Mini-workshop objectives: The goal of this mini-workshop is to define the critical atmospheric parameters required for an optimum design of future AO-MCAO systems for 8 to 100m telescopes and to identify existing and potential instruments and facilities to measure in-situ these parameters.

8:45 Welcome

(Alphabetic order)

D.Coburn, D.Garnier, J.C.Dainty. National University of Ireland, Galway.

We are currently working on the development of a generalized SCIDAR system for characterizing atmospheric parameters using single star targets. The instrument, which is based on a commercially available 25cm diameter telescope, offers the potential for characterizing atmospheric parameters for wide areas of the sky. Here, we describe the system and outline the challenges in the data reduction and in solving the inverse problem needed to estimate the altitude dependence of refractive index structure constant. With the system development well underway we hope to test the instrument some time between November 2003 and March 2004.

J.-M. Conan*, T. Fusco*, R. Conan**, G. Rousset, *Onera, Chatillon, France, ** LAOG, Grenoble, France.

We propose a list of key atmospheric parameters for the future AO developments. Their relevance for the different class of AO instruments and telescopes is justified.

We discuss several ways of measuring these parameters, mainly through a statistical processing of data provided by existing or planned instruments (ASM, GSM, MASS, scidar, NAOS, MACAO, MAD, VLTI...). We then propose a multi-instrument campaign, for instance at Paranal, to improve our knowledge of the atmospheric models and parameters. The use of different instruments will limit misinterpretations, and will give simultaneous spatial and temporal information at various scales. The results of such a campaign would help defining a strategy for future site testing. They are also of great interest for the design of the new generation AO systems.

Di Folco E., Koehler B., Kervella P., Sarazin M., Schoeller M., Glindemann A.

Infrared interferometric observations on long baselines are strongly affected by the atmosphere. While the sensitivity of the fiber-fed near-IR instruments is limited by the atmospheric seeing, the precision of the visibility measurements is degraded by the differential piston. These effects can be compensated for with adaptive optics and fringe tracking facilities, but their performance still depends on the atmospheric characteristics (seeing, coherence time, outer scale length). We have used the commissioning instrument of the VLTI, VINCI, with two test-siderostats on various baselines (up to 140m in length) in order to estimate the interferometric coherence time from the temporal spectra of the observed differential piston. It proved to be correlated with the independent estimation provided by the DIMM monitor. In addition, we have obtained the first estimation of the outer scale measured at Cerro Paranal. We will report on the first measurement campaign at the VLTI and will describe plans for further measurements of the relevant atmospheric parameters with the VLTI.

Brent Ellerbroek, AURA New Initatives Office

The spatial cross-correlations between the measurements from distinct wavefront sensors in an MCAO system may be used to determine weighted integrals of the atmospheric Cn2(h) profile. We derive the functional form of these weighting functions and present sample numerical values for the Gemini-South MCAO configuration, in which the guide stars are separated by 0.7 to 1.4 arc minutes and the subaperture size is 0.5 meters. Implementation issues (noise, closed-loop operation, data rates, and LGS tilt uncertainty) are briefly discussed.

S.Esposito, INAF-Arcetri

A device to measure the cn2 profile of the atmosphere is presented. We tried to design a relatively simple unit that can give information on the vertical distribution of the atmosphere. The proposed method could be used in some different configuration to measure the turbulence on spatial scale larger than 10m.

N.Hubin, ESO

We will present the status of the investigated AO-MCAO concepts and characteristics for ELTs. We will underline whenever possible the open questions related to the knowledge of the key atmospheric parameters.

M. Le Louarn, ESO

In this presentation, I will show some examples on how atmospheric parameters influence the design of AO systems. For example, the MUSE AO system, will aim at correcting the ground layer of turbulence in the visible to get an "improved seeing" over a 1’ FOV. It will be shown how the Cn2 distribution is critical. Also the influences of tau0 and r0 on the design are demonstrated by the use of current Paranal statistics. I will also briefly show what MCAO systems might need in terms of information on the atmospheric parameters for the control (regularization) and operation (DM conjugation heights). Some simulations on the effects of AO performance as a function of telescope diameter will show the effect of the outer scale of turbulence.

A. Quirrenbach, Leiden Observatory

R. Ragazzoni

I outline a couple of techniques to perform wavefront sensing measurement on specific layers or volumes of atmosphere that, with existing telescopic facilities, can achieve direct wavefront sensing on scales of the order of 100m.

These techniques are maybe, representative of a class of techniques and do not want to mean they are the only ways to achieve such a goal. Further to outline the used approaches and some initial calculation and simulations about their feasibility, I would like to summarize the outcome of such a measurement.

It is clear that 100m-scale WF sensing cannot be achieved routinely, on several sites, and hence is not strictly aimed to compete with any site-testing campaign. However it can provide unique information on parameters like the spectrum of the turbulence, the conformity of the atmosphere to the Taylor hypothesis, the maximum stroke required to DMs, the coherence of wind flow in such large footprints.

M. Sarazin

The AO relevant parameters routinely monitored at Paranal and La Silla observatories are reviewed and their statistics is updated.

Then follows a short non-exhaustive review of the to-date accumulated information about the outer scale in various sites and using various instruments and methods, including some information collected from VLT science operation data. To conclude, highlights of what remains to be answered about its vertical structure in particular.

The CELT/GSMT site testing program (talk cancelled)

Matthias Schoeck, CELT, Caltech, Pasadena, CA, USA

Last year, after a previous period of informal collaboration, the California Extremely Large Telescope (CELT) and AURA's Giant Segmented Mirror Telescope (GSMT) started a formal collaboration with respect to site testing for their 30-m telescope projects. I briefly describe the current state of and the plans for the site characterization effort. I will concentrate on the on-site measurements, which are centered around a DIMM/MASS system, but will also include other instruments such as SODARs and microthermal probes.

A short introduction to the principle of multi-aperture scintillation sensor (MASS) and its combination with DIMM is given. This instrument measures seeing, free-atmosphere seeing, isoplanatic angle, AO time constant, and low-resolution turbulence profile. Characteristics of MASS and results of its inter-comparisons with other instruments are reviewed. Properties of turbulence relevant to AO operation that were revealed by MASS are listed, and the use of MASS data for predicting the performance of ground-layer AO is presented. Plans for use of MASS-DIMM in site testing and monitoring are outlined. Mass database is: mass.ctio.noao.edu

J. Vernin, LUAN, Nice University

We will present a definition of the outer scale of optical turbulence which explains the large discrepancy between our definition (From Tatarski Lo ~ 1m) and the coherence outer scale (From von Karman Lo ~ 20m). We will summarize the instruments able to measure the above mentioned atmospheric parameters. We will present the new concept of Single Star Scidar and recall an old paper (Coulman, Vernin 1991) in which the phase structure function is expected to rise again, after a saturation region.

Richard Wilson

The SLODAR method for profiling of the atmospheric optical turbulence strength with altitude and velocity will be reviewed. Example and statistical results from recent observations at the WHT and Mercator telescopes will be presented, as well as details of the instrumentation deployed. The design and predicted performance of a proposed portable SLODAR instrument for site characterization will be discussed.