In the aerospace industry, CubeSats have emerged as a low-cost, easily manufacturable solution for space-based optical systems. They offer a unique opportunity to develop a production line approach for a space-based product through the manufacture of a constellation of smaller, more affordable systems.
Companies that manufacture CubeSat optical systems need an accurate and reliable method for developing an optical design, opto-mechanically packaging the system, as well as modeling structural and thermal impacts that the system will experience in-orbit. This article series will walk through the high-level development of a CubeSat system by leveraging the Zemax and Ansys software suites. We will illustrate how an integrated software toolset can streamline the design and analysis workflow.
Authored By Jordan Teich & Flurin Herren
Introduction
For decades, optical systems have been developed for operation in low, medium, and high Earth orbit. For many optical systems, the packaging form factor and the opto-mechanics that stemmed from this form factor were designed on a system-by-system basis. CubeSats are a class of lightweight nanosatellite that can house optical systems for applications ranging from laser communications to earth imaging. They are unique in that they use a standardized size and form factor.
For this article series, the paper Optical Design of a Reflecting Telescope for CubeSat1 was used as a reference for developing the CubeSat optical design.
In Part 2 of this series, we will cover the conversion of the optical design into Non-Sequential mode and then walkthrough the process of taking the system into OpticsBuilder. We will then demonstrate how OpticsBuilder was used to generate the CubeSat opto-mechanics and discuss how the optics were housed within the CubeSat form factor.
Using Non-Sequential Mode to Prepare for OpticsBuilder
Many optical systems can be exported directly from OpticStudio’s Sequential mode into the OpticsBuilder environment. When an optical design is imported from Sequential mode to OpticsBuilder, the Prepare for OpticsBuilder tool automates the transfer of the design to Non-Sequential before saving it as a ZBD file. However, if the file does not trace as expected in Non-Sequential mode, the model may not be converted properly. Due to the nature of this specific CubeSat design, edits must be manually implemented in Non-Sequential.
For this design, rays need to trace through a cut-out located at the bottom of the primary mirror to reach the image plane. Since this cut-out was not created in Sequential, the rays will not trace to the image in Non-Sequential.
Figure 1: Initial Non-Sequential Import
Due to the nature of Non-Sequential ray tracing, rays bounce off the primary mirror since the mirror is still a solid object. To create a cut-out, we can utilize Boolean logic with the Boolean Native object type. You can read more about the Boolean Native object and how to use it in the following Knowledgebase article: How to use the Boolean CAD, Boolean Native and Compound Lens objects, and the Combine Objects tool
To implement the necessary logic, a Cylinder Volume object can be positioned such that it overlaps a portion of the primary mirror. The Boolean Native object can then be directed to generate an object that results from subtracting the Cylinder Volume from the primary mirror. The result is a semicircular cut-out that allows rays to trace to the image plane uninhibited.
Figure 2: Implementing Primary Mirror Aperture
With the addition of this cut-out, the base optical design is now finalized. To verify that system performance hasn’t changed since the export from Sequential, the spot size at every defined field point can be viewed in Non-Sequential with the Detector Viewer. When a model is brought into Non-Sequential via the Convert to NSC tool, sources and detectors will be generated such that each detector corresponds to a field point’s location on the image plane in Sequential. By performing a ray trace and analyzing the spot on every detector, the general shape and size can be compared against the Spot Diagram in Sequential mode.
To provide an example, the pictures below illustrate the spot size and shape for Field Point 1 (on-axis) in Sequential vs the generated spot on a Detector Viewer in Non-Sequential.
Figure 3: Sequential (left) vs Non-Sequential Spot Size (Right & Bottom)
It can be seen that the spot size compares favorably between both modes for the on-axis field point. The spot size in Non-Sequential mode can also be confirmed by looking at the Beam Info tab in the Detector Viewer (bottom photo). Note that in the Sequential spot diagram, the units are in um while for the Non-Sequential Detector Viewer the units are in mm. For every field point, the spot shape and size compare favorably. Across all field points, the delta RMS spot radius between modes was at most 0.14um. This gives confidence that the optical design was successfully transferred from Sequential to Non-Sequential mode and that the performance remains unchanged after modifying the system in Non-Sequential.
Exporting Optical Design into OpticsBuilder
With the optical design work completed in OpticStudio, development of the opto-mechanics and external CubeSat packaging can begin. Due to the standardized form factor of CubeSat systems, size constraints become a major consideration of the opto-mechanical design. Since this optical system was designed to fit within a 3U CubeSat form factor, limited space is left for the opto-mechanics. It is also important to consider that while this structure will secure and shield the optical train, the opto-mechanical structure itself can induce stress into the design.
To begin this development process, the system first needs to be exported into a CAD environment. For this design, the Creo Parametric 4 environment was utilized. The Prepare for OpticsBuilder tool is a native feature within OpticStudio that can accurately import an optical design directly into a CAD environment.
When importing an optical system into CAD software, OpticStudio packages the relevant information into a ZBD file. To properly convert the optical system into a ZBD file that is compatible with CAD software, the Prepare for OpticsBuilder tool automates a few tasks for the user. You can learn more about the Prepare for OpticsBuilder tool in the following Knowledgebase article: Prepare for OpticsBuilder.
Since the optical design is already in Non-Sequential, the process for generating the initial ZBD file is simplified. The tool will confirm that all objects are compatible with the target CAD program before running a ray trace. The results of this ray trace will be used as an important reference once the ZBD file is brought into OpticsBuilder.
