LIDAR for measuring atmospheric extinction

Abstract

The Georgia Tech Research Institute and the University of New Mexico are developing a compact, rugged, eye safe lidar (laser radar) to be used specifically for measuring atmospheric extinction in support of the second generation of the CCD/Transit Instrument (CTI-II). The CTI-II is a 1.8 meter telescope that will be used to accomplish a precise timedomain imaging photometric and astrometric survey at the McDonald Observatory in West Texas. The supporting lidar will enable more precise photometry by providing real-time measurements of the amount of atmospheric extinction as well as its cause, i.e. low-lying aerosols, dust or smoke in the free troposphere, or high cirrus. The goal of this project is to develop reliable, cost-effective lidar technology for any observatory. The lidar data can be used to efficiently allocate observatory time and to provide greater integrity for ground-based data. The design is described in this paper along with estimates of the lidar's performance. RESEARCH INSTRUMENTATION AND NEEDSAstronomers using ground-based telescopes are necessarily engaged in negating the effects of the earth's atmosphere. The atmosphere is, in fact, a wavelength dependent, spatially and temporally non-stationary refractive optical element in any ground based telescope system. The atmosphere creates two deleterious physical observational effects: it absorbs and scatters light in a wavelength dependent manner, and it blurs images by turbulence-induced refractive wave front perturbations. The latter effect is being addressed by adaptive optics; here we are addressing the former.A general, all-too-common scenario is that an observer makes an assessment of sky conditions based upon current state of the art techniques, such as an all-sky camera, or weather satellite imagery. Astronomical data are obtained, and when reduced by the astronomer, an unacceptably large uncertainty in, say, the spectral index or photometric calibration is encountered. The data are never used for their intended purpose. On the other hand, the telescope might have been usefully scheduled for another program where the measured atmospheric effects would have no significant impact. Decision making like this optimizes the use of expensive telescope time, resulting in cost effective astronomy.While astronomical sites incorporate ingenious techniques for estimating atmospheric extinction, such as dedicated photometric telescopes and radiometers, none of these observatories directly and continuously measures the atmospheric transparency by measuring the atmosphere itself. Often the transparency estimates are provided to astronomers by the simple expedient of looking at a wide field image of the sky from which extinction is estimated. This is but one electronic step removed from the age-old tradition of actually looking at the night sky with the naked eye. These are typically "10% estimators" that astronomers often wish to apply to "sub-1% measurements." Extinction in the atmosphere is due to both gasses and particulate matter. The gases that make u...

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