Differential Image Motion Monitor Research Articles

Scientific potential of the Indo-Belgian 3.6-m DOT in the field of Galactic Astronomy

India and Belgium have jointly established two 4-meter class optical telescopes at Devasthal located in Nainital, India. After successful installation of the 3.6-m modern new technology Devasthal Optical Telescope (DOT) in 2015, it was technically activated by premiers of both countries from Brussels on March 30, 2016. Since then, the 3.6-m DOT has been used for both optical and near-Infrared (NIR) observations for a number of research proposals. The best angular resolution achieved is 0″.4 indicating that the optics of the 3.6-m DOT is good and capable of providing images of celestial bodies with sub-arc-second resolution. The observations provide proof that the care taken in the construction of the telescope dome building has paid a rich dividend as their thermal mass is so low that it has not degraded the natural atmospheric seeing at Devasthal measured about two decades ago during 1997 to 1999 using differential image motion monitor. A few preliminary scientific results obtained from recent observations are presented along with performance and global potential of the 3.6-m DOT in the field of galactic astronomy. The 3.6-m DOT is capable of providing internationally competitive science once high resolution spectrograph and other planned modern back-end instruments become operational. Geographical location of the observatory has global importance for time domain and multi-wavelength astrophysical studies.

Automatic Solar Seeing Observations at Mt. Wumingshan in Western China

Mountain Wumingshan (Mt. WMS) is located in the southeastern foot of the Tibet Plateau with an altitude of 4,800 m. It is one of the candidate sites to place China's next-generation large-scale solar telescope. A temporary observation platform has been built at Mt. WMS, but there is still a great need of a stable solar-seeing monitoring for long-term observations. Based on the preliminary studies on the Solar Differential Image Motion Monitor (SDIMM) of the Yunnan Observatories, we built an improved version of this early prototype, which is called Wumingshan Mountain Automated Solar Differential Image Motion Monitor (WMA-SDIMM). We develop the automatic system of the WMA-SDIMM, investigate the reliability and precision of our measurements by error analysis and comparison testing, and present the statistical results from October 2016 to September 2017 at Mt. WMS. WMA-SDIMM works very well at Mt. WMS and is quite suitable for long-period daytime seeing observations.

RETRACTED ARTICLE: Characterization of atmospheric optical turbulence by (GDIMM) telescope

In this paper, we present the measurements that have been made to determine some of the parameters governing the transmission of a light wave through the atmosphere. Three parameters are studied: the parameter of Fried r0, the angle of isoplanatic θ0 and the external scale l0. The data for this purpose have acquired by a generalized differential image motion monitor. These data are used to measure the position and scintillation of a star on two windows. The device is composed of 3 sub-apertures instead of 2 two pupils of a diameter d = 6 cm, detached by a distance x0 = 20 cm horizontally. The difference between the variations of the position of the star in each window makes it possible to determine r0 and l0, while the variation in the window (3) makes it possible to obtain the scintillation used to calculate θ0. The data used dated from the night of October 24, 2016. A series of measures were performed at the Calern plateau in Nice–France. The series between 5:55 and 9:01 pm, with two focal lengths F = 6.11 m, the star have been observed is Vega. The measured wavelength was taken at 0.5 μm.

Satellite-to-ground optical communications using small optical transponder (SOTA) – received-power fluctuations

In this paper, experimental results related to the received-power fluctuations in a LEO satellite-to-ground laser-communication link using the Small Optical TrAnsponder (SOTA) are presented. The data is compared to the theoretical expectations based on the Hufnagel-Valley model. The discrepancies in the results are discussed by spectrum analysis and by a comparison with the Differential Image Motion Monitor (DIMM) measurements of the Fried parameter. Finally, the experimentally-derived coherence time is shown and discussed.

Theoretical and experimental study of atmospheric turbulence measurement using two-aperture differential scintillation method

We report the basic theory and first horizontal results of a method called two-aperture differential scintillation method which is aimed at monitoring the vertical profile of atmospheric optical turbulence strength. The method is based on irradiance fluctuation of active light source, but can extract the optical turbulence information in the single-passage path. In this paper, the theoretical principle of two-aperture differential scintillation method is derived in detail. A concise expression is proposed for irradiance fluctuation structure function with differential aperture in the Rytov approximation under a weak fluctuation regime based on the cross-path theory. The mathematic relationship between irradiance fluctuation structure function and atmospheric optical turbulence strength is then developed. The effects of beacon aperture and beacon altitude on path weighting function of this method are analyzed for Kolmogorov turbulence. In order to test the validity of the new method, the experiments are conducted to compare the two-aperture differential scintillation method and single-aperture scintillation method in atmospheric boundary layer over 2 km horizontal single-passage path. In this arrangement, we employ a differential image motion monitor system to measure differential scintillation. Simultaneously, a large aperture scintillation instrument is placed 5 m away at the same altitude to measure the single-aperture scintillation. It is shown that the results of atmospheric refractive index structure constant deduced from the two methods are in good agreement. The measurements of atmospheric coherence length for spherical wave corresponding to the two methods indicate a linear correction factor (R2) of 0.96, in a slope of 0.98 with an offset of -0.09 cm. Feasibility and effectiveness of two-aperture differential scintillation method are thus verified experimentally. The novel method can separate single-passage scintillation information of active beacon double-passage propagation, thereby providing an accurate technique for measuring the atmospheric turbulence of active beacon.

