Astronomers get ultrasharp images with large telescope in Arizona

Astronomers have successfully tested a new method to remove atmospheric
blurring from large ground based telescopes.

The experiments were made in November 2002 and January 2003 at the 6.5-meter
(21-foot) telescope at the MMT Observatory on Mount Hopkins, Ariz. The
project is a collaboration of the University of Arizona’s Steward
Observatory and Italy’s Osservatorio Astrofisico di Arcetri in Florence. It
uses revolutionary new technology developed with support from the U.S. Air
Force.

Large ground based telescopes can make images as sharp as or sharper than
the Hubble Space Telescope, but only if atmospheric blurring is corrected.
Previously, the deformable mirrors available to do this were small, flat,
and relatively inflexible. They could be used only as part of complex
instruments attached to conventional telescopes.

But in this new work, one of the two mirrors that make up the telescope
optics is used to make the correction directly. The new secondary mirror
makes the entire correction with no other optics required, making for a more
efficient and cleaner system.

Like other secondary mirrors, this one is made of glass over 2 feet in
diameter and is a steeply curved dome shape. But under the surface, it is
like no other. The glass is less than 2 millimeters thick (less than
eight-hundredths of an inch). It literally floats in a magnetic field and
changes shape in milliseconds, virtually real-time. Electro-magnetically
gripped by 336 computer-controlled “actuators” that tweak it into place,
nanometer by nanometer, the adaptive secondary mirror focuses star light as
steadily as if Earth had no atmosphere. Astronomers can study precisely
sharpened objects rather than blurry blobs of twinkling light.

“The reason no one has done this before is because it turns out to be
enormously technically challenging,” said Michael Lloyd-Hart of the UA
Center for Astronomical Adaptive Optics (CAAO), project scientist for the
MMT adaptive optics system. Lloyd-Hart and chief system engineer Francois
Wildi led the design and engineering effort. “We found out the hard way just
how difficult it is. It’s taken us a number of years to build a mirror with
a shape that deforms in real time,” Lloyd-Hart said.

One of the system’s unique features is its ability to make corrections
according to the detail of the distortion measurements. If very faint stars
provide no useful data, the shape can be fixed to mimic a conventional
mirror of solid glass. The mirror can also be used to rapidly “chop” the
viewpoint of the telescope, as required for infrared imaging.

“It’s been 25 years or so since anyone’s tried a radically new way of
building a deformable mirror, and this technology really is different, ”
said Laird Close, UA assistant professor of astronomy and CAAO scientist.
“But the ability to build large, curved, deformable mirrors for adaptive
optics is a boon to astronomers using the giant new ground-based
telescopes.”

“The adaptive secondary mirror made for the UA and Smithsonian Institution’s
6.5-meter telescope at the MMT Observatory is a tremendous advance over
conventional adaptive optics, which are systems that involve extra relay
optics and mirrors in a box separate from the telescope, Lloyd-Hart said.

“One key feature is that we correct for blurring effects of the atmosphere
at a mirror which is an integral part of the telescope,” Lloyd-Hart said.
“The conventional approach has been to build the telescope, then build a box
of optical tricks to improve the resolving power of the telescope.”

The new system solves a big problem astronomers face when they try to
observe at longer infrared wavelengths, where special targets like
Jupiter-like planets and circumstellar disks are brightest.

“Everything in the world glows with thermal energy, or heat. When you try to
do astronomy at those wavelengths, even the optics that you are looking
through glow. So the more optical surfaces and telescope parts you can
eliminate =AD the simpler the system is optically, the better it is for doing
infrared astronomy,” Lloyd-Hart said.

The Steward Observatory Mirror Lab made the MMT’s large deformable convex,
aspheric secondary mirror. Learning how to make glass 2mm thick so that it’s
“infinitely floppy” was a major challenge to building the system, Lloyd-Hart
said.

The biggest equivalent flexible mirror available commercially at this time
is 12 cm, or about 4 and 3/4 inches, across, Close said.

But once Steward Observatory researchers realized how to do it, they also
realized they could make big deformable mirrors for use in space. Steward
Observatory has been developing new space-based optics as a spin-off of this
ground-based technology.

The Italian partners designed very powerful computer electronics that drive
the MMT’s adaptive secondary mirror with a cluster of 168 microprocessors,
which are all packed in an electronics crate that is mounted behind the
secondary mirror. The computer cluster is essentially a supercomputer, more
powerful than any computer available during the Apollo space era. It senses
the positions of and drives the actuators.

