number of solar flares. Methods . Faculae near both solar poles have been visually observed using a commercial off-the-shelf 20 cm Schmidt-Cassegrain telescope since 1995. The monthly averages were corrected for varying seeing conditions and the heliographic latitude of the center of the solar disk B 0 . From the deduced relationship between the smoothed number of monthly polar faculae during the solar cycle minimum, and the subsequent maximum of the monthly sunspot number, a prediction has been made for the amplitude of the next solar cycle. The methodology used can be considered as a precursor technique. The expected number of M- and X-class flares was calculated based on a statistical approach. Results. The maximum of SC25 is predicted to be 118 +/- 29, of similar strength than the previous SC24. Also the number of M5 or stronger flares is expected to be comparable to that of the previous solar cycle.

High Resolution 3-D Imaging of Mesospheric Sodium (Na) Layer Utilizing a Novel Multilayer ICCD Imager and a Na Lidar

Equipped with a 1-meter Cassegrain telescope with 6.2 meter focal length and an electronically gated Intensified Charge-Coupled Device (ICCD), a multilayer Na imager is designed and developed at Wuhan in China. This novel instrument has successfully achieved the first preliminary 3-D image of the mesospheric Sodium (Na) layer when running alongside a Na lidar. The vertical Na layer profile is measured by the lidar, while the horizontal structure of the layer at different altitudes is measured by the ICCD imaging with a horizontal resolution of ~3.7 urad. In this experiment, controlled by the delay and width of the ICCD gating signal, the images of the layer are taken with three-second temporal resolution for every 5 km. The results show highly variable structures in both the vertical and horizontal directions within the Na layer. Horizontal images of the Na layer at different altitudes near both the permanent layer (80–100 km) and a sporadic Na layer at 117.5 km are obtained simultaneously for the first time. The Na number density profiles measured by the lidar and those derived from this imaging technique show excellent agreement, demonstrating the success of this observational technique and the first 3-D imaging of the mesospheric Na layer.

Ariel Phase B

<p class="Sectiontext"><span lang="EN-US">Ariel was selected as the fourth medium-class mission in ESA’s Cosmic Vision programme in the spring 2018. This paper provides an overall summary of the science and baseline design derived during the phase A and consolidated during the phase B1.</span></p> <p class="Sectiontext"><span lang="EN-US">During its 4-year mission, Ariel will study what exoplanets are made of, how they formed and how they evolve, by surveying a diverse sample of about 1000 extrasolar planets, simultaneously in visible and infrared wavelengths. It is the first mission dedicated to measuring the chemical composition and thermal structures of hundreds of transiting exoplanets, enabling planetary science far beyond the boundaries of the Solar System.</span></p> <p class="Sectiontext"><span lang="EN-US">Transit, eclipse and phase-curve spectroscopy means that no angular resolution is required. The satellite is best placed into an L2 orbit to maximise the thermal stability and the field of regard. Detailed performance studies have demonstrated that the current mission design will achieve the necessary precision to observe all the Ariel target candidates within the mission lifetime.  </span></p> <p class="Sectiontext">The baseline integrated payload consists of 1-metre class, all-aluminium, off-axis Cassegrain telescope, feeding a collimated beam into two separate instrument modules. A combined Fine Guidance System / VIS-Photometer / NIR-Spectrometer contains 3 channels of photometry between 0.50 µm and 1.1 µm, of which two will also be used as a redundant system for provided guidance and closed-loop control to the AOCS. One further low resolution (R = ~15 spectrometer in the 1.1 µm – 1.95 µm waveband is also accommodated here. The other instrument module, the ARIEL IR Spectrometer (AIRS), provides spectral resolutions of between 30 – 100 for a waveband between 1.95 µm and 7.8 µm. The payload module is passively cooled to ~55 K by isolation from the spacecraft bus via a series of V-Groove radiators; the detectors for the AIRS are the only items that require active cooling to <42 K via an active Ne JT cooler. </p> <p>The Ariel mission payload is developed by a consortium of more than 50 institutes from 17 ESA countries, which include the UK, France, Italy, Poland, Spain, Belgium, the Netherlands, Austria, Denmark, Ireland, Czech Republic, Hungary, Portugal, Norway, Estonia, Germany and Sweden. A NASA contribution was approved in November 2019.</p>