■ The mine would brighten dark skies over most of the Santa Ritas, but not as much as originally thought. A lighting reduction plan would use LED lights with spectral controls to minimize lighting and shield mobile lights to minimize their upward flow.
■ Sky brightness would increase 3.4 percent at a 20 degree angle above the horizon – the lowest useful area for astronomical observations. The mine has also agreed to a monitoring plan at $218,000 to start and $100,000 a year.
■ The mine will generate about three-quarters the light that glows from tiny Benson, down from the original projection of two-thirds as much light as much larger Nogales, Ariz.
■ Astronomers at the Whipple Observatory on Mount Hopkins, run by the Smithsonian Astrophysical Laboratory, say the new lighting plan is better, but they say the monitoring plan is inadequate. “The negative public perception of having a copper mine next to an observatory has already impacted future observatory revenues,” the Smithsonian wrote. One example, it says, was its failure to land the $700 million giant Magellan Telescope.