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.