227 research outputs found
Astronomical seeing and ground-layer turbulence in the Canadian High Arctic
We report results of a two-year campaign of measurements, during arctic
winter darkness, of optical turbulence in the atmospheric boundary-layer above
the Polar Environment Atmospheric Laboratory in northern Ellesmere Island
(latitude +80 deg N). The data reveal that the ground-layer turbulence in the
Arctic is often quite weak, even at the comparatively-low 610 m altitude of
this site. The median and 25th percentile ground-layer seeing, at a height of
20 m, are found to be 0.57 and 0.25 arcsec, respectively. When combined with a
free-atmosphere component of 0.30 arcsec, the median and 25th percentile total
seeing for this height is 0.68 and 0.42 arcsec respectively. The median total
seeing from a height of 7 m is estimated to be 0.81 arcsec. These values are
comparable to those found at the best high-altitude astronomical sites
First Assessment of Mountains on Northwestern Ellesmere Island, Nunavut, as Potential Astronomical Observing Sites
Ellesmere Island, at the most northerly tip of Canada, possesses the highest
mountain peaks within 10 degrees of the pole. The highest is 2616 m, with many
summits over 1000 m, high enough to place them above a stable low-elevation
thermal inversion that persists through winter darkness. Our group has studied
four mountains along the northwestern coast which have the additional benefit
of smooth onshore airflow from the ice-locked Arctic Ocean. We deployed small
robotic site testing stations at three sites, the highest of which is over 1600
m and within 8 degrees of the pole. Basic weather and sky clarity data for over
three years beginning in 2006 are presented here, and compared with available
nearby sea-level data and one manned mid-elevation site. Our results point to
coastal mountain sites experiencing good weather: low median wind speed, high
clear-sky fraction and the expectation of excellent seeing. Some practical
aspects of access to these remote locations and operation and maintenance of
equipment there are also discussed.Comment: 21 pages, 2 tables, 15 figures; accepted for publication in PAS
Clear sky fraction above Indonesia: an analysis for astronomical site selection
We report a study of cloud cover over Indonesia based on meteorological
satellite data, spanning over the past 15 years (from 1996 to 2010) in order to
be able to select a new astronomical site capable to host a multi-wavelength
astronomical observatory. High spatial resolution of meteorological satellite
data acquired from {\it Geostationary Meteorological Satellite 5} ({\it GMS
5}), {\it Geostationary Operational Environmental Satellite 9} ({\it GOES 9}),
and {\it Multi-functional Transport Satellite-1R} ({\it MTSAT-1R}) are used to
derive yearly average clear fractions over the regions of Indonesia. This
parameter is determined from temperature measurement of the IR3 channel (water
vapor, 6.7 m) for high altitude clouds (cirrus) and from the IR1 channel
(10.7 m) for lower altitude clouds. Accordingly, an algorithm is developed
to detect the corresponding clouds. The results of this study are then adopted
to select the best possible sites in Indonesia to be analysed further by
performing in situ measurements planned for the coming years. The results
suggest that regions of East Nusa Tenggara, located in south-eastern part of
Indonesia, are the most promising candidates for such an astronomical site.
Yearly clear sky fraction of this regions may reach better than 70 per cent
with an uncertainty of 10 per cent.Comment: 15 pages, 13 figures, and 4 table
FACT - Long-term stability and observations during strong Moon light
The First G-APD Cherenkov Telescope (FACT) is the first Cherenkov telescope
equipped with a camera made of silicon photon detectors (G-APD aka. SiPM).
Since October 2011, it is regularly taking data on the Canary Island of La
Palma. G-APDs are ideal detectors for Cherenkov telescopes as they are robust
and stable. Furthermore, the insensitivity of G-APDs towards strong ambient
light allows to conduct observations during bright Moon and twilight. This gain
in observation time is essential for the long-term monitoring of bright TeV
blazars. During the commissioning phase, hundreds of hours of data (including
data from the the Crab Nebula) were taken in order to understand the
performance and sensitivity of the instrument. The data cover a wide range of
observation conditions including different weather conditions, different zenith
angles and different light conditions (ranging from dark night to direct full
Moon). We use a new parmetrisation of the Moon light background to enhance our
scheduling and to monitor the atmosphere. With the data from 1.5 years, the
long-term stability and the performance of the camera during Moon light is
studied and compared to that achieved with photomultiplier tubes so far.Comment: 3 pages, 3 figures, FACT Contribution to the 33rd International
Cosmic Ray Conference (ICRC), Rio de Janeir
FACT - Threshold prediction for higher duty cycle and improved scheduling
The First G-APD Cherenkov telescope (FACT) is the first telescope using
silicon photon detectors (G-APD aka. SiPM). The use of Silicon devices promise
a higher photon detection efficiency, more robustness and higher precision than
photo-multiplier tubes. Being operated during different light-conditions, the
threshold settings of a Cherenkov telescope have to be adapted to feature the
lowest possible threshold but also an efficient suppression of triggers from
night-sky background photons. Usually this threshold is set either by
experience or a mini-ratescan. Since the measured current through the sensors
is directly correlated with the noise level, the current can be used to set the
best threshold at any time. Due to the correlation between the physical
threshold and the final energy threshold, the current can also be used as a
measure for the energy threshold of any observation. This presentation
introduces a method which uses the properties of the moon and the source
position to predict the currents and the corresponding energy threshold for
every upcoming observation allowing to adapt the observation schedule
accordingly
FACT - Long-term Monitoring of Bright TeV-Blazars
Since October 2011, the First G-APD Cherenkov Telescope (FACT) is operated
successfully on the Canary Island of La Palma. Apart from the proof of
principle for the use of G-APDs in Cherenkov telescopes, the major goal of the
project is the dedicated long-term monitoring of a small sample of bright TeV
blazars. The unique properties of G-APDs permit stable observations also during
strong moon light. Thus a superior sampling density is provided on time scales
at which the blazar variability amplitudes are expected to be largest, as
exemplified by the spectacular variations of Mrk 501 observed in June 2012.
