12 research outputs found
Combined MASS-DIMM instrument for atmospheric turbulence studies
Several site-testing programs and observatories currently use combined
MASS-DIMM instruments for monitoring parameters of optical turbulence. The
instrument is described here. After a short recall of the measured quantities
and operational principles, the optics and electronics of MASS-DIMM,
interfacing to telescopes and detectors, and operation are covered in some
detail. Particular attention is given to the correct measurement and control of
instrumental parameters to ensure valid and well-calibrated data, to the data
quality and filtering. Examples of MASS-DIMM data are given, followed by the
list of present and future applications.Comment: Accepted by MNRAS, 11 pages, 8 figure
Comparison of the scintillation noise above different observatories measured with MASS instruments
Scintillation noise is a major limitation of ground base photometric
precision. An extensive dataset of stellar scintillation collected at 11
astronomical sites world-wide with MASS instruments was used to estimate the
scintillation noise of large telescopes in the case of fast photometry and
traditional long-exposure regime. Statistical distributions of the
corresponding parameters are given. The scintillation noise is mostly
determined by turbulence and wind in the upper atmosphere and comparable at all
sites, with slightly smaller values at Mauna Kea and largest noise at Tolonchar
in Chile. We show that the classical Young's formula under-estimates the
scintillation noise.The temporal variations of the scintillation noise are also
similar at all sites, showing short-term variability at time scales of 1 -- 2
hours and slower variations, including marked seasonal trends (stronger
scintillation and less clear sky during local winter). Some correlation was
found between nearby observatories.Comment: Accepted for publication in Astronomy and Astrophysics, 14 pages, 11
figure
First results of site testing program at Mt. Shatdzhatmaz in 2007 - 2009
We present the first results of the site testing performed at
Mt.~Shatdzhatmaz at Northern Caucasus, where the new Sternberg astronomical
institute 2.5-m telescope will be installed. An automatic site monitor
instrumentation and functionality are described together with the methods of
measurement of the basic astroclimate and weather parameters. The clear night
sky time derived on the basis of 2006 -- 2009 data amounts to 1340 hours per
year. Principle attention is given to the measurement of the optical turbulence
altitude distribution which is the most important characteristic affecting
optical telescopes performance. For the period from November 2007 to October
2009 more than 85\,000 turbulence profiles were collected using the combined
MASS/DIMM instrument. The statistical properties of turbulent atmosphere above
the summit are derived and the median values for seeing ~arcsec
and free-atmosphere seeing ~arcsec are determined.
Together with the estimations of isoplanatic angle ~arcsec and
time constant \tau_0 = 2.58 \mbox{ ms}, these are the first representative
results obtained for Russian sites which are necessary for development of
modern astronomical observation techniques like adaptive optics.Comment: Accepted for publication in MNRAS, 17 pages, 15 figure
The formation of the milky way halo and its dwarf satellites: a NLTE-1D abundance analysis. IV. Segue 1, Triangulumii, and Coma Berenices UFDs
We present atmospheric parameters and abundances for chemical elements from carbon to barium in metal-poor stars in Segue 1 (seven stars), Coma Berenices (three stars), and Triangulum II (one star) ultrafaint dwarf galaxies (UFDs). The effective temperatures rely on new photometric observations in the visible and infra-red bands, obtained with the 2.5 m telescope of the SAI MSU Caucasian observatory. Abundances of up to fourteen chemical elements were derived under the non-local thermodynamic equilibrium (NLTE) line formation, and LTE abundances were obtained for up to five more elements. For the first time, we present abundance of oxygen in Seg 1 S1 and S4, silicon in ComaBer S2 and Tri II S40, potassium in Seg 1 S1-S6 and ComaBer S1-S3, and barium in Seg 1 S7. Three stars in Segue 1, two stars in Coma Berenices, and Triangulumii star have very low [Na/Mg] of -1.08 to -1.67 dex, which is usually attributed in the literature to an odd-even effect produced by nucleosynthesis in massive metal-free stars. We interpret this chemical property as a footprint of first stars, which is not blurred due to a small number of nucleosynthesis events that contributed to chemical abundance patterns of the sample stars. Our NLTE abundances of Sr and Ba in Coma Berenices, Segue 1, and Triangulumii report on lower [Sr/Ba] abundance ratio in the UFDs compared to that in classical dwarf spheroidal galaxies and the Milky Way halo. However, in UFDs, just as in massive galaxies, [Sr/Ba] is not constant and it can be higher than the pure r-process ratio. We suggest a hypothesis of Sr production in metal-poor binaries at the earliest epoch of galactic evolution