Construction of the mirror began in 2004 and was completed in 2011; scientific instruments were built and installed between 2013 and 2016, and then both parts were united at the Goddard Space Flight Center in Greenbelt, Maryland. The integrated module is called OTIS: it comprises the OTE (Optical Telescope Element) and the ISIM (Integrated Science Instrument Module), and is the functional heart of the entire observatory. Before OTIS was connected to a sunshield and the spacecraft bus, it underwent Force Limited Vibration Testing between November 2016 and February 2017.
For the FLVT, we used 28 Kistler force transducers of three different types. They were mounted on a fixture attached to a shaker table. Although we encountered an anomaly with the first axis early in December, the whole procedure went quite smoothly, and it was only due to the very special nature of the UUT that this happened to be the longest test phase of my career.
Brian Ross, Group Lead Structural Dynamics Test Group at Goddard and leading engineer at the time
The OTIS structure The OTIS structure of the James Webb Space Telescope was vibration-tested in the three orthogonal axes with a closed-loop control (CLC) system based on precise force measurements. The measuring chain also included charge amplifiers such as the 5080A multichannel lab amplifier from Kistler. Thanks to signal summing and sophisticated calculation, a CLC was successfully established to prevent any overtesting that might jeopardize OTIS, the precious unit under test (UUT).
Cryogenic accelerometers help to achieve utmost precision
The James Webb Space Telescope is an instrument that requires utmost precision: the alignment of the mirror elements, for example, must be absolutely exact. This level of accuracy is achieved by 132 actuators capable of positioning at nanometer level; any unwanted micro-vibrations must also be taken into account. Kistler accelerometers of six different types were used for the various vibration tests (sine vibration, acoustic and shock testing); they include the 8793A500 sensors that are now in space as part of the Webb telescope.
“These triaxial accelerometers have a measuring range of up to 500 g. They were chosen for their ability to operate at very low temperatures down to 25 K,” according to Sandra Irish, Lead Structures Engineer for the James Webb Space Telescope at Goddard. “They worked very well for test-n-fly, but we are also very satisfied with all the other accelerometers we used for test-and-remove. Many of those products were procured and employed only and specifically for the JWST program,” she adds.
Following successful testing at Goddard, OTIS was shipped to Johnson Space Center in Houston where the optical testing took place in a giant thermal vacuum chamber during 2017. By 2019, the OTIS unit was assembled with the sunshield and spacecraft bus, the completed James Webb Space Telescope then entered the final testing and preparation phase prior to its successful launch in late 2021. Now, scientists and members of the general public are eagerly awaiting insights from Webb about our universe – and especially its past history, which was shrouded in darkness until now. After only a few more months, the first images, results and data will be available to the scientific community worldwide.
Another L2 infrared telescope already in the pipeline
In the meantime, another great telescope is set to launch soon: the Nancy Grace Roman Space Telescope (“Roman” for short, formerly known as the Wide Field Infrared Survey Telescope) is named for one of NASA's most famous chief astronomers of the twentieth century. Like Webb, Roman is also an infrared telescope; it has a smaller mirror than Webb, but a very wide view that will focus especially on dark matter and exoplanet research. Its space launch is planned for 2026 – to the same L2 destination as Webb. The major development, construction and testing phases will take place at Goddard Space Center, where all the main components will be built and tested: not only the telescope this time, but also the protective equipment and the spacecraft bus. Brian Ross again: “Procedures are already planned for about two years from now and in the meantime, our facilities will need some upgrading to be capable of testing the whole observatory structure. We're going to perform three FLVT tests for each part of Roman, and we will certainly rely on Kistler technology again to repeat our success with Webb.”