Earth and laser beams in space

Scanning the Skies in Visible Light

A revolutionary effort to monitor the sky for laser flashes is ramping up. Based on a new approach to detecting such flashes, the SETI Institute’s LaserSETI program will build a network of instruments to monitor the entire night sky. This would be an unprecedented capability to search for manifestations of sophisticated intelligence beyond Earth.

It has long been speculated that powerful lasers might be used by advanced societies for interstellar communication. Messaging by light has a fundamental advantage over radio in that it can, in principle, convey far more bits per second – typically a half-million times as many. This increased bandwidth is a characteristic that would make lasers useful for communicating with off-world colonies, for example. Powerful lasers can also propel spacecraft to near-relativistic speeds and could conceivably be in widespread use for this purpose.

For decades, a small number of so-called optical SETI projects have examined star systems for extremely brief (nanosecond) flashes of light. However, the instruments used for these projects have all relied on photomultiplier tubes to detect the flashes, making them essentially one-pixel cameras. Very little of the sky has been examined.

By capitalizing on the assumption that the flashes could be monochromatic, an intrinsic characteristic of lasers, it’s possible to use relatively slow, two-dimensional solid-state detectors for optical SETI. Such detectors are available commercially as sensors for video cameras, and it is this idea (originally conceived by Alan Holmes of the Santa Barbara Instrument Group) that has been developed and built out by Eliot Gillum at the SETI Institute.

Gillum has constructed a SETI device that uses cameras with a commercial lens that images approximately 75 degrees of sky onto off-the-shelf solid-state detectors. Behind the lens is a grating that transforms any light source in the camera’s field-of-view into a double rainbow-like spectrum. While stars will produce a complete spectrum from blue to red, a laser will only show up at its characteristic wavelength (think of your red laser pointer.)

Each LaserSETI detection device consists of two identical cameras, rotated 90 degrees to one another along the viewing axis, an arrangement that, together with the double rainbow response, helps eliminate false alarms due to cosmic rays, etc.

Able to distinguish different colors of light, Gillum’s LaserSETI instrumentation is not limited to extremely short flashes as conventional SETI searches have been. And because the devices are “wide angle,” it’s possible to cover the entire (night) sky with a relatively small number of them, thereby keeping costs down.

Ferguson Observatory
Robert Ferguson Observatory, Sugarloaf Ridge State Park, Sonoma, California  

This project is proceeding apace. A crowd-funding campaign provided monies for instrument development and an initial deployment. The first two sites to be graced with equipment will be in California, at the Ferguson Observatory in Sonoma County, and on Haleakala on Maui. Cameras at the latter site will be aimed east and the California devices will be aimed west. This provides redundant coverage of the sky over the Pacific. While the hardware has been ready to go, the COVID 19 pandemic has delayed installation in Hawaii. However, that work is now underway as the country re-opens.

In addition, ten more instruments are under construction for deployment in Puerto Rico, the Canary Islands, and Chile. When up and running, the system will be able to monitor the nighttime sky in roughly half of the western hemisphere. The budget for this “discovery” deployment phase is $540 thousand, of which approximately half has been donated so far.

 “The plan is to build out the discovery network, run it, and then determine what capabilities the next generation of cameras should have,” Gillum says. “What should we build for the ultimate network that will cover both hemispheres? Do we want more spectral or spatial resolution? Perhaps more sensitivity?”  

With the completion of the discovery phase, and assuming that funding is available, the construction and installation of enough cameras to cover the entire night sky would begin, at an estimated total cost of $5 million.

SETI is an enterprise of the last six decades only. Until recently, it has been almost exclusively a search for radio signals. But that’s largely an accident of history. A new and promising horse is joining the race to find intelligence elsewhere.

-Seth Shostak

Read More:
Reaching Out on a Beam of Light

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