Multi Aperture Scintillation Sensor Research Articles

Atmospheric turbulence profiling with multi-aperture scintillation of a Shack–Hartmann sensor

ABSTRACT Adaptive optics (AO) systems that use tomographic estimation of the three-dimensional structure of atmospheric turbulence require the vertical atmospheric turbulence profile, which describes turbulence strength as a function of altitude as prior information. We propose a novel method to reconstruct the profile by applying a multi-aperture scintillation sensor (MASS) method to scintillation data obtained by a Shack–Hartmann wavefront sensor (SH-WFS). Compared with a traditional MASS, which uses atmospheric scintillation within four concentric annular apertures, the new method utilizes scintillation in several hundreds of spatial patterns, which are created by combinations of SH-WFS subapertures. Accuracy of the turbulence profile reconstruction is evaluated with Bayesian inference, and it is confirmed that the turbulence profile with more than 10 layers can be reconstructed because of the large number of constraints. We demonstrate the new method with a SH-WFS attached to the 51-cm telescope at Tohoku University and we confirm that the general characteristics of the atmospheric turbulence profile are reproduced.

Forecasting seeing for the Maunakea Observatories

ABSTRACT Optical turbulence greatly impacts the range and quality of astronomical observations. Advanced knowledge of the expected atmospheric optical turbulence provides important guidance that helps astronomers decide which instrument to schedule and enables them to optimize the adaptive optics technology that improves image resolution. Along with forecasts of weather conditions, prediction of the optical observing quality on the Maunakea summit has been a goal for the Maunakea Weather Center (MKWC) since its inception more than 20 yr ago. Forecasting optical turbulence, and its derivative, ‘seeing’, has proven to be quite challenging because optical turbulence is too small and complex to directly capture with a regional weather model. Fortunately, the permanent installation of a Differential Image Motion Monitor (DIMM) and Multi-Aperture Scintillation Sensor (MASS) at the summit of Maunakea has made seeing observations available during the last decade, providing valuable feedback to the MKWC. This paper summarizes the experience at MKWC in anticipating optical turbulence for the summit of Maunakea accrued through years of daily operational forecasting, and continuous comparison between MKWC official forecasts, model guidance, and observational measures of seeing. Access to a decade seeing observations has allowed quantification the factors that impact seeing, including wind shear, atmospheric stability patterns, and optical turbulence, and to document the seasonal and intra-seasonal variations in seeing. Consequently, the combination of experience gained, and custom model guidance has led to more accurate seeing forecasts (rms errors averaging <0.25 arcsec since 2012) for the Maunakea astronomical observatories.

Night-sky brightness and extinction at Mt Shatdzhatmaz

The photometric sky quality of Mt. Shatdzhatmaz, the site of Sternberg Astronomical Institute Caucasian Observatory 2.5 m telescope, is characterized here by the statistics of the night-time sky brightness and extinction. The data were obtained as a by-product of atmospheric optical turbulence measurements with the MASS (Multi-Aperture Scintillation Sensor) device conducted in 2007--2013. The factors biasing night-sky brightness measurements are considered and a technique to reduce their impact on the statistics is proposed. The single-band photometric estimations provided by MASS are easy to transform to the standard photometric bands. The median moonless night-sky brightness is 22.1, 21.1, 20.3, and 19.0 mag per square arcsec for the $B$, $V$, $R$, and $I$ spectral bands, respectively. The median extinction coefficients for the same photometric bands are 0.28, 0.17, 0.13, and 0.09 mag. The best atmospheric transparency is observed in winter.

European Extremely Large Telescope: Isopistonic Angle Measurements at Aklim Site

As part of the Extremely Large Telescope (ELT) project, the sitetesting team at Morocco has measured from April 2008 to December 2009. The atmospheric properties at Aklim which was one of the candidates sites pre-selected. In this paper we present the isoplanatic angle 0 and the isopistonic angle p measurements at Aklim site. Statistics of the mentioned parameters are obtained from the whole data recorded using a Multi Aperture Scintillation Sensor (MASS) and a Differential Image Motion Monitor (DIMM) system. More representative results and statistics are shown hereafter.

Study on Atmospheric Optical Turbulence above Mount Shatdzhatmaz in 2007–2013

We present the results of the atmospheric optical turbulence (OT) measurements performed atop Mount Shatdzhatmaz at the installation site of new 2.5 m telescope of the Sternberg Astronomical Institute. Nearly 300,000 vertical OT profiles from the ground up to an altitude of 23 km were obtained in the period of 2007 November–2013 June 2 with the combined multiaperture scintillation sensor (MASS) and differential image motion monitor (DIMM) instrument. The medians of the main OT characteristics, computed over the whole dataset are as follows: the integral seeing β0 = 0.96″, the free-atmosphere seeing βFA = 0.43″, and the isoplanatic angle θ0 = 2.07″. The median atmospheric time constant is τ0 = 6.57 ms. The revealed long-term variability of these parameters on scales of months and years implies the need to take it into account in astroclimatic campaign planning. For example, the annual variation in the monthly θ0 estimate amounts to 30%, while the time constant τ0 changes by a factor of 2.5. Evaluation of the potential of Mount Shatdzhatmaz in terms of high angular resolution observations indicates that in October–November, this site is as good as the best of studied summits in the world.

