Photon Sieves for Better Telescopes

This image shows how the photon sieve brings red laser light to a pinpoint focus on its optical axis, but produces exotic diffraction patterns when viewed from the side (Source: NASA / W. Hrybyk)

This image shows how the photon sieve brings red laser light to a pinpoint focus on its optical axis, but produces exotic diffraction patterns when viewed from the side (Source: NASA / W. Hrybyk)

To resolve smaller and smaller physical details of distant celestial objects, scientists need larger and larger light-col­lecting mirrors. This challenge is not easily overcome given the high cost and imprac­ticality of building and in the case of space obser­vatories launching large-aperture telescopes. However, a team of scientists and engineers at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, has begun testing a potentially more affordable alter­native called the photon sieve. This new-fangled tele­scope optic could give scientists the reso­lution they need to see finer details still invi­sible with current observing tools – a jump in reso­lution that could help answer a 50-year-old question about the physical processes heating the sun’s million-degree corona.

Although poten­tially useful at all wavelengths, the team speci­fically is developing the photon sieve for studies of the sun in the ultra­violet, the wave­lengths needed to dis­entangle the coronal heating mystery. The team has fabricated three sieves and now plans to begin testing to see if it can withstand the rigors of operating in space. “This is already a success,” said Doug Rabin, who is leading the R&D ini­tiative.

The optic is a variant of a Fresnel zone plate. Rather than focusing light as most tele­scopes do through refraction or reflection, Fresnel plates cause light to diffract. This causes the light waves on the other side to reinforce or cancel each other out in precise patterns. Fresnel plates consist of a tightly spaced set of rings, alter­natingly trans­parent or opaque. Light travels through the spaces between the opaque zones, which are precisely spaced so that the diffracted light overlaps and focuses at a specific point, creating an image that can be recorded by a solid-state sensor.

The photon sieve operates largely the same. However, the sieve is dotted with millions of holes precisely placed on silicon in a circular pattern that takes the place of conventional Fresnel zones. The team wants to build a photon sieve at least three feet, or one meter, in diameter — a size they think could achieve up to 100 times better angular resolution in the ultra­violet than NASA’s high-re­solution space telescope, the Solar Dynamics Obser­vatory. “For more than 50 years, the central unanswered question in solar coronal science has been to under­stand how energy transported from below is able to heat the corona,” Rabin said. “Current instru­ments have spatial reso­lutions about 100 times larger than the features that must be observed to understand this process.” Rabin believes his team is well along the way in building an optic that can help answer the question.

In just a few months’ time, his team built three devices measuring three inches wide — five times larger than the initial 17-millimeter optic developed four years ago under a previous R&D-funded effort. Each device contains 16 million holes whose sizes and loca­tions were determined by team member Adrian Daw. Another team member, Kevin Denis, then etched the holes in a silicon wafer to Daw’s exacting specifications using a fabri­cation technique called photo­litho­graphy. Team members Anne-Marie Novo-Gradac and John O’Neill have acquired optical images with the new photon sieves, while Tom Widmyer and Greg Woytko have prepared them for vibration testing to make sure they can survive harsh g-forces en­countered during launch.

“This testing is to prove that the photon sieve will work as well as theory predicts,” Rabin said. Although the team has already accomplished nearly all the goals it set forth when work began late last year, Rabin believes the team can enlarge the optics by a factor of two before the end of the fiscal year. But the work likely won’t end there. In the nearer term, Rabin believes his team can mature the techno­logy for a potential sounding-rocket demon­stration. In the longer term, he and team member Joe Davila envision the optic flying on a two-space­craft for­mation-flying CubeSat-type mission designed specifically to study the sun’s corona. “The scientific payoff is a feasible and cost-effective means of achieving the reso­lution necessary to answer a key problem in solar physics,” he said. (Source: NASA)

Link: NASA’s Goddard Space Flight Center, Greenbelt, USA

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