Multi Aperture Scintillation Sensor Research Articles

Thirty Meter Telescope Site Testing VII: Turbulence Coherence Time

ABSTRACTAs one of the atmospheric turbulence figures of merit, the Thirty Meter Telescope (TMT) site-testing campaign is aimed at measuring the time constant that dictates adaptive optics observations. We hence present the coherence time statistics of the TMT candidate sites, measured using a Multi Aperture Scintillation sensor (MASS) and a Differential Image Motion Monitor (DIMM). We find, using radiosonde and NCEP reanalysis data, that the MASS underestimates the free atmosphere component of the coherence time by a factor of 1.73. After debias of the MASS data and including the ground layer component, we find median coherence times ranging from 4.2 to 5.6 ms for our five candidate sites. This is the seventh article in a series discussing the TMT site-testing project.

Study on the precision of the multiaperture scintillation sensor turbulence profiler (MASS) employed in the site testing campaign for the Thirty Meter Telescope

The multiaperture scintillation sensor (MASS) has become a device widely employed to measure the altitude distribution of atmospheric turbulence. An empirical study is reported that investigates the dependence of the MASS results on the knowledge of the instrumental parameters. Also, the results of a side-by-side comparison of two MASS instruments are presented, indicating that MASS instruments permit measurements of the integrated seeing to a precision better than 0.05 arc sec and of the individual turbulence layer strength C(n)(2)(h)dh to better than 10(-14) m(1/3).

Accurate seeing measurements with MASS and DIMM

Astronomical seeing is quantified by a single parameter, the turbulence integral, in the framework of the Kolmogorov turbulence model. This parameter can be routinely measured by a Differential Image Motion Monitor (DIMM). A new instrument, the multi‐aperture scintillation sensor (MASS), permits one to measure the seeing in the free atmosphere above ∼0.5 km and, together with a DIMM, to estimate the ground‐layer seeing. The absolute accuracy of both methods is studied here using analytical theory, numerical simulation and experiments. A modification of the MASS data processing to compensate for partially saturated scintillation is developed. We find that the DIMM can be severely biased by optical aberrations (e.g. defocus) and propagation. Seeing measurements with DIMM and MASS can reach absolute accuracy of ∼10 per cent when their biases are carefully controlled. Pushing this limit to 1 per cent appears unrealistic because the seeing itself is just a model‐dependent parameter of a non‐stationary random process.

Optical Turbulence Profiles at Mauna Kea Measured by MASS and SCIDAR

ABSTRACTThe vertical distribution of turbulence over Mauna Kea has been measured on four nights in 2002 October, simultaneously using two different instruments based on stellar scintillation—the generalized SCIDAR (scintillation detection and ranging) and MASS (multiaperture scintillation sensor). The turbulence integrals match within 20%, and the low‐resolution profiles delivered by MASS correctly reveal the localization of the strongest high‐altitude turbulent layers. As deduced from DIMM (differential image motion monitor), MASS, and SCIDAR measurements, optical turbulence in the first 0.7 km above the summit contributed typically half of the total integral, the latter corresponding to a seeing of 0.″5. The ground layer and free atmosphere are not correlated.

Mass seeing measurements from Dome C

The astronomical seeing from the Antarctic high plateau site Dome C has been a matter of speculation for over a decade. During 2004 we made the first wintertime measurements of the seeing using a Multi Aperture Scintillation Sensor (MASS), a device that is well-suited for automated unattended operation. The mean seeing was found to be 0.27 arcsec, and the seeing is better than 0.15 arcsec for 25% of the time. These extraordinarily low levels of turbulence make Dome C a superb site for future optical and infrared telescopes.

Restoration of turbulence profile from scintillation indices

An algorithm that permits one to measure atmospheric turbulence by statistical analysis of light flux fluctuations in four concentric-ring apertures is described in detail. It consists of computing the scintillation indices for each aperture and pairwise aperture combination and in fitting the set of measured indices to a model with a small number of turbulent layers. The performance of this method is analysed by means of simulations and using the real data from a multi-aperture scintillation sensor. It is shown that a turbulence profile with a vertical resolution of Δh/h∼ 0.5 can be reconstructed and that the errors of the measured intensities of turbulent layers are typically around 10 per cent of the integrated intensity. The integral parameters such as the seeing and the isoplanatic angle are measured with few per cent accuracy.