Primary and Secondary Mirror Engineering Design Units (EDUs) have recently arrived at NASA's Goddard Space Flight Center in Greenbelt, Md. from Northrop Grumman Aerospace Systems in Redondo Beach, Calif. and are undergoing examination and testing. When used on the James Webb Space Telescope those two types of mirrors will allow scientists to make those observations.
Why are the mirrors hexagonal shaped? Because a hexagon allows a segmented mirror to fit together without gaps. When Webb's primary mirror is focused on a distant star for example, that image will appear in all 18 mirror segments. To focus on the star and get one image, the mirror segments can then be tilted to align the 18 separate images into a single image.
Although there are 18 segments, there are three different optical prescriptions for the 18 segments: six segments of each prescription. The segment received is the first of the "A" prescription segments for which a total of 7 will be made - 6 flight and 1 spare. A prescription is similar to an eyeglass prescription and specifies a unique mirror curvature. Like eyeglasses, mirrors with the same prescription are interchangeable.
The primary mirror EDU that arrived at Goddard is also a flight spare. That means it can be used on the actual telescope. In fact, it could even be put on the telescope now if needed.
The primary mirror segment has already been cleaned and coated. Ball Aerospace & Technologies cleaned the mirror segment and Quantum Coating, Inc., in Moorestown, N.J., coated it. Ball Aerospace then took the mirror segment back, reassembled it with mounts and actuators and conducted final vibration testing.
Afterward, the mirror segment went back to the X-ray and Cryogenic Facility (XRCF) in Huntsville, Ala., where Ball performed final cryogenic acceptance testing on the segment before it came to NASA Goddard.
The secondary mirror on the Webb telescope will direct the light from the primary mirror to where it can be collected by the Webb's instruments. The secondary mirror is connected to "arms" that position it in front of the 18 primary mirror segments. It will focus all of the light from the 18 primary mirrors.
The secondary EDU at Goddard is not coated but can be, so it can be a flight spare once coated.
Eventually, the final flight mirrors will all come to NASA Goddard and be assembled on the telescope and the instrument module. Then, as a complete unit it will undergo acoustic and vibration testing at Goddard.
The James Webb Space Telescope is the world's next-generation space observatory and successor to the Hubble Space Telescope. The most powerful space telescope ever built, Webb will observe the most distant objects in the universe, provide images of the very first galaxies ever formed and see unexplored planets around distant stars. The Webb Telescope is a joint project of NASA, the European Space Agency and the Canadian Space Agency.
Rob Gutro | EurekAlert!
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