Astronomical Observing Conditions at Xinglong Observatory from 2007 to 2014

Xinglong Observatory of the National Astronomical Observatories, Chinese Academy of Sciences (NAOC), is one of the major optical observatories in China, which hosts nine optical telescopes including the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) and the 2.16 m reflector. Scientific research from these telescopes is focused on stars, galaxies, and exoplanets using multicolor photometry and spectroscopic observations. Therefore, it is important to provide the observing conditions of the site, in detail, to the astronomers for an efficient use of these facilities. In this article, we present the characterization of observing conditions at Xinglong Observatory based on the monitoring of meteorology, seeing and sky brightness during the period from 2007 to 2014. Meteorological data were collected from a commercial Automatic Weather Station (AWS), calibrated by China Meteorological Administration. Mean and median wind speed are almost constant during the period analyzed and ranged from 1.0 to 3.5 m s-1. However, high wind speed (≥15 m s-1) interrupts observations, mainly, during the winter and spring. Statistical analysis of air temperature showed the temperature difference between daytime and nighttime, which can be solved by opening the ventilation device and the slit of the dome at least 1 hr before observations. Analysis resulted in average percentage of photometric nights and spectroscopic nights are 32% and 63% per year, respectively. The distribution of photometric nights and spectroscopic nights has a significant seasonal tendency, worse in summer due to clouds, dust, and high humidity. Seeing measurements were obtained using the Differential Image Motion Monitor (DIMM). Mean and median values of seeing over 1 year are around 1.9″ and 1.7″, respectively. Eighty percent of nights with seeing values are below 2.6″, whereas the distribution peaks around 1.8″. The measurements of sky brightness are acquired from the Sky Quality Meter (SQM) and photometric observations. Analysis shows that sky brightness at the zenith is around 21.1 mag arcsec-2 and becomes brighter with a larger zenith angle. Sky brightness increases due to the light pollution of the surrounding cities, Beijing, Tangshan, and Chengde. Significant influence toward the direction of Beijing, at an altitude of 30°, can increase the sky brightness up to 20.0 mag arcsec-2. Sky brightness reduces after midnight, mainly because of the influence of city lights and the artificial acts. The above results suggest that Xinglong Observatory is still a good site for astronomical observations. Our analysis of the observing conditions at Xinglong Observatory can be used as a reference to the observers on targets selection, observing strategy, and telescope operation.

Optical turbulence characterization at LAMOST site: observations and models

Atmospheric optical turbulence seriously limits the performance of high angular resolution instruments. An 8-night campaign of measurements was carried out at the LAMOST site in 2011, to characterize the optical turbulence. Two instruments were set up during the campaign: a Differential Image Motion Monitor (DIMM) used to measure the total atmospheric seeing, and a Single Star Scidar (SSS) to measure the vertical profiles of the turbulence C_n^2(h) and the horizontal wind velocity V(h). The optical turbulence parameters are also calculated with the Weather Research and Forecasting (WRF) model coupled with the Trinquet-Vernin model, which describes optical effects of atmospheric turbulence by using the local meteorological parameters. This paper presents assessment of the optical parameters involved in high angular resolution astronomy. Its includes seeing, isoplanatic angle, coherence time, coherence etendue, vertical profiles of optical turbulence intensity _n^2(h)$ and horizontal wind speed V(h). The median seeing is respectively 1.01 arcsec, 1.17 arcsec and 1.07arcsec as measured with the DIMM, the SSS and predicted with WRF model. The history of seeing measurements at the LAMOST site are reviewed, and the turbulence measurements in this campaign are compared with other astronomical observatories in the world.

Towards a fully automatic and robust DIMM (DIMMA)

Quantitative seeing measurements have been provided at the Canarian Observatories since 1990 by differential image motion monitors (DIMMs). Image quality needs to be studied in long term (routine) measurements. This is important, for instance, in deciding on the siting of large telescopes or in the development of adaptive optics programmes, not to mention the development and design of new instruments. On the other hand, the continuous real time monitoring is essential in the day-to-day operation of telescopes.These routine measurements have to be carried out by standard, easy-to-operate and cross- calibrated instruments that required to be be operational with minimum intervention over many years. The DIMMA (Automatic Differential Image Motion Monitor) is the next step, a fully automated seeing monitor that is capable of providing data without manual operation and in remote locations. Currently, the IAC has two DIMMs working at Roque de los Muchachos Observatory (ORM) and Teide Observatory (OT). They are robotic and require an operator to start and initialize the program, focus the telescope, change the star when needed and turn off at the end of the night, all of which is done remotely. With a view to automation, we have designed a code for monitoring image quality (avoiding spurious data) and a program for autofocus, which is presented here. The data quality control protocol is also given.

