Rosemont mine lighting plan seeks to minimize impact on night sky

By Patrick McNamara | Posted: Friday, March 1, 2013 8:00 am

Among the concerns over the proposed Rosemont Copper mine in the Santa Rita Mountains has been the effects of lights. Once opened, the mine would operate around the clock, meaning artificial lighting and what it would do to the night sky and astronomy interests.

To minimize the impact, Rosemont Copper commissioned an analysis of ways to limit the amount of light that would escape from its facility.

“In wanting to be a good neighbor, we wanted to do these mitigations,” said Fermin Samorano, mine manager with Rosemont Copper.

Last week, mine officials and consultants invited members of the regional astronomy community to visit the site and see mockups of a lighting system that would be used once copper extraction starts.

The proposed mine would be less than 50 miles from Kitt Peak National Observatory in the Quinlan Mountains southwest of Tucson. Even closer is the Smithsonian Astrophysical Observatory’s Fred Lawrence Whipple Observatory on Mount Hopkins on the other side of the Santa Rita Mountains ridgeline.

The lighting plan that Rosemont has proposed is expected to reduce the amount of lumens from the mine from an estimated 21 million to 6 million. A lumen is a measure of visible light based on candelas or candlepower. Roughly speaking, one candela equals the light intensity of one candle.

Because of the amount of light needed to operate the mine safely at night, there have been concerns among professional and amateur astronomers that the project would allow too much light to escape brightening the night sky.

To mitigate the impact, Rosemont Copper hired Tucson-based Monrad Engineering Inc. to come up with a plan.

“Given its proximity to the Smithsonian observatory complex, we’re trying to eliminate as much stray light as possible,” said Chris Monrad, principal and vice president of Monrad Engineering.

He said the effects of light pollution usually are seen in three ways:

• Spill light that crosses over a property line.

• Direct visible glare that causes a visible glare and skyglow.

• The cumulated effects of escaped lighting from an urban setting that makes the night sky appear to glow orange and red.

Among strategies that Monrad developed was to have fully shielded and aimed lighting fixtures to minimize the amount of light that escapes or is reflected from the mine into the atmosphere.

The plan also calls for the use of light-emitting diodes, or LED lighting, which uses less power to produce more light. LED lighting also can be aimed and concentrated more efficiently to minimize the amount of light that escapes into the atmosphere.

Most importantly, at least to astronomers, is the color of the lights the mine plans to use.

“Anything you can do to curtail light on the blue end of the spectrum minimizes the amount of light scattered into the atmosphere,” Monrad said.

The lighting plan Monrad developed limits the amounts of cool colors – blues, greens and purples — in favor of colors on the amber spectrum, which have less impact on the night sky.

“That’s really critical to Southern Arizona astronomy,” said Richard Green, assistant director of government relations with the University of Arizona’s Steward Observatory.

Green, who was among stakeholders touring the mine site last week, said the approach to lighting Monrad Engineering devised was the best the astronomical community could hope for.

“I think this would be a pioneering effort,” he said.

Another stakeholder on the tour, Scott Kardel, managing director of the International Dark-Sky Association, said the plan Monrad proposed appeared to be a good compromise.

“I think the Monrad plan has a much lower impact than what was proposed originally,” Kardel said, referring to Rosemont Copper’s original proposal for lighting.

Kardel noted that the mine still would significantly illuminate an area that traditionally has been dark.

The astronomical sciences have a significant impact on the state’s economy. Industry figures estimate that more than $1 billion has been invested in infrastructure and equipment. The total estimated annual economic impact to the state was estimated at $250 million in 2008.

Rosemont’s Samorano said the efforts to create a lighting plan that minimize the impacts on the scientific community fit with his company’s overall philosophy.

“It’s part of putting together a modern mine,” he said.

Contact reporter Patrick McNamara at pmcnamara@azbiz.com or (520) 295-4259.