Abstract
We have constructed and operated an automated instrument for measuring ground-level microthermal seeing at the Magdalena Ridge Observatory (MRO). The MRO is located at an altitude of 10500' in the Cibola National Forest in New Mexico, USA. It is the planned site for the MRO Optical Interferometer (MROI) planned for up to 10 collecting elements, each with a diameter of 1.4 m, and baselines eventually up to approximately 400 m. As part of the preparation for construction we deployed a system to characterize the ground-level seeing across the observatory site. The instrument is built largely of off-the-shelf components, with only the sensor head and power supply requiring electronic board assembly. Even in those cases the board architecture is very simple. The first proof-of-concept system was deployed for several weeks in the autumn of 2004, and has since undergone several iterations. The latest configuration operates entirely off batteries, incorporates wireless data acquisition, and is thus able to operate in an area with no shelter, power, or communications. In this paper we present the design of the instrument, and show initial data. The microthermal tower has four sensor pairs at heights from 0.8 to 4.41 m, significantly lower than other microthermal experiments, because of the need to characterize the seeing near the ground. We find significant variation in the contribution of this range of heights to the seeing, contributing up to 03 of the seeing at some times and only 002 at other times. The individual sensor power spectra have a slope in the range of 1.4--1.5, which is lower than the 1.67 slope predicted by Kolmogorov turbulence theory. We measure the well known effect of improved seeing immediately around sunset. While we find significant variation in the microthermal seeing, we did not find a pattern of corresponding variations in weather conditions, suggesting that a complicated set of factors control microthermal turbulence.
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