Results 1 - 10 of 84
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[en] We give a simple differential geometric proof of the conformal transformation of the night sky under change of observer. The proof does not use the four dimensionality of spacetime or spinor methods. Furthermore, it really shows that the result does not depend on Lorentz transformations. This approach, by giving a transparent covariant expression to the conformal factor, shows that in most situations it is possible to define a thermal sky metric independent of the observer. (paper)
[en] Plans for the future Cherenkov telescope array CTA include replacing the monolithic camera designs used in H.E.S.S. and MAGIC-I by one that is built up from a number of identical segments. These so-called clusters will be relatively autonomous, each containing its own triggering and readout hardware. While this choice was made for reasons of flexibility and ease of manufacture and maintenance, such a concept with semi-independent sub-units lends itself quite naturally to the possibility of new, and more flexible, readout modes. In all previously-used concepts, triggering and readout of the camera is centralised, with a single camera trigger per event that starts the readout of all pixels in the camera at the same time and within the same integration time window. The limitations of such a trigger system can reduce the performance of a large array such as CTA, due to the huge amount of useless data created by night-sky background if trigger thresholds are set low enough to achieve the desired 20 GeV energy threshold, and to image losses at high energies due to the rigid readout window. In this study, an alternative concept (''COLIBRI'' = Concept for an Optimised Local Image Building and Readout Infrastructure) is presented, where only those parts of the camera which are likely to actually contain image data (usually a small percentage of the total pixels) are read out. This leads to a significant reduction of the expected data rate and the dead-times incurred in the camera. Furthermore, the quasi-independence of the individual clusters can be used to read different parts of the camera at slightly different times, thus allowing the readout to follow the slow development of the shower image across the camera field of view. This concept of flexible, partial camera readout is presented in the following, together with a description of Monte-Carlo studies performed to evaluate its performance as well as a hardware implementation proposed for CTA.
[en] One of the greatest mysteries in the cosmos is that it is mostly dark. That is, not only is the night sky dark, but also most of the matter and the energy in the universe is dark. For every atom visible in planets, stars and galaxies today there exists at least five or six times as much 'Dark Matter' in the universe. Astronomers and particle physicists today are seeking to unravel the nature of this mysterious but pervasive dark matter, which has profoundly influenced the formation of structure in the universe. Dark energy remains even more elusive, as we lack candidate fields that emerge from well established physics. I will describe various attempts to measure dark matter by direct and indirect means, and discuss the prospects for progress in unravelling dark energy.
[en] We discuss the imaging properties of Hybrid Photon Detectors (HPDs) in the framework of an Imaging Air Cherenkov Telescope experiment. We present also some preliminary results on Night Sky Background measurements, made with an HPD located in the focal plane of a single CLUE telescope
[en] How often do bright optical transients occur on the sky but go unreported? To constrain the bright end of the astronomical transient function, a systematic search for transients that become bright enough to be noticed by the unaided eye was conducted using the all-sky monitors of the Night Sky Live network. Two fisheye CONtinuous CAMeras operating over three years created a database that was searched for transients that appeared in time-contiguous CCD frames. Although a single candidate transient was found, the lack of more transients is used here to deduce upper limits to the general frequency of bright transients. To be detected, a transient must have increased by over three visual magnitudes to become brighter than visual magnitude 5.5 on the timescale of minutes to years. It is concluded that, on the average, fewer than 0.0040 (t dur/60 s) transients with duration t dur between minutes and hours, occur anywhere on the sky at any one time. For transients on the order of months to years, fewer than 160 (t dur/1 year) occur, while for transients on the order of years to millennia, fewer than 50 (t dur/1 year)2 occur.
[en] The day-night variation of cosmic rays (CR) intensity at sea level has been observed by a simple G-M counter telescope. We perform two 5 hours counting during the day and the night and find that the pattern of variation is closely related to the atmospheric disturbance. The normal pattern shows that the day counts is a few percents lower than that of the night counts. This can be simply explained by the temperature effect. But when our observation station is being affected by a meteorological front or trough, a reversal occurs. At times the reversal pattern occurs even up to a few days ahead the formation of the troughs at a few hundred kilometers away. Such a reversal may lasts for a few days until the influence of the front or trough has significantly weakened. This effect also overrides the barometric effect, except when there is a sharp change of air pressure, such as during the approach of a tropical cyclone. Our further investigation shows that the pattern of variation may be negatively correlated to the altitude of the 0degC level in the atmosphere. A 150 m upward shift of the 0degC level reduces the CR intensity by about 3-4%. Our postulation is that this variation of CR intensity is related to the relativistic effect of the muon decay, and is an indication of the height of the mid-level pressure distribution. The phenomenon should be further investigated for possible application in weather forecasting and better understanding of the phenomenon could be of use in differentiating the atmospheric effect and the solar effects in CR data. (author)
[en] The classical formulation of 'Olbers' paradox' consists in looking for an explanation of the fact that the sky at night is dark. We use the experimental datum of the nocturnal darkness in order to put constraints on a Newtonian cosmological model. We infer then that the stellar system in such a model should have had an origin at a finite time in the past
[en] Using a sky brightness monitor at the Xinglong station of National Astronomical Observatories, Chinese Academy of Sciences, we collected data from 22 dark clear nights and 90 moon nights. We first measured the sky brightness variation with time for dark nights and found a clear correlation between sky brightness and human activity. Then with a modified sky brightness model of moon nights and data from these nights, we derived the typical value for several important parameters in the model. With these results, we calculated the sky brightness distribution under a given moon condition for the Xinglong station. Furthermore, we simulated the sky brightness distribution of a moon night for a telescope with a 5° field of view (such as LAMOST). These simulations will be helpful for determining the limiting magnitude and exposure time, as well as planning the survey for LAMOST during moon nights