Stellar Façades
26 NOVEMBER 2019 • SKY & TELESCOPE
on the Canary Island of La Palma, is 10.4 meters (34.1 feet)
across and could just make out a baseball diamond on the
lunar terrain under optimal conditions. Seeing a mattress
would require a telescope about 12 times as wide.
There are exceptions, if the star is big enough and close
enough. Betelgeuse, the famous red supergiant on the shoul-
der of Orion, leads the pack here. In 1921, American physicist
Albert Michelson and astronomer Francis Pease relied on
Betelgeuse to make the fi rst direct measurement of a star’s
size, coming up with a diameter of 390 million km (240
million miles) — a bit wider than the orbit of Earth. In 1995,
the Hubble Space Telescope snapped a couple of fuzzy images
of Betelgeuse’s surface, the fi rst direct image of a star other
than the Sun (S&T: May 2019, p. 34). But if astronomers have
to rely on the few stellar heavyweights in our neighborhood,
they’re not going to make much progress.
Fortunately, there is a workaround. One telescope hun-
dreds of meters across is impractical. But intertwining the
light from several small telescopes scattered across hundreds
of meters of terrain delivers the same resolution. “We’ve
cheated to build a big telescope without paying for it,” says
Gerard van Belle (Lowell Observatory).Interfering with Light
A conventional telescope is a big bucket that collects light
and focuses it to an eyepiece or detector, forming an image.Before it gets focused, the light from a single star washes
over the telescope’s entire mirror (or lens), so you can
remove a chunk of the mirror and still see the star. Keep
smashing away until just a few patches of mirror remain,
and the focused image won’t be a picture in the traditional
sense. Interfering light waves from the disparate mirrors will
create a kaleidoscope of light that looks more like the rain-
speckled surface of a pond than a cosmic vista. But those
speckles encode much of the same information that would
have ended up in the traditional image.
“We get the information that is connected to the picture,
but we kind of get it in dribbles and drabs,” says van Belle.
With the help of software working through some clever math,
it’s possible to deduce what the image would have looked like.
Now, there’s no reason the small mirrors have to sit in the
same telescope. As long as the light waves from each mirror
travel the same distance to their meeting point — dilly-dally-
ing by no more than one millionth of a meter — the mirrors
can sit anywhere.
So, place those mirrors hundreds of meters apart in tele-
scopes of their own. Direct the light from each mirror into
long vacuum tubes that use additional mirrors on sliding carts
to maintain constant light travel times as the star traverses
the sky. Steer the light into a central hub where the waves
from each mirror can overlap and interfere with one another.
Congratulations: You just built an astronomical interfer-
ometer.
Radio astronomers have been doing this for decades,
culminating recently in the fi rst direct image of a black hole
(S&T: Sept. 2019, p. 18). This feat was made possible by hook-
ing up seven radio telescopes across the globe to simulate a
single antenna as wide as the planet.
But radio astronomers have it a bit easier than optical ones
do. To keep the radio waves from all telescopes in perfect
synchrony, astronomers can record the full swell of each
wave and line them up in a computer later. Interferometers
that capture visible and infrared light don’t have that luxury:
The waves wiggle too fast to capture every crest and trough,
so they must be lined up and merged on the fl y.
“The light has traveled sometimes for thousands of light-
years, and now you want to very precisely correct the path,”
Baron explains. “That has taken many years to refi ne the
technique.”
Only a few facilities have this ability. Atop Mount Wilson,
just north of Los Angeles, lies the Center for High Angu-
lar Resolution Astronomy (CHARA). Run by Georgia State
University, CHARA hosts six 1-meter telescopes that can
simulate a single telescope up to 331 meters wide. It was here
that Rachael Roettenbacher saw spots on Zeta Andromedae
as well as on Sigma Geminorum, a comparable star also in its100-inch
telescope60 metersBeam-combining
lab and synthesis
facilityCHARA 1-meter
telescopesCHARA facilitiesLight pipes to
central facilityqSPLAYED ARRAY The CHARA Array combines the light from six
1-meter telescopes, laid out in a Y formation on Mount Wilson.CHARA:^GREGG^ DINDERMAN^ /S&T