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.

差分图像运动监测仪的数值模拟与对比实验研究

差分图像运动监测仪的数值模拟与对比实验研究

Medidas de calidad de cielo (seeing) usando la técnica DIMM en el Observatorio de Huancayo

Mostramos la implementación de un instrumento de medición de la calidad de cielo (seeing) basado en la técnica del Monitoreo del Movimiento Diferencial de Imagen (DIMM). Este instrumento permite la medición precisa y sistemática del seeing, así como su evolución temporal. Fue monitoreado el seeing en el Observatorio de Huancayo durante siete noches consecutivas de Agosto de 2012. El seeing promedio fue de 1.34” ± 0 .33”.En 14% del tiempo observado el seeing estuvo por debajo de 1”, y en 71% del tiempo, por debajo de 1.5”. El dominio de la técnica y su aplicabilidad nos permitirá usar el instrumento en varios lugares donde campañas de medición de seeing sean programadas dentro del Perú. La selección de una facilidad astronómica óptica profesional (observatorio astronómico), será favorecida con medidas DIMM futuras.

Laser differential image-motion monitor for characterization of turbulence during free-space optical communication tests

A laser differential image-motion monitor (DIMM) system was designed and constructed as part of a turbulence characterization suite during the DARPA free-space optical experimental network experiment (FOENEX) program. The developed link measurement system measures the atmospheric coherence length (r0), atmospheric scintillation, and power in the bucket for the 1550 nm band. DIMM measurements are made with two separate apertures coupled to a single InGaAs camera. The angle of arrival (AoA) for the wavefront at each aperture can be calculated based on focal spot movements imaged by the camera. By utilizing a single camera for the simultaneous measurement of the focal spots, the correlation of the variance in the AoA allows a straightforward computation of r0 as in traditional DIMM systems. Standard measurements of scintillation and power in the bucket are made with the same apertures by redirecting a percentage of the incoming signals to InGaAs detectors integrated with logarithmic amplifiers for high sensitivity and high dynamic range. By leveraging two, small apertures, the instrument forms a small size and weight configuration for mounting to actively tracking laser communication terminals for characterizing link performance.

Experimental measurement of the atmospheric primary aberrations by a four-aperture differential image motion monitor

In this paper, the ability of the four-aperture differential image motion monitor (DIMM) to measure the atmospheric primary aberrations is verified experimentally as in agreement with the simulation results. It is shown that the four-aperture DIMM is able to measure the defocus term of atmospheric aberration with reasonable accuracy while the astigmatism term is less accurate and the coma term is not reliable.

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.

Development of a differential column image motion light detection and ranging for measuring turbulence profiles

We have developed a differential column image motion (DCIM) lidar for measuring real-time vertical profiles of Fried's transverse coherence length (r0) and testing it against a differential image motion (DIM) lidar and a DIM monitor by observing stars throughout a range of turbulent conditions. With the DCIM lidar system parameters elaborately designed and the detector installed with an angle corresponding to the receiving telescope axis, the focal position of the laser guide star from a range of altitudes that coincide with the CCD's receiving area, r0 of different altitudes can be obtained simultaneously. The experiment results support the DCIM lidar and confirm that a high temporal and spatial resolution r0 profile can be measured with this method.

Method of Modeling Atmospheric Optical Turbulence

The intensity and distribution of optical turbulence are important parameters to evaluate an observing station. A modeling method to obtain n atmospheric optical turbulence parameters, including the C2n profile, coherent length, coherent time, seeing, isoplanatic angle, and scintillation index by using meteorological parameters are presented. The accomplishment of this method, the configuration of the WRF model (Weather Research and Forecasting Model), and the verification of calculated results are introduced. The reliability of the method is confirmed preliminarily by comparing the calculated results with the DIMM (Differential Image Motion Monitor) measurements.

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.

Excellent daytime seeing at Dome Fuji on the Antarctic plateau

Context. Dome Fuji, the second highest region on the Antarctic plateau, is expected to have some of the best astronomical seeing on Earth. However, site testing at Dome Fuji is still in its very early stages. Aims. To investigate the astronomical seeing in the free atmosphere above Dome Fuji, and to determine the height of the surface boundary layer. Methods. A Differential Image Motion Monitor was used to measure the seeing in the visible (472 nm) at a height of 11 m above the snow surface at Dome Fuji during the austral summer of 2012/2013. Results. Seeing below 0.2'' has been observed. The seeing often has a local minimum of ~0.3'' near 18 h local time. Some periods of excellent seeing, 0.3'' or smaller, were also observed, sometimes extending for several hours at local midnight. The median seeing is higher, at 0.52''---this large value is believed to be caused by periods when the telescope was within the turbulent boundary layer. Conclusions. The diurnal variation of the daytime seeing at Dome Fuji is similar to that reported for Dome C, and the height of the surface boundary layer is consistent with previous simulations for Dome Fuji. The free atmosphere seeing is ~0.2'', and the height of the surface boundary layer can be as low as ~11 m.