Ground Layer Laser Seeing Meter

The seeing calculation and its evolution during the night is a key point for the operation of telescopes and adaptive optics systems. Currently, there are various instruments able to measure the seeing, for example, the DIMM (differential image motion monitor) and the MASS (multi aperture scintillation sensor). This paper describes a new tool for the local ground layer seeing measurement. In particular, we want to derive the Fried parameter r0 through a laser beam horizontal propagation. This is a new method for the experimental study of low-altitude atmospheric turbulence. Finally, we sketch an experimental setup for the Asiago Ekar Observatory and its possible applications.

On the comparison between MASS and generalized-SCIDAR techniques

The Multi Aperture Scintillation Sensor (MASS) and the Generalized-Scintillation Detection and Ranging (Generalized SCIDAR) are two instruments conceived to measure the optical turbulence (OT) vertical distribution on the whole troposphere and low stratosphere (~ 20 km) widely used in the astronomical context. In this paper we perform a detailed analysis/comparison of measurements provided by the two instruments and taken during the extended site testing campaign carried out on 2007 at Cerro Paranal and promoted by the European Southern Observatory (ESO). The main and final goal of the study is to provide a detailed estimation of the measurements reliability i.e dispersion of turbulence measurements done by the two instruments at different heights above the ground. This information is directly related to our ability in estimating the absolute value of the turbulence stratification. To better analyse the uncertainties between the MASS and the GS we took advantage of the availability of measurements taken during the same campaign by a third independent instrument (DIMM - Differential Imaging Motion Monitor) measuring the integrated turbulence extended on the whole 20 km. Such a cross-check comparison permitted us to define the reliability of the instruments and their measurements, their limits and the contexts in which their use can present some risk.

Optical turbulence characterization at the SAAO Sutherland site

We present results from the first year of a campaign to characterize and monitor the optical turbulence profile at the SAAO Sutherland observing station in South Africa. A MASS-DIMM (MultiAperture Scintillation Sensor - Differential Image Motion Monitor) was commissioned in March 2010 to provide continuous monitoring of the seeing conditions. Over the first month of the campaign, a SLODAR (SLOpe Detection And Ranging) from Durham University was also installed allowing an independent verification of the performance of the MASS-DIMM device. After the first year of data collection, the overall median seeing value is found to be 1.32" as measured at ground level. The ground layer which includes all layers below 1 km accounts for 84% of the turbulence, while the free atmosphere above 1 km accounts for 16% with a median value of 0.41". The median isoplanatic angle value is 1.92", which is similar to other major astronomical sites. The median coherence time, calculated from corrected MASS measurements, is 2.85 ms. The seeing conditions at the site do show a strong correlation with wind direction with bad seeing conditions being associated with winds from the South-East.

Preliminary DIMM and MASS Nighttime Seeing Measurements at PEARL in the Canadian High Arctic

ABSTRACTResults of deploying a differential image motion monitor (DIMM) and a DIMM combined with a multiaperture scintillation sensor (MASS/DIMM) are reported for campaigns in 2011 and 2012 on the roof of the Polar Environment Atmospheric Research Laboratory (PEARL). This facility is on a 610 m high ridge at latitude 80°N, near the Eureka weather station on Ellesmere Island, Canada. The median seeing at 8 m elevation is 0″.85 or better based on DIMM data alone, but is dependent on wind direction and likely includes a component due to the PEARL building itself. Results with MASS/DIMM yield a median seeing less than 0″.76. A semiempirical model of seeing versus ground wind speed is introduced which allows agreement between these datasets, and with previous boundary-layer profiling by lunar scintillometry from the same location. This further suggests that best 20th percentile seeing reaches 0″.53, of which typically 0″.30 is due to the free atmosphere. Some discussion for guiding future seeing instrumentation and characterization at this site is provided.

