Sky & Telescope - November 2021 - 32

80 YEARS: WHERE WE'RE GOING by Javier Barbuzano
Through a Lens, Brightly
Will giant refractors make a comeback as space-based telescopes?
I
f lenses could be built as large as pick-up trucks and thinner
than their windshields, then we would probably still find
them in many big telescopes, including those in space. Lenses
are more resilient to misalignment and small defects than
mirrors are, making lens-based devices potentially simpler,
sturdier, and more reliable - as well as cheaper to launch.
But astronomers largely abandoned refractor designs for large
telescopes early in the 20th century, mainly because of the
lenses' massive weight.
However, a group of astronomers and engineers at the
University of Arizona might have found the key to making
large, lens-based space telescopes a reality. Led by Daniel Apai
and Tom Milster, they have come up with a lens design that
combines refractive and diffractive elements and can reach
several meters in diameter - with a thickness of just three
millimeters. Compare that to the Yerkes Great Refractor, the
world's largest refractor, whose 40-inch convex lens is about
2½ inches thick at its center.
The team is developing ways of mass-producing these
lenses cheaply and quickly with molds. In fact, researchers are
already making plans for a space array with dozens of telescopes,
which might cost less than the soon-to-be-launched
James Webb Space Telescope (JWST, see page 20).
" This is potentially game-changing optics technology, "
says space telescope expert Garth Illingworth (University of
California, Santa Cruz). Illingworth recently reviewed the
array's progress for the Moore Foundation, which has granted
the project $1.1 million. " Really lightweight optics will make
a major difference in the size [of space telescopes] that can be
built for a given cost. "
A Fresh Look at Lenses
Called multi-order diffractive engineered (MODE) lenses, the
new lenses are inspired by Fresnel lenses, like those typically
found in lighthouses. Instead of being a thick, convex monolith,
the surface of Fresnel lenses has a series of concentric
circles. The pattern preserves the curvature of the lens while
removing excess material from its back.
This flattened lens surface, however, produces terrible
chromatic aberration - a shift in the focus of different wavelengths
that distorts images. To fix it, the team adds a diffractive
layer on the back of the lens, similar to gratings used to
À LA MODE A prototype glass MODE lens. Unlike conventional, fully
refractive lenses, MODE lenses combine diffractive and refractive properties
to achieve high optical performance - one largely independent of
the lens's thickness.
32 NOVEMBER 2021 * SKY & TELESCOPE
DANIEL APAI / UNIV. OF ARIZONA

Sky & Telescope - November 2021

Table of Contents for the Digital Edition of Sky & Telescope - November 2021

Contents
Sky & Telescope - November 2021 - Cover1
Sky & Telescope - November 2021 - Cover2
Sky & Telescope - November 2021 - 1
Sky & Telescope - November 2021 - Contents
Sky & Telescope - November 2021 - 3
Sky & Telescope - November 2021 - 4
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