The Advanced Technology Solar Telescope site survey Sky Brightness Monitor

The Advanced Technology Solar Telescope (ATST) will be a 4 m aperture off-axis telescope with advanced high-resolution and low scattered light capabilities for the observation of the solar photosphere and corona. The site characteristics that are critical to the success of the ATST coronal observations are the sky brightness, the precipitable water vapor content, and the number and size distributions of the dust particles. Therefore, part of the ATST site survey effort is to obtain measurements of these atmospheric properties at all the potential ATST sites. The ATST site survey Sky Brightness Monitor (SBM) is a new instrument specifically developed for this task. The SBM is a modified externally occulted coronagraph capable of imaging the solar disk and sky simultaneously. The ability to image the Sun and the sky simultaneously greatly simplifies the calibration of the sky-brightness measurements. The SBM has a very simple optical configuration that makes it a compact and low-maintenance instrument. The SBM is sensitive to sky brightness below 1 x 10(-6) disk center intensity, with a field of view extending from 4 to 8 R-.. It measures the solar disk and sky brightness at three continuum bandpasses located at 450, 530, and 890 nm. A fourth bandpass is centered at the 940 nm water vapor absorption band. With measurements of disk and sky brightness at these four wavelengths, site characteristics such as extinctions, aerosol content, and precipitable water vapor content can be derived. This paper documents the design, specifications, calibration procedures, and performance of the SBM.