Astroclimate at San Pedro Mártir - I. Seeing statistics between 2004 and 2008 from the Thirty Meter Telescope site-testing data

We present comprehensive seeing statistics for the San Pedro M\'artir site derived from the Thirty Meter Telescope site selection data. The observations were obtained between 2004 and 2008 with a Differential Image Motion Monitor (DIMM) and a Multi Aperture Scintillation Sensor (MASS) combined instrument (MASS--DIMM). The parameters that are statistically analised here are: whole atmosphere seeing -measured by the DIMM-; free atmosphere seeing --measured by the MASS--; and ground-layer seeing (GL) --difference between the total and free-atmosphere seeing--. We made a careful data coverage study along with statistical distributions of simultaneous MASS--DIMM seeing measurements, in order to investigate the nightly, monthly, seasonal, annual and global behaviour, as well as possible hourly seeing trends. Although this campaign covers five years, the sampling is uneven, being 2006 and 2007 the best sampled years in terms of seasonal coverage. The overall results yield a median seeing of 0.78 (DIMM), 0.37 (MASS) and 0.59 arcsec (GL). The strongest contribution to the whole atmosphere seeing comes, therefore, from a strong ground layer. We find that the best season is summer, while the worst one is winter, in accordance with previous studies. It is worth noting that the best yearly results are correlated with the best sampled years. The hourly analysis shows that there is no statistically significant tendency of seeing degradation towards dawn. The seeing values are slightly larger than those reported before. This may be caused by climate changes.

Angular correlation of the stellar scintillation for large telescopes

The stellar scintillation is one of the fundamental limitation to the precision of ground-based photometry. The paper examines the problem of correlation of the scintillation of two close stars at the focus of a large telescope. The derived correlation functions were applied to data of the long-term study of the optical turbulence (OT) in the Northern Caucasus with MASS (Multi-Aperture Scintillation Sensor) instrument to predict the angular correlation of the scintillation at the Sternberg institute 2.5 m telescope currently in construction. A median angular radius of the correlation as large as 20 arcsec was found for the case of Kolmogorov OT. On the basis of the obtained relations we also analyze the correlation impact in ensemble photometry and conjugate plane photometry. It is shown that a reduction of the scintillation noise up to 8 times can be achieved when using a crowded ensemble of comparison stars. The calculation of the angular correlation can be repeated for any large telescope at the site where the OT vertical profiles are known.

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.

Using a Sodar to Measure Optical Turbulence and Wind Speed for the Thirty Meter Telescope Site Testing. Part II: Comparison with Independent Instruments

In the second part of this study, we compare both the wind speed and turbulence given by the Sodars with independent sets of measurements. In the case of the wind speed we compare the lowest Sodar data bin with a sonic anemometer located on a 7-m tower. The agreement between the two instruments was convincing with a regression slope near unity. The integrated turbulence measurements of the Sodars are compared with those obtained with a combined multi-aperture scintillation sensor and differential image motion monitor (MASS/DIMM) unit. It was found that the Sodars are indeed capable of quantitatively measuring optical turbulence, and agree with the MASS/DIMM measurements with a correlation coefficient of approximately 80% and a regression slope within 10% of unity. Additional acoustic noise in the Sodar data was identified using this comparison and removed from the data.

Stellar scintillation in the short exposure regime and atmospheric coherence time evaluation

Aims. Accurately measuring the atmospheric coherence time is still a significant problem despite a variety of applicable methods. The Multi-Aperture Scintillation Sensor (MASS) designed for the vertical profiling of optical turbulence also provides a measurements of coherence time, but its results were found to be biased. Hence there is a need for a more robust method to determine τ0.

First results of a site-testing programme at Mount Shatdzhatmaz during 2007-2009

We present the first results of site testing performed at Mt Shatdzhatmaz in the 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 h per year. Principal attention is given to measurement of the optical-turbulence altitude distribution, which is the most important characteristic affecting the performance of optical telescopes. For the period from 2007 November–2009 October, more than 85 000 turbulence profiles were collected using the combined Multi Aperture Scintillation Sensor and Differential Image Motion Monitor (MASS/DIMM) instrument. The statistical properties of the turbulent atmosphere above the summit are derived and the median values for seeing β0= 0.93 arcsec and free-atmosphere seeing βfree= 0.51 arcsec are determined. Together with the estimations of isoplanatic angle θ0= 2.07 arcsec and time constant τ0= 2.58 ms, these are the first representative results obtained for Russian sites, and are necessary for the development of modern astronomical observation techniques like adaptive optics.

The PLATO observatory: robotic astronomy from the Antarctic plateau

AbstractPLATO is a 6 tonne completely self-contained robotic observatory that provides its own heat, electricity, and satellite communications. It was deployed to Dome A in Antarctica in January 2008 by the Chinese expedition team, and is now in its second year of operation. PLATO is operating four 14.5cm optical telescopes with 1k × 1k CCDs, a wide-field sky camera with a 2k × 2k CCD and Sloan g, r, i filters, a fibre-fed spectrograph to measure the UV to near-IR sky spectrum, a 0.2m terahertz telescope, two sonic radars giving 1m resolution data on the boundary layer to a height of 180m, a 15m tower, meteorological sensors, and 8 web cameras. Beginning in 2010/11 PLATO will be upgraded to support a Multi Aperture Scintillation Sensor and three AST3 0.5m schmidt telescopes, with 10k × 10 CCDs and 100TB/annum data requirements.