The packaged ZBD file contains allowable deltas for three different system metrics: the overall spot size, beam clipping, and image contamination. After importing a ZBD file into OpticsBuilder, a simulation is performed using a saved ray set which verifies that every metric is within an allowable delta set by the user. This confirms that the system performance of the imported optical system has not changed. The picture below showcases the CubeSat optical design after it was initially imported into OpticsBuilder within CREO Parametric.
Figure 4: Simulation After Import into OpticsBuilder
After running the simulation, we can see that all three metrics have been met and that the system has been successfully imported. With the full optical system now represented in OpticsBuilder, the design can be modified as needed to create the necessary opto-mechanics. Any changes made to the design will be saved to the ZBD file. The ZBD file format allows for easy transfer between OpticStudio and OpticsBuilder. With this streamlined workflow, it is straightforward for optical and optomechanical engineers to make iterative adjustments to the design.
Opto-Mechanical Considerations for a CubeSat Design
When designing a space payload, factors such as the in-orbit operational temperature and the vibration loads the payload will experience during launch need to be accounted for. For the purposes of this example, the operational temperature conditions were the main design factor that was considered.
When designing opto-mechanics for operation in low earth orbit, the opto-mechanical structures and optics will be subject to fluctuations in temperature. As the CubeSat payload experiences different temperatures in-orbit, expansion and contraction of the optics and opto-mechanics can degrade optical performance. Thus, material selection for the mirror substrates and opto-mechanics should be carefully considered to minimize CTE mismatch. This degradation in optical performance can be assessed with FEA analysis and the OpticStudio STAR module in a future modeling step.
For space payloads, stay light effects are also important to account for. Accounting for stray light can result in the development of baffling within the opto-mechanical model. For the purposes of this design, we have simplified stray light considerations by assuming that the CubeSat’s solar panels shield the detector from most stray light effects.
The positioning of opto-mechanics in relation to the light path must also be considered. The smaller form factor of CubeSats can cause challenges due to the limited real estate available for opto-mechanical structures. Whether the opto-mechanical design is successful in this regard can be assessed with the OpticsBuilder Simulation tool. This feature will run a ray trace within the CAD environment and the trace will consider all opto-mechanics of interest. The Region of Interest feature in OpticsBuilder can be utilized to exclude specific mechanical components from the simulation if they will not affect the results. The allowable deltas for spot size, beam clipping, and image contamination can be re-computed to analyze changes in performance due to the addition of opto-mechanical support structures. If these three metrics are within the allowable deltas that were previously set, the opto-mechanics can be considered to not negatively impact performance. These design considerations are integral to the final opto-mechanical design that was created for this CubeSat example.
Design of CubeSat Opto-Mechanics
First, the external frame of the CubeSat was developed. The external frame was designed to the standard form factor of a 3U design. 2U’s of space are devoted to the optical design and opto-mechanical structures. The final 1U of space is allocated to the electronics and detector.
To design the external frame of the CubeSat, a specification drawing created by the California Polytechnic State University was utilized as a resource.
Figure 5: External Frame Specification for 3U CubeSat2
Using this specification as a reference, the external 3U CubeSat was sketched in CREO Parametric. The following picture showcases the external frame without any optics present.
Figure 6: External Frame for 3U CubeSat
With the external frame developed, the ZBD file was then placed within the structure. Opto-mechanics were then built to retain the optics and mate them to the external frame. With the considerations described above in mind, an opto-mechanical design for the 3U CubeSat was created.
Figure 7: CubeSat Opto-Mechanical Design
The main frame (C and B in the above drawing) is made from a combination of carbon fiber (C) and 36 Invar rods (B) to prevent the entire system from expanding. To compensate the expansion of the mirrors under temperature, the optics are retained with spring loaded bolts (D). To prevent beam clipping, the secondary mirror is retained with an angled spider structure (A). With the opto-mechanical design in place, the influence of these components on optical performance can be directly tested in CREO Parametric with the OpticsBuilder Simulation tool. For the final simulation shown in Figure 8, the full model was encased with shielding.
From running the simulation, we can see that all design metrics were met. With the opto-mechanical model finalized in OpticsBuilder, the fully built system can now be exported to finite element analysis (FEA) software. FEA software can be used to generate structural deformation datasets for both mirrors. Finally, this data can be exported to OpticStudio’s STAR module for further analysis.
Figure 8: Simulation of Final Opto-Mechanical Model
Conclusion
In this article, we covered how the optical performance of the CubeSat was verified after being imported into Non-Sequential mode. We demonstrated how the final optical design was imported into OpticsBuilder and detailed the opto-mechanical structure used to package the 3U CubeSat. Finally, we detailed how to verify optical performance with the OpticsBuilder simulation tool once the opto-mechanics have been finalized.
References
- Jin H, Lim J, Kim Y, Kim S. Optical Design of a Reflecting Telescope for CubeSat. J Opt Soc Korea. 2013;17(6):533-537. doi:10.3807/josk.2013.17.6.533
- Cubesat Specification Drawings.; 2020. https://static1.squarespace.com/static/5418c831e4b0fa4ecac1bacd/t/621941d8e53eb916a609611d/1645822427304/CDS+Rev14_1+Drawings.pdf. Accessed May 19, 2022.
Next article: From Concept to CubeSat Part 3: Using Ansys Zemax Software to Develop a CubeSat System