Webb Telescope Optics Meet Expectations

The James Webb Space Telescope team has announced that they’ve completed a key stage in deploying its primary mirror. Find out why NASA now confirms that the telescope’s audacious design will deliver on its demanding science goals.

I still have the first telescope I bought at the now defunct Efston Science store in Toronto about thirty years ago. The Edmund Scientific Company (also now defunct) produced that basic but easy to use 75mm (3 inch) Newtonian reflector.

Columnist Alex Berezov has made the case that Sir Isaac Newton was “the smartest person who ever lived.” He contributed to many fields, but for this story, we’ll focus on his Newtonian telescope design.

The Newtonian telescope uses mirrors instead of lenses. There’s a large, round, concave mirror at the back of the tube that reflects the light to a smaller mirror near the front.

Newtonian Telescope Uses Mirrors Instead of Lenses

The light then passes vertically through an eyepiece to the observer. Newton’s design eliminates the distorted colours we tend to see with lenses.

It’s also less expensive because it only needs one precisely shaped surface to work. It offers a wider field of view and you look into it from the side instead of the back end, which is usually more comfortable, at least on a backyard telescope.

These advantages have proven to be timeless. Nearly all major telescopes in observatories today are Newtonian Reflectors.

Webb Telescope Includes Bold, Fascinating Innovation

That also goes for NASA’s James Webb Space Telescope, the replacement for the venerable Hubble. Even so, the Webb Telescope includes a bold and fascinating innovation.

The 6.5 metre (21 foot) primary mirror on the Webb Telescope consists of 18 separate hexagonal segments that folded up into the rocket for launch. Each segment is 1.32 metres (4.3 feet) wide.

The Webb Telescope is 2.7 times larger than the mirror in the Hubble Telescope. It’s the largest mirror and the first folding mirror ever launched into space.

“A New Way to Build Space Telescopes”

Lee Feinberg, Webb optical telescope element manager at NASA’s Goddard Space Flight Center explained the achievement this way. “The teams that designed, built, tested, launched, and now operate this observatory have pioneered a new way to build space telescopes.”

Readers may be asking, “If the mirror needs to be so precise, how do they make sure each segment lines up properly?” That’s a very astute question and, as my dad used to say about challenging tasks, “It ain’t easy!”

The Webb Telescope team has been focusing on the mirror alignment tasks for the past six weeks. The segments have to match each other to within 50 nanometers, where a nanometer is one-millionth of a millimeter.

At or Above Expections for Every Defined Optical Parameter

Last week, NASA announced that the team had completed the “fine phasing” stage of the mirror alignment process. The team reports that the assembled telescope is performing at or above expectations for every defined optical parameter.

Thomas Zurbuchen is NASA’s associate administrator for the Science Mission Directorate. He described the team’s success this way.

“More than 20 years ago, the Webb team set out to build the most powerful telescope that anyone has ever put in space and came up with an audacious optical design to meet demanding science goals. Today we can say that design is going to deliver.”

Mirrors Now Aligned with Near-Infrared Camera

With fine phasing complete, the Webb Telescope’s mirrors are now aligned with the Near-Infrared Camera, which is the main imaging instrument. However, aligning all the other imaging tools will take another six weeks.

These other devices include the Near-Infrared Spectograph, the Mid-Infrared Instrument, the Near Infrared Imager, and the Slitless Spectrograph. Then there will be a final step where the team will fine tune any remaining small discrepancies in the positions of the mirror segements.

The Webb Telescope team expects to complete all of this final alignment work in early May. However, that’s still not the end of the instrument preparations.

First Full-Reolution Images and Data This Summer

The plan calls for the team to release the first full-resolution images and data this summer. NASA expects the Webb to become “the world’s premier space science observatory.”

At that point, the Webb Telescope will enable thousands of scientists to better understand the Solar System, find planets orbiting other stars, examine the first galaxies that formed shortly after the Big Bang, and learn how galaxies assemble over time.

It’s important to note that, for all the attention that crewed and uncrewed spacecraft missions receive, most of what we know about space has come from telescopes.

Most Space Discoveries Have Come From Telescopes

The telescope is the instrument that has told us the story of the Universe and our place in it. It’s also a kind of time machine, because when we look out into space, we also look back in time.

We now know that the folding mirror on the Webb Telescope is a triumph of engineering. NASA is calling it “a giant leap forward in our quest to understand the Universe and our origins.”

We always have more to learn if we dare to know.
Learn more:
NASA’s Webb Reaches Alignment Milestone, Optics Working Successfully
James Webb Space Telescope Observatory Overview
NASA Discovery Program – 4 Bids to Explore Solar System
James Webb Space Telescope Unfolds Its Mirror
James Webb Space Telescope – “Faster, Better, Cheaper”





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