Guido Brusa, CAAO/Large Binocular Telescope adaptive optics scientist, said
that he is “personally very happy” with the results of effort, achieved
during the past 7 to 8 years through “patient and persevering work of many
people in both Arizona and Italy….It is great to see that the adaptive
secondary mirror performs beautifully, even in the presence of relatively
strong wind (20 to 30 mph) and in environmental conditions very different
from those in the lab.”

Brusa called the success “an important step” in integrating adaptive optics
into an astronomical telescope. An adaptive primary telescope mirror is “a
foreseeable future development,” he said.

In the MMT’s new adaptive optics, a wavefront sensor camera mounted at the
base of the telescope senses atmospheric turbulence and sends that
information to the MMT adaptive secondary mirror. The powerful computer
cluster behind the secondary mirror sends electronic current through coils
so that each of 336 actuator magnets spaced across the mirror is instantly
moved to the desired position. The result is a “flat,” non-wavy wavefront
seen by the astronomer’s science camera.

The unique adaptive optics system also includes its own plumbing. Its
half-water, half-methanol liquid cooling system can dissipate up to a
kilowatt of heat.

DETWINKLING AND BLOCKING STARLIGHT

UA astronomer Phil Hinz’ observing run Jan. 22 illustrates why the new
adaptive optics system is ideal if you’re looking for planetary disks or
planets around bright, nearby stars.

On Jan. 22, Hinz and UA astronomer emeritus Bill Hoffmann took Hinz’ nulling
interferometer called “BLINC” and Hoffman’s infrared “MIRAC” camera to the
MMT, while UA, Italian and Smithsonian Institution MMTO staff ran the new
adaptive optics.

“Without adaptive optics, nulling interferometry is able to suppress the
star to only 5 to 10 percent of its original brightness,” Hinz said. “In
addition, the intensity of the star rapidly changes because of atmospheric
turbulence, so the star appears to blink on and off.”

Nulling interferometry works by creating two “sub- telescopes,” both looking
at the same bright star, but positioned so starlight from each sub-telescope
travels in slightly different paths before hitting the detector. When
properly aligned, crests of lightwaves from one sub-telescope will line up
with the troughs of the lightwaves from the other, in effect canceling the
light of the bright star.

Hinz, who used the 6.5-meter MMT as two 3-meter sub- telescopes, said the
initial Jan.22 observations were successful in showing the power of adaptive
optics to stabilize the star and suppress all but two percent of its light.

“Once we’ve refined this technique, we should be able to stabilize and
suppress all but one-tenth of a percent, down to three-hundredths of a
percent of the starlight and see faint, planetary dust disks much like our
own solar system around nearby stars,” Hinz said.

“Our own dust disk is about one-hundredth of one percent of the brightness
of the sun, which sets the ultimate goal of this technique. This is the
level of suppression we’re aiming for with the Large Binocular Telescope
Interferometer,” he added.

ADAPTIVE OPTICS FOR THE LBT

University of Arizona scientists are developing two adaptive secondary
mirrors for the Large Binocular Telescope (LBT) on Mount Graham, said UA
astronomer John Hill, who directs the LBT project. The LBT won’t have
conventional secondary mirrors, Hill said. Each of the LBT’s 8.4-primary
mirrors will have an adaptive concave (rather than convex) secondary mirror
91 cm (36 inches) across, held by 672 actuators that will bend it moment by
moment to the required shape.

In principle, even a 6.5-meter ground-based telescope could be used to image
a Jupiter-like planet in a solar system like our own within the 8 parsec
neighborhood, Mirror Lab director and CAAO director Roger Angel has noted in
research papers. (Eight parsecs is about 26 light-years, or more than 153
trillion miles.)

As for the really giant telescopes of the future =AD telescopes with
20-meter-or-more diameter primary mirrors =AD ground based telescopes with
adaptive secondary mirrors should be able to directly detect and study
nearby Earth-like planets, Angel predicts.

Success in making deformable adaptive secondary mirrors for large telescopes
is “a natural stepping stone to so-called ‘multiple-conjugate adaptive
optics,'” Lloyd-Hart said. This system would use several deformable mirrors
in series and correct for atmospheric turbulence in 3 dimensions.

“You could cancel the atmospheric error anywhere you look. You’d have a very
large field of view with high resolution all at once. And when you can
capture huge fields of view and see them with extreme clarity, then you’re
talking real scientific progress,” Lloyd-Hart said.