While still in commissioning, FACT monitored bright blazars like Mrk 421 and
Mrk 501 during the past 1.5 years so far. Preliminary results including the Mrk
501 flare from June 2012 will be presented.Comment: 4 pages, 4 figures, presented at the 33rd ICRC (2013
High Precision Astrometry with MICADO at the European Extremely Large Telescope
In this article we identify and discuss various statistical and systematic
effects influencing the astrometric accuracy achievable with MICADO, the
near-infrared imaging camera proposed for the 42-metre European Extremely Large
Telescope (E-ELT). These effects are instrumental (e.g. geometric distortion),
atmospheric (e.g. chromatic differential refraction), and astronomical
(reference source selection). We find that there are several phenomena having
impact on ~100 micro-arcsec scales, meaning they can be substantially larger
than the theoretical statistical astrometric accuracy of an optical/NIR
42m-telescope. Depending on type, these effects need to be controlled via
dedicated instrumental design properties or via dedicated calibration
procedures. We conclude that if this is done properly, astrometric accuracies
of 40 micro-arcsec or better - with 40 micro-arcsec/year in proper motions
corresponding to ~20 km/s at 100 kpc distance - can be achieved in one epoch of
actual observationsComment: 15 pages, 9 figures, 3 tables. Accepted by MNRA
FACT - How stable are the silicon photon detectors?
The First G-APD Cherenkov telescope (FACT) is the first telescope using
silicon photon detectors (G-APD aka. SiPM). The use of Silicon devices promise
a higher photon detection efficiency, more robustness and higher precision than
photo-multiplier tubes. Since the properties of G-APDs depend on auxiliary
parameters like temperature, a feedback system adapting the applied voltage
accordingly is mandatory.
In this presentation, the feedback system, developed and in operation for
FACT, is presented. Using the extraction of a single photon-equivalent (pe)
spectrum as a reference, it can be proven that the sensors can be operated with
very high precision. The extraction of the single-pe, its spectrum up to
10\,pe, its properties and their precision, as well as their long-term behavior
during operation are discussed. As a by product a single pulse template is
obtained. It is shown that with the presented method, an additional external
calibration device can be omitted. The presented method is essential for the
application of G-APDs in future projects in Cherenkov astronomy and is supposed
to result in a more stable and precise operation than possible with
photo-multiplier tubes
Radio Galaxies at z = 1.1 to 3.8: Adaptive-Optics Imaging and Archival Hubble Space Telescope Data
We have undertaken a program of high-resolution imaging of high-redshift
radio galaxies (HzRGs) using adaptive optics on the Canada-France-Hawaii
Telescope. We report on deep imaging in J, H,and K bands of 6 HzRGs in the
redshift range 1.1 to 3.8. At these redshifts, near-infrared bandpasses sample
the rest-frame visible galaxian light. The radio galaxy is resolved in all the
fields and is generally elongated along the axis of the radio lobes. These
images are compared to archival Hubble Space Telescope Wide-Field Planetary
Camera 2 optical observations of the same fields and show the HzRG morphology
in rest-frame ultraviolet and visible light is generally very similar: a string
of bright compact knots. Furthermore, this sample - although very small -
suggests the colors of the knots are consistent with light from young stellar
populations. If true, a plausible explanation is that these objects are being
assembled by mergers at high redshift.Comment: 32 pages, 8 figures, accepted for publication in the Astrophysical
Journa
Data compression for the First G-APD Cherenkov Telescope
The First Geiger-mode Avalanche photodiode (G-APD) Cherenkov Telescope (FACT)
has been operating on the Canary island of La Palma since October 2011.
Operations were automated so that the system can be operated remotely. Manual
interaction is required only when the observation schedule is modified due to
weather conditions or in case of unexpected events such as a mechanical
failure. Automatic operations enabled high data taking efficiency, which
resulted in up to two terabytes of FITS files being recorded nightly and
transferred from La Palma to the FACT archive at ISDC in Switzerland. Since
long term storage of hundreds of terabytes of observations data is costly, data
compression is mandatory. This paper discusses the design choices that were
made to increase the compression ratio and speed of writing of the data with
respect to existing compression algorithms.
Following a more detailed motivation, the FACT compression algorithm along
with the associated I/O layer is discussed. Eventually, the performances of the
algorithm is compared to other approaches.Comment: 17 pages, accepted to Astronomy and Computing special issue on
astronomical file format
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