Lunar Scintillometer Research Articles

Optical turbulence at Ali, China – results from the first year of lunar scintillometer observations

ABSTRACT The location of an astronomical observatory is a key factor that affects its scientific productivity. The best astronomical sites are generally those found at high altitudes. Several such sites in western China and the Tibetan plateau are presently under development for astronomy. One of these is Ali, which, at over 5000 m, is one of the highest astronomical sites in the world. In order to further investigate the astronomical potential of Ali, we have installed a lunar scintillometer, for the primary purpose of profiling atmospheric turbulence. This paper describes the instrument and technique, and reports results from the first year of observations. We find that ground layer (GL) turbulence at Ali is remarkably weak and relatively thin. The median seeing, from turbulence in the range 11–500 m above ground, is 0.34 arcsec, with seeing better than 0.26 arcsec occurring 25 per cent of the time. Under median conditions, half of the GL turbulence lies below a height of 62 m. These initial results, and the high altitude and relatively low temperatures, suggest that Ali could prove to be an outstanding site for ground-based astronomy.

Development of a Lunar Scintillometer as part of the national large optical telescope site survey

Ground layer turbulence is a very important site characterization parameter used to assess the quality of an astronomical site. The Lunar Scintillometer is a simple and effective site-testing device for measuring the ground layer turbulence. It consists of a linear array of photodiodes which are sensitive to the slight variations in the moon's brightness due to scintillation by the lower layers of the Earth's atmosphere. The covariance of intensity values between the non-redundant photodiode baselines can be used to measure the turbulence profile from the ground up to a height determined by the furthest pair of detectors. The six channel lunar scintillometer that has been developed at the Indian Institute of Astrophysics is based closely on an instrument built by the team led by Andrei Tokovinin of Cerro Tololo Inter-American Observatory (CTIO), Chile. We have fabricated the instrument based on the existing electronic design, and have worked on the noise analysis, an EMI (Electromagnetic Induction) resistant PCB design and the software pipeline for analyzing the data from the same. The results from the instrument's multi-year campaign at Mount Saraswati, Hanle is also presented.

Characterizing Optical Turbulence at the GMT Site with MooSci and MASS-DIMM

ABSTRACTIn order to guide the adaptive optics design and aid in performance predictions, optical turbulence at the site of the future Giant Magellan Telescope (GMT) is characterized using MooSci, a lunar scintillometer, and MASS-DIMM, a combination differential image motion monitor and multiaperture scintillation sensor. As a new instrument, MooSci, is verified as a reliable ground-layer turbulence profiler. The GMT can expect an improvement of approximately 0.1″ over the site testing results as measured with a DIMM. Turbulence below 30 m is horizontally nonhomogeneous, dependent on wind speed and direction, and on average accounts for 60% of the full ground-layer (up to 500 m) turbulence.

First results from optical turbulence measurements at Cerro Las Campanas in 2010

We report preliminary results from optical turbulence measurements carried out in 2010 at Cerro Las Campanas, the future site for the Giant Magellan Telescope (GMT). The instruments involved are MooSci, a lunar scintillometer for the near-ground optical turbulence profile, Differential Image Motion Monitor (DIMM) for the whole atmosphere total seeing, and MASS Multiple Aperture Scintillation Sensor (MASS) for high-altitude optical turbulence estimation. The main purpose of these measurements is to anticipate the optical turbulence strength above the future GMT enclosure, and to provide a means to model the future adaptive optics performance. We also discuss the significance of such a combination of instruments and some hypothetical limitations.

Measuring Atmospheric Turbulence with a Lunar Scintillometer Array

ABSTRACTAn array of photodetectors, responding to scintillation of light from the Moon, provides a probe of the turbulence structure along the line of sight through the atmosphere. The measured quantities are time series of intensity fluctuations received by the individual detectors in the array. From the covariance of these fluctuations, taken between all possible detector pairs, one can determine the C2N turbulence profile from the ground up to a maximum height determined by the separation between the detectors. This paper outlines the theoretical basis of the technique and discusses methods of data analysis. We then describe the design and construction of a prototype eight‐element lunar scintillometer array and present first results of observations using this system.