Analytical expressions for optimum alignment modes of highly segmented mirrors

The major sources causing deterioration of optical quality in extremely large optical telescopes are misadjustments of the mirrors, deformations of monolithic mirrors, and misalignments of segments in segmented mirrors. For active optics corrections, all three errors, which can partially compensate each other, are measured simultaneously. It is therefore of interest to understand the similarities and differences between the three corresponding types of modes which describe these errors. The first two types are best represented by Zernike polynomials and elastic modes respectively, both of them being continuous and smooth functions. The segment misaligment modes, which are derived by singular value decomposition, are by their nature not smooth and in general discontinuous. However, for mirrors with a large number of segments, the lowest modes become effectively both smooth and continuous. This paper derives analytical expressions for these modes, using differential operators and their adjoints, for the limit case of infinitesimally small segments. For segmented mirrors with approximately 1000 segments, it is shown that these modes agree well with the corresponding lowest singular value decomposition modes. Furthermore, the analytical expressions reveal the nature of the segment misalignment modes and allow for a detailed comparison with the elastic modes of monolithic mirrors. Some mathematical features emerge as identical in the two cases.Comment: 24 pages, 13 figures, accepted for publication in Journal of Modern Optic

Optical measurements of phase steps in segmented mirrors - fundamental precision limits

Phase steps are an important type of wavefront aberrations generated by large telescopes with segmented mirrors. In a closed-loop correction cycle these phase steps have to be measured with the highest possible precision using natural reference stars, that is with a small number of photons. In this paper the classical Fisher information of statistics is used for calculating the Cramer-Rao bound, which determines the limit to the precision with which the height of the steps can be estimated in an unbiased fashion with a given number of photons and a given measuring device. Four types of measurement devices are discussed: a Shack-Hartmann sensor with one small cylindrical lenslet covering a sub-aperture centred over a border, a modified Mach-Zehnder interferometer, a Foucault test, and a curvature sensor. The Cramer-Rao bound is calculated for all sensors under ideal conditions, that is narrowband measurements without additional noise or disturbances apart from the photon shot noise. This limit is compared with the ultimate quantum statistical limit for the estimate of such a step which is independent of the measuring device. For the Shack-Hartmann sensor, the effects on the Cramer-Rao bound of broadband measurements, finite sampling, and disturbances such as atmospheric seeing and detector readout noise are also investigated. The methods presented here can be used to compare the precision limits of various devices for measuring phase steps and for optimising the parameters of the devices. Under ideal conditions the Shack-Hartmann and the Foucault devices nearly attain the ultimate quantum statistical limits, whereas the Mach-Zehnder and the curvature devices each require approximately twenty times as many photons in order to reach the same precision.Comment: 23 pages, 19 figures, to be submitted to Journal of Modern Optic

Mirror Position Determination for the Alignment of Cherenkov Telescopes

Imaging Atmospheric Cherenkov Telescopes (IACTs) need imaging optics with large apertures to map the faint Cherenkov light emitted in extensive air showers onto their image sensors. Segmented reflectors fulfill these needs using mass produced and light weight mirror facets. However, as the overall image is the sum of the individual mirror facet images, alignment is important. Here we present a method to determine the mirror facet positions on a segmented reflector in a very direct way. Our method reconstructs the mirror facet positions from photographs and a laser distance meter measurement which goes from the center of the image sensor plane to the center of each mirror facet. We use our method to both align the mirror facet positions and to feed the measured positions into our IACT simulation. We demonstrate our implementation on the 4 m First Geiger-mode Avalanche Cherenkov Telescope (FACT).Comment: 11 figures, small ray tracing performance simulation, and implementation demonstratio

miR449 Protects Airway Regeneration by Controlling AURKA/HDAC6-Mediated Ciliary Disassembly

Airway mucociliary regeneration and function are key players for airway defense and are impaired in chronic obstructive pulmonary disease (COPD). Using transcriptome analysis in COPD-derived bronchial biopsies, we observed a positive correlation between cilia-related genes and microRNA-449 (miR449). In vitro, miR449 was strongly increased during airway epithelial mucociliary differentiation. In vivo, miR449 was upregulated during recovery from chemical or infective insults. miR0449−/− mice (both alleles are deleted) showed impaired ciliated epithelial regeneration after naphthalene and Haemophilus influenzae exposure, accompanied by more intense inflammation and emphysematous manifestations of COPD. The latter occurred spontaneously in aged miR449−/− mice. We identified Aurora kinase A and its effector target HDAC6 as key mediators in miR449-regulated ciliary homeostasis and epithelial regeneration. Aurora kinase A is downregulated upon miR449 overexpression in vitro and upregulated in miR449−/− mouse lungs. Accordingly, imaging studies showed profoundly altered cilia length and morphology accompanied by reduced mucociliary clearance. Pharmacological inhibition of HDAC6 rescued cilia length and coverage in miR449−/− cells, consistent with its tubulin-deacetylating function. Altogether, our study establishes a link between miR449, ciliary dysfunction, and COPD pathogenesis

