Methods and instruments to measure/monitor major site properties
The VLT Dealing With The Atmosphere, A Night Operation point of view
October 4, 09:00 – 09:40Julio Navarrete (ESO)
The Science Operation Department is composed of Astronomers, Telescope Instruments Operators(TIOs) and Data Handling Administrators(DHAs). Their main goal is to produce top-quality astronomical data by operating a suite of 9 telescopes, 14 instruments and related systems, supporting the execution of Visitor Mode Observations or executing Service Mode Observations. Astronomers and TIOs have to deal with atmospheric parameters like seeing, coherence time, isoplanatic angle, precipitable water vapor, etc. in order to take in real time the best decisions on the best programs to be executed according to the current conditions. We describe the tools available in the control room provided by the environmental monitoring and forecast systems.
Measurement of the Atmospheric Primary Aberrations by 4-Apertures HARTMANN
October 4, 09:40 – 10:20Ramin Shomali (ZNU), Davoud Afshari (ZNU), Sadollah Nasiri (ZNU), Ahmad Darudi (ZNU)
Atmospheric turbulence is the most important limitation in the astronomical high resolution imaging. Astronomers have proposed several methods to quantitatively measure the amount of turbulence above the telescope, which the most common ones is Differential Image Motion Monitor (DIMM).
In optical testing, the Hartmann test is the common method to test the quality of optical components. The simplest Hartmann test can be performed by a Hartmann screen with four holes near the screenòÀÙs edge, in front of optical component for measurement of some primary aberrations. DIMM is a like Hartmann test with two apertures and it canmeasure tilts terms, but with four apertures DIMM, we could calculate 8 terms of zernike aberrations.
In this paper, we present modification to the DIMM method data analysis and in addition to r0 , we determine 5 primary aberration of atmosphere which contain Defocusing, Astigmatism with axis at 0 or 90, Astigmatism with axis at ˆ‘45 , Coma along the y-axis and Coma along the x-axis by the DIMM system contain 4 apertures.
This paper is organized as follows: section 2 introduces the theory of Hartmann test using a screen with four holes and its application at law order aberration calculation. In section 3 we describe the computer simulation of Four Aperture DIMM and then we discuss the ability of this instrument for atmospheric primary aberrations measurement, section 4 describes the Four Apertures DIMM layout and data acquisition method. Data processing, monitoring results and error analysis are presented in section 5. Finally, we conclude in section 6.
Dynamic Shack-Hartman System "RAPID"
October 4, 10:20 – 11:00Valeri Orlov (UNAM), Valerii Votsekhovich (UNAM)
Rapid allows for the measuring of the spatial-temporal structure of turbulence-induced phase distortions at the telescope aperture. The phase distortion at the telescope aperture is mapped at the Hartmann mask. Hartmann mask consists of 48 lenses. Each of mask lenses produces a Hartmann spot at the mask focal plane. The Hartmann picture is projected to the entrance of the light intensifier which amplifies the picture intensity and passes it to the entrance of the CCD camera. The CCD-camera is a high-speed camera allowing for the frame recording with the speed of 25, 50, 100, 200 frames/sec. Because instead of the whole Hartmann picture keeping only the spot offsets are recorded, the equipment allows accumulating long data sets (up to 40000 frames).
Coffee break
October 4, 11:00 – 11:30Cn2 profile measurement from Shack-Hartmann data
October 4, 11:30 – 12:10Laurent Mugnier (ONERA), Clelia Robert (ONERA), Nicolas V†édrenne (ONERA), Juliette Voyez (ONERA)
Cn2 profile monitoring usually makes use of wavefront slope correlations or of scintillation pattern correlations. Scintillation is rather sensitive to high turbulence layers whereas wavefront slope correlations are mainly due to layers close to the receiving plane. Wavefront slope and scintillation correlations are therefore complementary. A maximum-likelihood method is developed to estimate precisely the slopes and flux corresponding to each Shack-Hartmann wavefront sensor (SHWFS) spot. This method can take into account the undersampling of the SHWFS as well as the statistics of the measurement noise. Slopes and scintillation being recorded simultaneously with a SHWFS, we here exploit their correlations in order to retrieve the Cn2 profile using the method named COupled SLodar scIDAR (CO-SLIDAR, Vedrenne 07). Cn2 profiles estimated with CO-SLIDAR are then presented, both on simulated data and on experimental data from a binary star.
Surface-layer turbulence measurement with lunar scintillometers
October 4, 12:10 – 12:50Andrei Tokovinin (NOAO)
Turbulence profiles in the first few hundred meters above ground have been measured with the lunar scintillometer, LuSci, at a number of sites. Typically, they show $C_n^2(h)$ decline proportional to 1/h and $C_n^2(30m) \sim 10^{-15} m^{-2/3}$. Such turbulence is too weak to explain strong ground-layer seeing measured by site monitors. We infer that monitors may introduce local man-made turbulence biasing their data, especially under excellent conditions. If the seeing can be better than we think it is, there is further room for telescope optimization to gain in resolution. Wide-field ground-layer AO is envisioned.
