APS News

Air Force Restrictions Impact Adaptive Optics

By Michael Lucibella

Restrictions imposed by the Air Force on the use of lasers are significantly diminishing the utility of adaptive optics for studying the cosmos, according to a number of astronomers.

“At one time, four or five years ago, we were getting very few restrictions, but more recently that has increased,” said Julian Christou, the adaptive optics technician for the Gemini North Observatory in Hawaii. “The impact is we are losing time to do long integrations…It’s an accumulated time loss.”

Laser guide star adaptive optics involves shining powerful lasers into the lower atmosphere to correct for atmospheric distortions. The most common type refracts an orange beam off a 90 kilometer-high layer of sodium atoms to create a reference point in the sky. By tracking this reference point, known as an artificial guide star, astronomers can cancel out much of the atmospheric interference. Currently four telescopes in the US use these lasers, with more in development.

Air Force Space Command regulates their use to protect passing satellites. All uses of the lasers must be approved days ahead of time by the Laser Clearing House at Vandenberg Air Force base to prevent the beam from crossing paths with an approaching spacecraft.  The lasers cannot damage a craft’s hull, but they could potentially burn out sensitive optical equipment.

Astronomers submit a request to the Laser Clearing House two to three days before their observations, stating when and where they plan to aim the laser. The SPIRAL-3 program at the clearing house tracks the orbits of satellites and calculates “shutter times,” periods when the observatory must switch off the laser to avoid passing satellites. With these safety measures in place, there have been no known instances of a laser beam harming an orbiting satellite.

The Federal Aviation Administration also regulates the laser’s usage. It requires that spotters outside the observatory keep watch and shut off the laser if any aircraft nears the beam.

A study prepared last year by the Association of Universities for Research in Astronomy said that newly expanded restrictions by Space Command have had “a very significant impact on science operations” at Gemini North Observatory in Hawaii. The report, “AURA’s Assessment of Adaptive Optics” said that newly broadened zones of avoidance around satellites have caused more interruptions.

“The significant negative impacts of these new restrictions on scientific productivity are being felt now,” the report read, “Only 50-63% of science targets have suitable clearance windows on a given night.”

Craig Foltz, acting division director of astronomical sciences at the National Science Foundation, said he is aware of more instances where the guide star’s use was limited.

“Towards the end of 2007, the Laser Clearing House list of restrictions seemed to get a bit tighter,” Foltz said.

These shutter times vary nightly depending on the flight paths of passing satellites. Usually lasers need to be shuttered for five to fifteen seconds at a stretch. When observatories are studying areas of the sky near the slower-moving geosynchronous satellite band, blackout periods could last up to several minutes.

“The percentage of lost time to Space Command is around two percent,” said Randy Campbell, an astronomer at the Keck Telescope, adding that poor weather is a more serious concern, as it can cut observations by a third. He said also that overall the restrictions have had only a small impact on operations at Keck.

The net amount of time lost to shutter times is usually only a few minutes, but these interruptions can interfere with sensitive observations. “A three second interruption could mean interrupting a three hour long exposure,” said Antonin Bouchez, the adaptive optics head at the Palomar telescope.

Though there are no definitive studies on the dangers posed by these lasers, most laser operators agree that the odds of a laser damaging a satellite’s instruments are small. Satellites in low Earth orbit travel at average speeds around 17,000 miles per hour and would cross a beam for only a few milliseconds. If the two were to cross, the satellite would likely need to have its optical scanner aimed directly at the beam to cause any damage. Generally satellites observing Earth at night record infrared wavelengths outside the narrow visual spectrum of the lasers.

The Air Force did not respond to submitted questions about their policies before press time.

The long approval process has limited astronomers’ ability to respond to unexpected events. Phenomena that happen without warning, such as supernovae or gamma ray bursts, can be over by the time the LGS is approved.

“As it becomes more routine, and especially with this rapid response science where a lot of astronomers want to look at things that didn’t exist yesterday, it becomes a bigger issue,” said Bouchez.

Space Command on occasion transmits last minute orders to shutter beams. These unanticipated shutter times can be the most disruptive, as they come unexpectedly and usually last much longer than the predetermined times. Campbell estimated that the Keck telescope gets calls to switch off their beam for 30 minutes to a few hours on five to ten percent of observing nights.

Observatories have adapted to the restrictions by scheduling exposures around the interruptions. Additionally some observatories like Keck request that anyone using the telescope on a given night have a backup plan that doesn’t require a laser.   

“We’ll just power through some of these shorter closures,” Campbell said, “We just roll with the punches.”

There is no formal law requiring compliance, but all US and US-supported observatories voluntarily abide by the Air Force’s restrictions. The National Science Foundation does explicitly require any telescope using its funds to follow Air Force guidelines. The regulations also act as a liability shield for observatories, in the unlikely event a satellite is damaged.

None of the astronomers interviewed said that laser safety measures were unnecessary, but these measures have sparked frustration and a growing concern about the future of US astronomical endeavors. Observatories outside the United States are not bound by US Space Command’s guidelines, leading to concerns that the United States could lose some of its competitive edge in astronomy.

“If this keeps on continuing at more and more observatories, it puts us at a disadvantage,” said Christou, “It gives them [Europe] an advantage we don’t have,”

Foltz recently began a dialogue between the NSF and the Air Force about changing the laser restrictions. He said that the foundation has had some preliminary fact-finding talks with the Air Force, but has not yet discussed specific policy changes.

“We don’t want to misstep, we don’t want anybody to think the scientists are being arrogant,” said Foltz, “We really want to be good citizens with respect to the Air Force.” He added also that  while laser guide star use was not at the top of the NSF’s list of priorities, “It is an inefficiency, and I do think it is something we’re going to have to work out.”

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Editor: Alan Chodos