Measuring the optical point spread function of FACT using the Cherenkov camera

FACT, the First G-APD Cherenkov Telescope, is an Imaging Air Cherenkov Telescope (IACT) operating since 2011 at the Observatorio del Roque de los Muchachos on the Canary Island of La Palma. As typical for IACTs, its reflector is comprised of smaller mirror facets and not protected by a dome. In the case of FACT, 30 hexagonal facets form a total mirror area of 9:5m². Hence, it is crucial to monitor the optical properties of this system and realign the facets if necessary. Up to now, measuring the Point Spread Function of FACT required human interaction to mount a screen and an optical camera. In this contribution, a new method to measure the optical Point Spread Function using directly the Cherenkov camera of the telescope is presented. Inspired by the method radio telescopes use to determine their resolution, the telescope is pointed towards a fixed position on the trajectory of a star. During the star’s passage through the field of view, the camera is read out using a fixed rate. In each event, the pedestal variance is determined for each pixel. This value is directly correlated with the amount of night sky background light a pixel received. Translating the time of the measurement to the position of the star in the camera enables to determine the optical point spread function from this measurement. As the measurement is done for each pixel along the trajectory of the star, the Point Spread Function can be determined not only for the camera center but for the entire field of view. In this contribution, the new method will also be compared with the existing methods of determining the optical Point Spread Function: direct measurement with an optical camera and the width of Muon ring events.M. Noethe, J. Adam, M. L. Ahnen, D. Baack, M. Balbo, A. Biland, M. Blank, T. Bretz, K. Bruegge, J. Buss, A. Dmytriiev, D. Dorner, S. Einecke, D. Elsaesser, C. Hempfling, T. Herbst, D. Hildebrand, L. Kortmann, L. Linhoff, M. Mahlke, K. Mannheim, S. A. Mueller, D. Neise, A. Neronov, J. Oberkirch, A. Paravac, F. Pauss, W. Rhode, B. Schleicher, F. Schulz, A. Shukla, V. Sliusar, F. Temme, J. Thaele, R. Walte

FACT - Performance of the first cherenkov telescope observing with SiPMs

The First G-APD Cherenkov Telescope (FACT) is pioneering the usage of silicon photo multipliers (SIPMs also known as G-APDs) for the imaging atmospheric Cherenkov technique. It is located at the Observatorio Roque de los Muchachos on the Canary island of La Palma. Since first light in October 2011, it is monitoring bright TeV blazars in the northern sky. By now, FACT is the only imaging atmospheric Cherenkov telescope operating with SIPMs on a nightly basis. Over the course of the last five years, FACT has been demonstrating their reliability and excellent performance. Moreover, their robustness allowed for an increase of the duty cycle including nights with strong moon light without the need for UV-filters. In this contribution, we will present the performance of the first Cherenkov telescope using solid state photo sensors, which was determined in analysis of data from Crab Nebula, the so called standard candle in gamma-ray astronomy. The presented analysis chain utilizes modern data mining methods and unfolding techniques to obtain the energy spectrum of this source. The characteristical results of such an analysis will be reported providing, e.g., the angular and energy resolution of FACT, as well as, the energy spectrum of the Crab Nebula. Furthermore, these results are discussed in the context of the performance of coexisting Cherenkov telescopes.M. Noethe, J. Adam, M.L. Ahnen, D. Baack, M. Balbo, A. Biland, M. Blank, T. Bretz, K. Bruegge, J. Buss, A. Dmytriiev, D. Dorner, S. Einecke, D. Elsaesser, C. Hempfling, T. Herbst, D. Hildebrand, L. Kortmann, L. Linhoff, M. Mahlke, K. Mannheim, S. Mueller, D. Neise, A. Neronov, J. Oberkirch, A. Paravac, F. Pauss, W. Rhode, B. Schleicher, F. Schulz, A. Shukla, V. Sliusar, F. Temme, J. Thaele, R. Walte

The extreme HBL behaviour of Markarian 501 during 2012

A multiwavelength campaign was organized to take place between March and July of 2012. Excellent temporal coverage was obtained with more than 25 instruments, including the MAGIC, FACT and VERITAS Cherenkov telescopes, the instruments on board the Swift and Fermi spacecraft, and the telescopes operated by the GASP-WEBT collaboration. Mrk 501 showed a very high energy (VHE) gamma-ray flux above 0.2 TeV of $\sim$0.5 times the Crab Nebula flux (CU) for most of the campaign. The highest activity occurred on 2012 June 9, when the VHE flux was $\sim$3 CU, and the peak of the high-energy spectral component was found to be at $\sim$2 TeV. This study reports very hard X-ray spectra, and the hardest VHE spectra measured to date for Mrk 501. The fractional variability was found to increase with energy, with the highest variability occurring at VHE, and a significant correlation between the X-ray and VHE bands. The unprecedentedly hard X-ray and VHE spectra measured imply that their low- and high-energy components peaked above 5 keV and 0.5 TeV, respectively, during a large fraction of the observing campaign, and hence that Mrk 501 behaved like an extreme high-frequency- peaked blazar (EHBL) throughout the 2012 observing season. This suggests that being an EHBL may not be a permanent characteristic of a blazar, but rather a state which may change over time. The one-zone synchrotron self-Compton (SSC) scenario can successfully describe the segments of the SED where most energy is emitted, with a significant correlation between the electron energy density and the VHE gamma-ray activity, suggesting that most of the variability may be explained by the injection of high-energy electrons. The one-zone SSC scenario used reproduces the behaviour seen between the measured X-ray and VHE gamma-ray fluxes, and predicts that the correlation becomes stronger with increasing energy of the X-rays