Thirty Meter Telescope Site Testing V: Seeing and Isoplanatic Angle

ABSTRACTIn this article we present an analysis of the statistical and temporal properties of seeing and isoplanatic angle measurements obtained with combined Differential Image Motion Monitor (DIMM) and Multi-Aperture Scintillation Sensor (MASS) units at the Thirty Meter Telescope (TMT) candidate sites. For each of the five candidate sites we obtained multiyear, high-cadence, high-quality seeing measurements. These data allow for a broad and detailed analysis, giving us a good understanding of the characteristics of each of the sites. The overall seeing statistics for the five candidate sites are presented, broken into total seeing (measured by the DIMM), free-atmosphere seeing and isoplanatic angle (measured by the MASS), and ground-layer seeing (difference between the total and free-atmosphere seeing). We examine the statistical distributions of seeing measurements and investigate annual and nightly behavior. The properties of the seeing measurements are discussed in terms of the geography and meteorological conditions at each site. The temporal variability of the seeing measurements over timescales of minutes to hours is derived for each site. We find that each of the TMT candidate sites has its own strengths and weaknesses when compared against the other candidate sites. The results presented in this article form part of the full set of results that are used for the TMT site-selection process. This is the fifth article in a series discussing the TMT site-testing project.

The Durham/ESO SLODAR optical turbulence profiler

An instrument for monitoring of the vertical profile of atmospheric optical turbulence strength, employing the Slope Detection and Ranging (SLODAR) double star technique applied to a small telescope, has been developed by Durham University and the European South Observatory. The system has been deployed at the Cerro Paranal observatory in Chile for statistical characterization of the site. The instrument is configured to sample the turbulence at altitudes below 1.5 km with a vertical resolution of approximately 170 m. The system also functions as a general-purpose seeing monitor, measuring the integrated optical turbulence strength for the whole atmosphere, and hence the seeing width. We give technical details of the prototype and present data to characterize its performance. Comparisons with contemporaneous measurements from a differential image motion monitor (DIMM) and a multi-aperture scintillation sensor (MASS) are discussed. Statistical results for the optical turbulence profile at the Paranal site are presented. We find that, in the median case, 49 per cent of the total optical turbulence strength is associated with the surface layer (below 100 m), 35 per cent with the ‘free atmosphere’ (above 1500 m) and 16 per cent with the intermediate altitudes (100–1500 m).

High-altitude wind velocity at Oukaimeden observatory

This paper aims at studying the wind at 200 mbar over the Moroccan observatory Oukaimeden, as high-altitude winds have been adopted as a useful parameter for site characterization in terms of the suitability of a site for the development of some adaptive optics techniques. The data used come from the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data base, which is widely acknowledged as being reliable. Statistical analyses of 200-mbar wind speed since 1983 are performed. Comparison with some of the main observatory sites worldwide qualifies Oukaimeden as one of the best observatory sites in terms of 200-mbar wind statistics. Our analysis of a record of seeing measurements during the years 2003 and 2004 concludes that while 200-mbar wind speed can be used as a parameter for ranking astronomical sites in term of their suitability for adaptive optics, it cannot be used for the whole atmospheric seeing prediction. A comparison of monthly values of the seeing parameter at Oukaimeden, La Silla and Paranal demonstrates the high seeing quality of Oukaimeden, as the seeing values measured were lower than those of La Silla and Paranal for most of the time during the comparison period. Furthermore, a statistical analysis of atmospheric stratified seeing, wavefront coherence time and isoplanatic angle measured with a Multi-Aperture Scintillation Sensor instrument over Paranal from 2004 to 2007 have been performed. We found good correlations between 200-mbar wind velocity and levels 4, 5 and 6 seeing, wavefront coherence time and isoplanatic angle, with corresponding correlation coefficients of 0.74, 0.79, 0.70, 0.97 and 0.78.

Measurements of optical turbulence in the free atmosphere above Mount Maidanak in 2005–2007

Results of 2005-2007 campaign of measurement of the optical turbulence vertical distribution above Mt. Maidanak are presented. Measurements are performed with the MASS (Multi-Aperture Scintillation Sensor) device which is widely used in similar studies during last years at several observatories across the world. The data analysis shows that median seeing in free atmosphere (at altitudes above 0.5km) is 0.46 arcsec and median isoplanatic angle is 2.47 arcsec. Given a rather long atmospheric coherence time (about 7 ms when the seeing is good) such conditions are favorable for adaptive optics and interferometry in the visible and near-IR.