[en] Imaging Atmospheric Cherenkov Telescopes (IACTs) need fast and large imaging optics to map the faint Cherenkov light emitted in cosmic ray air showers onto their image sensors. Segmented reflectors are inexpensive, lightweight and offer good image quality. However, alignment of the mirror facets remains a challenge. A good alignment is crucial in IACT observations to separate gamma rays from hadronic cosmic rays. We present a star tracking alignment method which is not restricted to clear nights. It normalizes the mirror facet reflections to be independent of the reference star or the cloud coverage. It records asynchronously of the telescope drive which makes the method easy to integrate in existing telescopes. It can be combined with remote facet actuation, but it does not need one to work. Furthermore, it can reconstruct all individual mirror facet point spread functions. We present the method and alignment results on the First Geiger-mode Photo Diode Avalanche Cherenkov Telescope (FACT) on the Canary Island of La Palma, Spain.

[en] Imaging Atmospheric Cherenkov Telescopes (IACTs) observe extensive air showers during the night to probe the very high energetic gamma ray sky on an event to event basis. Both a Cherenkov photon and a Night Sky Background (NSB) photon generates an indistinguishable electric pulse in IACT cameras. However, the separation of Cherenkov photons from the pool of NSB photons is crucial for the air shower reconstruction. Using silicon sensors, the single photon performance of the First G-APD Cherenkov Telescope (FACT) is stable enough that at least in dark nights a single photon reconstruction can be applied. This results in a list of photon arrival times for each pixel in contrast to the common main-pulse amplitude and main-pulse arrival time for each pixel. In this contribution, we first present our single photon extractor for FACT and evaluate its performance, second we introduce an IACT event representation which stores only the arrival times of the individual photons in each pixel (Photon Stream). Finally, we give a brief outlook into possible improvements in air shower reconstruction and show why the Photon Stream is a very natural IACT event format and why it has potential to be very compact, efficient and interchangeable.

[en] Imaging Atmospheric Cherenkov Telescopes (IACTs) need fast and large imaging optics to map the faint Cherenkov light emitted in cosmic ray air showers onto their image sensors. Segmented reflectors are inexpensive, lightweight and offer good image quality. However, alignment of the mirror facets remains a challenge. A good alignment is crucial in IACT observations to separate gamma rays from hadronic cosmic rays. We present a simple, yet extendable method, to align segmented reflectors using their Bokeh. Bokeh alignment does not need a star or good weather nights but can be done anytime, even during the day. Bokeh alignment optimizes the facet orientations by comparing the segmented reflector's Bokeh to a predefined template. The Bokeh is observed using the out of focus image of a nearby point like light source in a distance of about ten times the focal lengths. We introduce Bokeh alignment on segmented reflectors and present its use on the First Geiger-mode Avalanche Cherenkov Telescope (FACT) on Canary Island La Palma, as well as on the Cherenkov Telescope Array (CTA) Medium Size Telescope (MST) prototype in Berlin Adlershof.

No abstract available

[en] Imaging Air Cherenkov Telescopes, including the First G-APD Cherenkov Telescope (FACT), use multiple mirror reflectors. These reflectors offer a great performance for very little resources. However one challange of multiple mirror reflectors is the alignment of the single mirrors to gain a good image. To align the FACT reflector a method developed by the VERITAS group was adopted and enhanced. Using a 1/10th scale model of FACT, the alignment system was developed off site in the lab which results in a highly telescope independent procedure. Finally FACT's reflector was aligned using all new Bokeh alignment and fine adjusted using enhanced SCCAN in May 2014. The basic alignment system and the alignment procedure on the telescope are presented. Alignment results are presented by comparison of star images using visible light and actual muon detection rates to compare the overall telescope performance before and after the reflector alignment.

[en] During the last four years, the First G-APD Cherenkov Telescope (FACT) established silicon-based photo detectors as a valid concept for the imaging atmospheric Cherenkov technique. These detectors, namely silicon photo multipliers (SiPMs), are more robust to bright light conditions than conventional photo multiplier tubes (PMTs). At the same time, SiPMs feature a high photon detection efficiency. As a consequence, this technology yields observations at bright light conditions where PMTs would be damaged. However, dark counts and night sky background light (NSB), in combination with optical crosstalk and after-pulses contribute to the extracted signal from Cherenkov photons. Therefore, they cause a bias on the photon charge extraction and any subsequent analysis steps. Consequently, it is necessary to understand their impact on the data of FACT. This presentation will show the influence of changing NSB and crosstalk conditions on the performance of FACT. Therefore, the influence on the analysis chain is investigated on basis of data that were taken at different NSB conditions as well as dedicated Monte Carlo simulations.

[en] The accelerated corrosion of Zircaloy-4, Zr-2.5%Nb pressure tube material (PT) and Zr-1%Nb alloys has been studied in concentrated lithium hydroxide (LiOH) solutions at high temperature and pressure. The microstructure of Zircaloy-4 consisted of α-Zr equiaxed grains whereas Zr-2.5%Nb and Zr-1%Nb alloys showed a two phase (α-Zr + β-Zr) microstructure. Autoclaving corrosion tests were performed in LiOH aqueous solutions ranging from 0.1M to 1M at 343 deg C. For the sake of comparison, these alloys were also tested in steam at 400 deg C according to ASTM G2/G2M-06 standard. The weight gain per unit area of the samples was determined and the oxide thickness and hydride distribution were analyzed through metallographic techniques. The results of 16-hour tests showed that Zr-2.5Nb% (PT) and Zr-1%Nb alloys suffer accelerated corrosion at LiOH concentrations higher than 0.6M, whereas in Zircaloy-4 the acceleration occurs at 0.45M LiOH. In 1M LiOH solution, the hydrogen uptake by Zr-2.5Nb% (PT) and Zr-1%Nb (about 1%) is substantially lower than the hydrogen uptake by Zircaloy-4 (approximately 70%). The less amount of β-Zr phase in Zr-1%Nb alloy generates thinner and compact oxide layers than those formed on Zr-2.5%Nb (PT). The good corrosion behaviour of Zr-1%Nb alloy in concentrated lithium hydroxide solutions points out that this alloy can be used as cladding material in replacement of Zircaloy-4 in High Performance Western PWRs. (author)

No abstract available

[en] We study the n-level spectral correlation functions of classically chaotic quantum systems without time-reversal symmetry. According to Bohigas, Giannoni and Schmit's universality conjecture, it is expected that the correlation functions are in agreement with the prediction of the circular unitary ensemble (CUE) of random matrices. A semiclassical resummation formalism allows us to express the correlation functions as sums over pseudo-orbits. Using an extended version of the diagonal approximation on the pseudo-orbit sums, we derive the n-level correlation functions identical to the n x n determinantal correlation functions of the CUE.

No abstract available