The James Webb Space Telescope Mirror Academy Boot Camp

How many of you are following the incredible James Webb Space Telescope? Well, I think we all can agree that it is truly a marvel and it is just now revealing the Cosmos like never before. But, have you ever wondered how the heart of the telescope, it’s mirrors, were created? Now, imagine, taking a behind-the-scenes look. Well, that’s just what we are going to do with the Webb Mirror Academy Boot Camp. I will share the incredible story, both the beauty and the challenges of JWST mirror making, something you will not find anywhere else.

AMSD (the Advanced Mirror System Demonstrator) was a joint NASA/DOD program. There was a competition between vendors to prove which specific approach to mirror making would provide what was needed for JWST. After the first phases of the competition, Ball Aerospace and Goodrich were the winning vendors. These mirrors were 1.4 meters in point-to-point - the size needed to produce the JWST segmented primary mirror – and it had what we call, an areal density of approximately 15 kg/m2. Ball developed a beryllium mirror and Goodrich developed a very lightweight all-glass cellular-core mirror.

First, you might ask, why the facility needed to be upgraded at all. There were several reasons. First, this was a Huge program. There were the eighteen primary mirror segments, the secondary, and the tertiary mirrors. We needed an array of large polishing machines for these. Next, we were direct-polishing the beryllium mirrors. So, with this came safety protocols, cryo-cycling, and other beryllium-specific requirements. In addition, we needed exceptional metrology to meet the JWST specifications. For all these reasons, we needed a complete overhaul of our capabilities for this challenging program.

Berylliosis is an allergic-type chronic lung disease caused by exposure to beryllium particles, a form of beryllium poisoning. The condition is incurable, but symptoms can be treated. Berylliosis can be an occupational disease. Generally, relevant occupations are those where beryllium is mined, processed or converted into metal alloys. In our case, we were removing small particles from our beryllium substrates in our optical fabrication grinding and polishing operations. With prolonged exposure, by inhalation, the lungs may become sensitized to beryllium. Some people who are sensitized to beryllium may not have symptoms. Continued exposure causes the development of small inflammatory nodules, called granulomas. The onset of symptoms can range from weeks up to tens of years from the initial exposure. In some individuals a single exposure can cause Berylliosis.

The primary mirror is composed of 18 hexagonal segments, with three unique prescriptions or surface shapes. We describes them as segment type A, segment type B and segment type C’s. In other words, JWST was a segmented telescope. But, this is not a “light bucket,” as we call some other telescopes. It is an imaging telescope. As such, we need to have the segments “co-phased” to each other. In other words, the 6.5 meter primary mirror is created with segments, but the light from the Cosmos reflects off of what is essentially a single-surface primary. Each of the segments is co-aligned to the others within a handful of nanometers of the prescriptive (or mathematical) shape.

The JWST mirror process flow starts with the beryllium blank and complete with the shipment of the finished polished mirrors. What is also indicated are the required measurements as the optical fabrication progresses. Even with all our planning and experience, the details of the flow Evolved as the Program advanced. For instance, as our refinement of the CMM was demonstrated, we eventually stopped using the Shack Hartmann in the grinding phases.

The Engineering Development Unit, or the EDU, was the workhorse for the process development and refinement. The idea was that the EDU would lead all the other segments by six weeks. This “EDU buffer,” as we called it, would help reduce the risk to the flight segments. The problem was, the EDU hit All of the unexpected potholes, or Anomalies, as we called them, along the journey.

In spite of the size, the blanks were easily lifted by two trained technicians. But, the tricky part was avoiding touching the thin ribs on the back side. We did plenty of practice with our “surrogate” mirror before prime hardware handling. In the long run, we needed a safer approach for handling the Primary Mirror Segments. Early on, we developed a comprehensive handling system for the 50,000 segment moves throughout the JWST mirror program.

The metrology station provided the final mirror-level metrology for the 18 primary mirror segments. The building and commissioning of the station was not trivial. The primary segment mount was a complex robotic positioner which held the segments on edge with minimal gravity-induced surface figure degradation. The granite base provided a stable and vibrationally-isolated foundation. The fold of the horizontal optical path was needed for reasonable packaging within the temperature-controlled, laminar flow test facility.