All-sky imaging systems are currently very popular. They are used in ground-based meteorological stations and as a crucial part of the weather monitors for autonomous robotic telescopes. Data from all-sky imaging cameras provide important information for controlling meteorological stations and telescopes, and they have specific characteristics different from widely-used imaging systems. A particularly promising and useful application of all-sky cameras is for remote sensing of cloud cover. Post-processing of the image data obtained from all-sky imaging cameras for automatic cloud detection and for cloud classification is a very demanding task. Accurate and rapid cloud detection can provide a good way to forecast weather events such as torrential rainfalls. However, the algorithms that are used must be specifically calibrated on data from the all-sky camera in order to set up an automatic cloud detection system. This paper presents an assessment of a modified k-means++ color-based segmentation algorithm specifically adjusted to the WILLIAM (WIde-field aLL-sky Image Analyzing Monitoring system) ground-based remote all-sky imaging system for cloud detection. The segmentation method is assessed in two different color-spaces (L*a*b and XYZ). Moreover, the proposed algorithm is tested on our public WMD database (WILLIAM Meteo Database) of annotated all-sky image data, which was created specifically for testing purposes. The WMD database is available for public use. In this paper, we present a comparison of selected color-spaces and assess their suitability for the cloud color segmentation based on all-sky images. In addition, we investigate the distribution of the segmented cloud phenomena present on the all-sky images based on the color-spaces channels. In the last part of this work, we propose and discuss the possible exploitation of the color-based k-means++ segmentation method as a preprocessing step towards cloud classification in all-sky images.

Ultra-wide-field of view (UWFOV) imaging systems are affected by various aberrations, most of which are highly angle-dependent. A description of UWFOV imaging systems, such as microscopy optics, security camera systems and other special space-variant imaging systems, is a difficult task that can be achieved by estimating the Point Spread Function (PSF) of the system. This paper proposes a novel method for modeling the space-variant PSF of an imaging system using the Zernike polynomials wavefront description. The PSF estimation algorithm is based on obtaining field-dependent expansion coefficients of the Zernike polynomials by fitting real image data of the analyzed imaging system using an iterative approach in an initial estimate of the fitting parameters to ensure convergence robustness. The method is promising as an alternative to the standard approach based on Shack–Hartmann interferometry, since the estimate of the aberration coefficients is processed directly in the image plane. This approach is tested on simulated and laboratory-acquired image data that generally show good agreement. The resulting data are compared with the results of other modeling methods. The proposed PSF estimation method provides around 5% accuracy of the optical system model.

Adaptive Optics with laser guide star (LGS-AO) has been in routine use at large astronomical telescope facilities since more than two decades^{1, 2}. However, it is only relatively recently that reliable and stable second generation LGS-AO systems have been installed and used routinely at major astronomical observatories, usually located at remote, high altitude sites, without requiring major efforts to operate them. This has been thanks to developments in the controls and in the LGS-AO subsystems. A great impact on the reliability and operability of the LGS-AO facilities has been given by the development of turn-key 589nm, solid state, narrow band lasers³ based on 36W CW Raman Fiber Amplifiers⁴, with powers of 22W CW or higher, incorporating features allowing efficient sodium optical pumping, including repumper photons and accurate wavelength stabilization within 2MHz rms. Recently MPBC has been able, in a collaborative effort with ESO, to produce 95W CW RFA, which leads to an LGS laser with more than 50W CW at 589nm.

We report on the novel ALASCA (Advanced LGS AO for Satellite Communication Assessment) facility being built for ESA by a consortium of industry and national research institutes under the ScyLight program. The aim of ALASCA is to create a facility for Optical Feeder Links (OFL) field tests, as well as to demonstrate at the ESA Optical Ground Station in Tenerife, starting in 2023, 24/7 reliable operation of optimal Optical Feeder Links based on Laser Guide Star Adaptive Optics (LGS-AO) to solve the point-ahead problem on ground-space laser communications. Space optical communication represents a technological challenge due to its specific requirements and merit parameters; the consortium’s extensive experience in LGS-AO in the astronomical field allows an expert technology transfer to earth-space communication. This will enhance the review of the ALASCA’s main requirements, their implementation by a proper tailoring of the modular solutions that will be adopted by the design, facing the new challenges at system level posed by the OFL applications compared to astronomical solutions. The ALASCA project will, last but not least, provide a technology assessment and a development roadmap towards the industrial exploitation of a 24/7 operational Optical Ground Station (OGS). We will provide an overview of the ALASCA project, its goals, phases and planned timeline up to the field experiments; the presentation will then focus on the project status, including also the simulations results of LGS-AO assisted OFL.

This paper is devoted to the application of the optical Fourier transform (OFT) for the study and evaluation of optical scattering in the latest generation of calomel single crystals ready for application in several possible devices such as IR polarizers and acoustooptic tunable filters (AOTF). There are numerous effects that are responsible for the scattering of optical wave passing through the crystal sample volume and surface layers because they affect the optical crystal quality. The scattering level is a crucial and limiting parameter in many technical applications of the evaluated crystal. The proposed approach is based upon the high dynamic range optical FT configuration, creating the amplitude spectrum in the focal plane and its spatial angular distribution analysis based on the spectrum sectorization. The optical scattering pattern was tested in nine locations within each crystal sample volume and on numerous crystal samples. The experimental results are presented and discussed.