1.1.

Origins Space Telescope

The Origins Space Telescope (Origins) traces our cosmic history, from the formation of the first galaxies and the rise of metals to the development of habitable worlds and present-day life. Origins does this through exquisite sensitivity to infrared radiation from ions, atoms, molecules, dust, water vapor, and ice, and observations of extra-solar planetary atmospheres, protoplanetary disks, and large-area extragalactic fields. Origins operates in the wavelength range 2.8 to $588\mu \mathrm{m}$ and is more than 1000 times more sensitive than its predecessors due to its large, cold (4.5 K) telescope and advanced instruments.¹

1.2.

Materials Evaluation

A materials evaluation was completed for the mission’s main optical and structural elements. The evaluation team, which included Origins team members and industry partner materials experts, developed an iterative process to identify suitable material candidates. The material candidates under consideration were chosen based on their material properties, spaceflight mission heritage, and knowledge of current manufacturing and processing capabilities. The evaluation criteria were determined by the driving requirements for Origins, material performance—particularly at cryogenic temperatures—and relevant manufacturing challenges.

An initial assessment of primary mirror material candidates yielded five potential options: beryllium, aluminum, AlBeMet^®, silicon carbide (SiC), and fused silica. The team conducted further trade studies to better understand the material performance needed to meet Origins’ requirements. Structural materials were also evaluated in a broader framework. This paper describes the approach to the evaluations and the results, ultimately leading to a materials selection.

2.1.

Driving Requirements

The telescope, including the backplane, is expected to be isothermal so it must be composed of materials with relatively high thermal conductivity, greater than $4\mathrm{W}/\mathrm{m}·\mathrm{K}$ at 4.5 K. It must have low overall mass, namely $<900\mathrm{kg}$, to be consistent with the 6643 kg payload mass established for the design,¹ a large primary mirror with a light collecting area $>25{\mathrm{m}}^2$, and the materials themselves should have a relatively high technology readiness level (TRL) $\ge 4$, where component validation exists in a laboratory environment.

2.2.

Evaluation Criteria

The evaluation criteria include key material properties that drive performance for the Origins’ primary mirror and additional considerations. The criteria include the following: density, stiffness, thermal conductivity, coefficient of thermal expansion (CTE), outgassing, and manufacturability.

3.1.

Preliminary Assessment

The mirror material evaluation was prioritized over the structural material evaluation due to the inherent complexity of assessing mirror manufacturing. A preliminary list of material candidates is shown in Table 1. Candidate mirror materials were systematically evaluated, resulting in five potential options. Major advantages and disadvantages were noted for each material with respect to manufacturability and material performance for Origins. The candidates were selected due to their material properties, mirror material heritage, and the knowledge of current manufacturing capabilities.

Table 1

Candidate mirror materials were systematically evaluated, resulting in five potential options.

Traditional mirror materials include glasses, ceramic materials, and fused quartz while nontraditional mirror materials include metals, metal alloys, SiC, and CFRP composites. Nontraditional mirror materials offer opportunities to reduce weight and cost. In the case of Origins, nontraditional mirror materials also provide thermal advantages over traditional mirror materials.⁴ The first assessment was primarily driven by Origins’ 4.5 K operating temperature. The first materials eliminated were any that were not ideally suited for cryogenic temperatures. This included the glasses and glass ceramics: Ultra Low Expansion (ULE) glass (titania-silicate glass), Zerodur (lithium-aluminosilicate glass-ceramic), and Borosilicate (glass with silica and boron trioxide). However, fused silica was not eliminated because of its prominent heritage as an optical substrate and its potential to perform in cryogenic temperatures. Other materials eliminated include titanium because of its extremely high density and CFRP based on its limited manufacturability technologies. Five mirror material candidates remained: beryllium, aluminum, fused silica, SiC, and aluminum, and beryllium metal matrix composite (AlBeMet®).

3.2.

Mirror Material Trade Matrix

A material trade matrix was created for the top choices among the remaining material candidates. This trade matrix was initially designed for consideration of passive mirror segments for a 9.1-m aperture, where the actuators are used once to optically phase the mirror segments. The matrix is shown in Table 2 and assesses each material on a scale of 1 to 5 for performance, schedule, and cost, using NASA standard values. The materials are listed in order of preference, showing fused silica and SiC as the top choices.

