The Gamma Cherenkov Telescope (GCT) is proposed to be part of the Small Size Telescope (SST) array of the Cherenkov Telescope Array (CTA). The GCT dual-mirror optical design allows the use of a compact camera of diameter roughly 0.4 m. The curved focal plane is equipped with 2048 pixels of ∼0.2 • angular size, resulting in a field of view of ∼9 • . The GCT camera is designed to record the flashes of Cherenkov light from electromagnetic cascades, which last only a few tens of nanoseconds. Modules based on custom ASICs provide the required fast electronics, facilitating sampling and digitisation as well as first level of triggering. The first GCT camera prototype is currently being commissioned in the UK. On-telescope tests are planned later this year. Here we give a detailed description of the camera prototype and present recent progress with testing and commissioning.
Abstract. This paper presents the retrieval method that was developed to derive tropospheric NO 2 columns from UV/VIS spectral measurements obtained with the Spectrolite Breadboard Instrument during the AROMAPEX campaign in Berlin (April 2016). A typical DOAS retrieval approach is followed. For the calculation of air mass factors this study specifically focuses on the impact of the surface reflectance, which varies considerably from pixel to pixel over this urban region. Groundbased aerosol optical thickness measurements are used as prior information. It is shown that retrieved surface reflectance shows 5 good agreement with those derived from Landsat 8 measurements performed on the same day. Furthermore we demonstrate that tropospheric NO 2 columns retrieved for pairs of adjacent pixels are self-consistent in the sense that they do not show a substantial systematic dependence on surface reflectance, in contrast to differential slant column densities. Also some cases are identified to illustrate this on a pixel-by-pixel level. An error budget is provided to quantify the impact of various assumptions on the accuracy of the retrieval of surface reflectance and tropospheric NO 2 columns. Both in the morning and afternoon flight
Since 1995 the Global Ozone Monitoring Experiment (GOME) is measuring ozone (total column and profile), nitrogen dioxide and other minor trace gases on-board of the European Space Agency (ESA) ERS-2 satellite. The European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) and ESA decided to fly an advanced GOME-2 instrument on the METOP satellites. Within the EUMETSAT Polar System (EPS), the GOME-2 measurements will provide the input for the ozone data record in the timeframe 2005 to 2020.The radiometric calibration of the polarisation sensitive GOME-2 instrument is significantly improved by the simultaneous measurement of s-and p-polarised light at moderate resolution and high temporal resolution. The Polarisation Monitoring Unit (PU) measures the spectral range between 312 and 790 nm in 15 narrow bands. The ground pixel size in the 960 km swath is 40 * 5km 2 .The paper describes in detail the polarisation measurement devices and their technical capabilities.
An international round-robin experiment has been conducted to test procedures and methods for the measurement of angle resolved light scattering. ASTM E2387-05 has been used as the main guide, while the experience gained should also contribute to the new ISO standard of angle resolved scattering currently under development (ISO/WD 19986:2016). Seven laboratories from Europe and the USA measured the angle resolved scattering from Al/SiO 2 coated substrates, transparent substrates, volume diffusors, quasi volume diffusors, white calibration standards, and grating samples at laser wavelengths in the UV, VIS and NIR spectrum. Results were sent to Fraunhofer IOF that coordinated the experiments and analyzed the data, while ESA-ESTEC, as the project donor, defined conditions and parameters. Depending mainly on the sample type, overall good to reasonable agreements were observed, with largest deviations at scattering angles very close to the specular beam. Volume diffusor characterization unexpectedly turned out to be challenging. Not all participants provided measurement uncertainty ranges according to GUM, often, a single general scatterometer-related measurement uncertainty value was stated. Although relative instrument measurement uncertainties close to 1% are sometimes claimed, the comparison results did not support these claims for specular scattering samples as mirrors, substrates, or gratings.
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