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About the Backplane


JWST's backplane is the large structure that holds and supports the big hexagonal mirrors of the telescope. The backplane has an important job as it must carry not only the 21 ft (6.5 m) diameter primary mirror plus other telescope optics but also the entire module of scientific instruments. All told, the backplane carries more than 2 1/2 tons (about 2400kg) of hardware.

Being the "spine" of the mirror requires it to essentially be motionless while the mirrors move to see far into deep space. Imagine holding the handle of a magnifying glass to see a tiny object. If your hand shakes a lot, it will be hard to focus on the object. Just as you would have to hold the magnifying glass handle steady with your hand, the JWST backplane has to hold the telescope mirrors steady, to allow them to focus.

JWST's Flight Backplane

This structure is also designed to provide unprecedented thermal stability performance at temperatures colder than -400°F (-240°C). At these temperatures, the backplane was engineered to be steady down to 32 nanometers, which is 1/10,000 the diameter of a human hair!

Orbital-ATK (formerly ATK) is responsible for the development of advanced graphite composite materials mated to titanium and invar fittings and interfaces. Invar is a nickel steel alloy notable for its uniquely low changes due to thermal expansion.

(Image credit: Northrop Grumman)

The Journey of the Backplane

There are actually two versions of the backplane. The first is a test version, called the Pathfinder. It is full-scale, and only missing the two side "wings" that hold three mirrors each, and fold to each side in launch configuration. The Pathfinder was shipped to NASA Goddard, where the spare secondary mirror, as well as two spare primary mirror segments were mounted to it. This allowed for rehersal of the mirror assembly process. The Pathfinder next went to NASA Johnson, to be tested in Chamber A - which is where the flight telescope will be cryogenically tested. The Pathfinder structure and the mirrors were put through a series of tests, which allowed for a full checkout of the equipment and test procedures, in preparation for the flight telescope.

You can view our photo archive, but here are some highlights.

The two spare mirrors on the Pathfinder at NASA Goddard:

Silver and Gold
Credit: NASA/Chris Gunn

The Pathfinder about to be moved into Chamber A for cryo tests:

JWST Pathfinder in front of Chamber A
Credit: NASA/Chris Gunn

The flight structure arrived at NASA Goddard - but as more than just the backplane. What we call the "telescope structure" includes the primary mirror backplane assembly; the secondary mirror support structure; the main backplane support fixture (BSF); and the deployable tower structure that lifts the telescope off of the spacecraft. The three arms at the top come together into a ring where the round secondary mirror resides.

Diagram showing the BSF:

NASA's Webb Telescope's Last Backbone Component Completed
Credit: Northrop Grumman

The center section of the flight backplane:

James Webb Space Telescope Flight Backplane Structure
Credit: Orbital-ATK

The wing sections of the flight backplane:

The James Webb Space Telescope gets its wings
Credit: Northrop Grumman/ATK

The Deployable Tower Array at the base of the telescope structure:

The Deployable Tower Assembly
Credit Northrop Grumman

The flight telescope structure arriving at NASA Goddard:

James Webb Space Telescope Telescope Structure Arrival
Credit: Maggie Masetti

The flight telescope structure at NASA Goddard:

NASA's James Webb Space Telescope Structure Stands Tall
Credit: NASA/Chris Gunn

The flight telescope structure in the assembly stand at NASA Goddard:

James Webb Space Telescope Structure Poised for Mirror Assembly
Credit: NASA/Chris Gunn

The first mirror is installed on the flight telescope structure:

NASA's Webb Space Telescope Receives First Mirror Installation
Credit: NASA/Chris Gunn

The completed primary mirror on the flight telescope structure. The black covers on the gold-coated mirror segments are temporary to keep the mirror surfaces protected:

NASA's James Webb Space Telescope Primary Mirror Fully Assembled
Credit: NASA/Chris Gunn

Once the full assembly of the telescope is complete, and some tests are done, the telescope will go to NASA Johnson for cryogenic testing, just like the Pathfinder went through. After that, the telescope goes to Northrop Grumman to be mated with the sunshield and spacecraft bus.

Here is the Behind the Webb video on the Backplane: