Why Webb's unassuming secondary mirror makes it a telescope
The James Webb telescope has a two-story concave mirror that looks like a golden honeycomb dish.
On Wednesday, that’s not what NASA opened.
What it did deploy was a much smaller mirror — a round convex piece only 2.4 feet in diameter, about that of a car tire. But the Webb team cautions against underestimating its significance. As program manager Bill Oakes said during the live broadcast of the deployment: “We’re about 600,000 miles from Earth, and we actually have a telescope.”
So what is it about the secondary mirror that makes this thing a telescope?
“This will allow the light to actually go into the telescope,” said Michelle Thaller, a NASA spokeswoman.
Without the small mirror there to gather and concentrate the light reflected off the primary honeycomb mirror, Webb would not be able to perform the science it’s expected to accomplish. The second mirror is positioned way in front of the honeycomb and will collect light from the 18 hexagonal honeycomb segments, focusing them into a beam. The light is concentrated by bouncing off the secondary’s curved surface.
Then, the beam is directed into a third mirror and other so-called “fine-steering” mirrors before being channeled into Webb’s scientific instruments. If the secondary mirror — the focusing mechanism — isn't in the right position, no light goes into the telescope.
“The beam is shaped and reflected back to the secondary (mirror), which is convex," said Julie Van Campen, Webb’s deputy commissioning manager, “and then it pushes it down through the center of the telescope.”
The deployment Wednesday involved a large tripod with 25-foot-long legs outstretching from the primary mirror, holding the smaller mirror in front of it. (The above video shows the maneuver being demonstrated in a ground test.)
Webb is a telescope now. If all goes well, it’ll be a much bigger telescope by the end of the week.
Elisha Sauers writes about space for Mashable, taking deep dives into NASA's moon and Mars missions, chatting up astronauts and history-making discoverers, and jetting above the clouds. Through 17 years of reporting, she's covered a variety of topics, including health, business, and government, with a penchant for public records requests. She previously worked for The Virginian-Pilot in Norfolk, Virginia, and The Capital in Annapolis, Maryland. Her work has earned numerous state awards, including the Virginia Press Association's top honor, Best in Show, and national recognition for narrative storytelling. For each year she has covered space, Sauers has won National Headliner Awards, including first place for her Sex in Space series. Send space tips and story ideas to [email protected] or text 443-684-2489. Follow her on X at @elishasauers.