Searching for VHE gamma-ray emission associated with IceCube neutrino alerts using FACT, H.E.S.S., MAGIC, and VERITAS

The realtime follow-up of neutrino events is a promising approach to searchfor astrophysical neutrino sources. It has so far provided compelling evidencefor a neutrino point source: the flaring gamma-ray blazar TXS 0506+056 observedin coincidence with the high-energy neutrino IceCube-170922A detected byIceCube. The detection of very-high-energy gamma rays (VHE, $\mathrm{E} >100\,\mathrm{GeV}$) from this source helped establish the coincidence andconstrained the modeling of the blazar emission at the time of the IceCubeevent. The four major imaging atmospheric Cherenkov telescope arrays (IACTs) -FACT, H.E.S.S., MAGIC, and VERITAS - operate an active follow-up program oftarget-of-opportunity observations of neutrino alerts sent by IceCube. Thisprogram has two main components. One are the observations of known gamma-raysources around which a cluster of candidate neutrino events has been identifiedby IceCube (Gamma-ray Follow-Up, GFU). Second one is the follow-up of singlehigh-energy neutrino candidate events of potential astrophysical origin such asIceCube-170922A. GFU has been recently upgraded by IceCube in collaborationwith the IACT groups. We present here recent results from the IACT follow-upprograms of IceCube neutrino alerts and a description of the upgraded IceCubeGFU system.<br

Multiwavelength variability and correlation studies of Mrk 421 during historically low X-ray and γ-ray activity in 2015-2016

We report a characterization of the multiband flux variability and correlations of the nearby (z = 0.031) blazar Markarian 421 (Mrk 421) using data from Metsahovi, Swift, Fermi-LAT, MAGIC, FACT, and other collaborations and instruments from 2014 November till 2016 June. Mrk 421 did not show any prominent flaring activity, but exhibited periods of historically low activity above 1 TeV (F->1 TeV 0.1 TeV) gamma-rays, which, despite the low activity, show a significant positive correlation with no time lag. The HRkeV and HRTeV show the harder-when-brighter trend observed in many blazars, but the trend flattens at the highest fluxes, which suggests a change in the processes dominating the blazar variability. Enlarging our data set with data from years 2007 to 2014, we measured a positive correlation between the optical and the GeV emission over a range of about 60 d centred at time lag zero, and a positive correlation between the optical/GeV and the radio emission over a range of about 60 d centred at a time lag of 43(-6)(+9) d. This observation is consistent with the radio-bright zone being located about 0.2 parsec downstream from the optical/GeV emission regions of the jet. The flux distributions are better described with a lognormal function in most of the energy bands probed, indicating that the variability in Mrk 421 is likely produced by a multiplicative process

Investigation of the correlation patterns and the Compton dominance variability of Mrk 421 in 2017

Aims. We present a detailed characterisation and theoretical interpretation of the broadband emission of the paradigmatic TeV blazar Mrk 421, with a special focus on the multi-band flux correlations.Methods. The dataset has been collected through an extensive multi-wavelength campaign organised between 2016 December and 2017 June. The instruments involved are MAGIC, FACT, Fermi-LAT, Swift, GASP-WEBT, OVRO, Medicina, and Metsahovi. Additionally, four deep exposures (several hours long) with simultaneous MAGIC and NuSTAR observations allowed a precise measurement of the falling segments of the two spectral components.Results. The very-high-energy (VHE; E > 100 GeV) gamma rays and X-rays are positively correlated at zero time lag, but the strength and characteristics of the correlation change substantially across the various energy bands probed. The VHE versus X-ray fluxes follow different patterns, partly due to substantial changes in the Compton dominance for a few days without a simultaneous increase in the X-ray flux (i.e., orphan gamma-ray activity). Studying the broadband spectral energy distribution (SED) during the days including NuSTAR observations, we show that these changes can be explained within a one-zone leptonic model with a blob that increases its size over time. The peak frequency of the synchrotron bump varies by two orders of magnitude throughout the campaign. Our multi-band correlation study also hints at an anti-correlation between UV-optical and X-ray at a significance higher than 3 sigma. A VHE flare observed on MJD 57788 (2017 February 4) shows gamma-ray variability on multi-hour timescales, with a factor ten increase in the TeV flux but only a moderate increase in the keV flux. The related broadband SED is better described by a two-zone leptonic scenario rather than by a one-zone scenario. We find that the flare can be produced by the appearance of a compact second blob populated by high energetic electrons spanning a narrow range of Lorentz factors, from gamma(min)' = 2 x 10(4) to gamma(max)' = 6 x 10(5).</p