Table 2

A material trade matrix assisted the Origins team in identifying the top primary mirror materials. (On a scale of 1 to 5 with the higher values being better.)

Additionally, the results of the mirror material trade matrix show beryllium as the highest performing, despite programmatic challenges. SiC also has high performance and is another viable option for Origins. AlBeMet^® offers the opportunity to improve the manufacturability of beryllium but would ultimately weigh more overall and have a lower stiffness than beryllium. Fused silica has the strongest optical heritage among the candidates, which would greatly reduce cost and schedule, but its lower thermal conductivity makes it less ideal for cooling than the other candidates. Aluminum has relatively good properties and machinability, but overall poorer thermal contraction performance and lower specific stiffness than the top candidates.

3.3.

Material Performance for Origins

The team created material performance plots to further evaluate the five potential material candidates. Figure 1 shows critical material properties plotted against specific stiffness at room temperature for beryllium, AlBeMet^®, SiC, fused silica, and aluminum. It is important to note that measured material properties vary at 4.5 K from test to test and alloy to alloy. The properties also vary based on purity, grade, and manufacturer. Where 4 K data is unavailable, values measured at temperatures in the range 4.5 to 30 K were used in these figures.

Fig. 1

(a) The CTE at 4 K plotted against specific stiffness shows beryllium (Be) is the best performer, followed by AlBeMet^®, SiC, fused silica, and aluminum (Al). (b) Steady-state gradient thermal stability plotted against specific stiffness shows Be, AlBeMet^®, and SiC as the best performers. (c) Long-term stability plotted against specific stiffness shows Be, AlBeMet^®, and SiC as the best performers. (d) Strain from room temperature (293 K) down to 4 K against specific stiffness shows trade-offs between Be, AlBeMet^®, SiC, and fused silica, with Al as the lowest performer.

4.1.

Final Materials Selection

For the 4.5 K structure, beryllium was selected. There are several potential issues with the use of beryllium that will be taken into account. Beryllium is more brittle compared to traditional materials such as aluminum so it requires special care in design (sharp corners, proper clearance holes, etc.). Beryllium dust is toxic, so fabrication is limited to certain places. These dangers do not exist after fabrication though, and a safety and handling plan would be created to address transport and handling of the parts.

Several organizations that have used large beryllium structures were consulted and offered guidance for Origins. These organizations would not hesitate to use beryllium again. Ball Aerospace developed the beryllium mirrors and the beryllium Aft Optics Subsystem (AOS) bench for the JWST⁹ as well as the beryllium mirror and structure for Spitzer.^10,11 Lockheed Martin developed the beryllium bench of near-infrared camera (NIRCam) on JWST.² Origins plans to work with vendors early to qualify workmanship and design details. A segment of the primary mirror would be used to validate performance and then again after integrating the segments to the mirror backplane. Origins recognizes the need to restore the beryllium processing facilities used by JWST.

For the spacecraft structure, the team selected CFRP whenever possible. Origins has critical structures on the warm side [spacecraft bus module (SBM)] and the cold side (mirror and instrument thermally isolating support structures). While the observatory is warm before cool down, water vapor will outgas from CFRP. On the cold side, this vapor could condense onto the mirror and instruments, impacting optical throughput. As a result, the team avoided using CFRP as much as practicable on the cold side, opting instead for metals.

Therefore, CFRP is an advantageous choice for the SBM, whereas metals are more suited for the cold side structures. An exception to this are the 4.5 K bipods, which are made of the CFRP M55J to provide the necessary stiffness to thermal conductivity in the range of 4.5 to 35 K. In contrast, high thermal conductance is critical for the backplane, which will be cooled conductively to 4.5 K, so the team sought a metal with good thermal conductivity down to 4.5 K. The mirror structures would also ideally be made from the same material as the mirror segments to avoid CTE mismatch between structures on the cold side. Thus, an athermal design of beryllium on the cold side is favorable.