The details of the webinar are below! This thread will be used to collect questions before the webinar, and to answer any questions we received during the webinar. Feel free to post your questions!
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Date: Thursday, February 24th
Time: 6:00am PST & 11:00am PST
@Jordan.Teich, Application Engineer II @Flurin Herren, Application Engineer II @Matthias.Schlich, R&D Engineer II
CubeSats are a class of nanosatellite that are designed to operate within standardized dimensions of 1U cubes (10 cm x 10cm x 10cm). They can vary in size from 1U to 6U payloads. In the aerospace market, CubeSats have emerged as a lower cost solution for space-based optical systems. To design a CubeSat system, a workflow needs to be defined for developing the optical design, opto-mechanically packaging the system, and modeling structural and thermal impacts that the system will experience in orbit. In this webinar, we will demonstrate how the Zemax software suite can be leveraged to take a CubeSat design through these modelling steps. Starting with an optical design in Zemax OpticStudio, we will showcase how to apply an opto-mechanical retaining system to the design with Zemax OpticsBuilder, run an FEA analysis with Ansys software, and how FEA data can be used to examine impacts on system performance with OpticStudio’s STAR module. Learn how these discrete modelling steps can be united with Zemax software.
Thank you everyone for joining this webinar.
In this thread, our speakers will answer the questions they didn’t get to during the live event.
If you have additional questions, post them here! The thread will be open until March 10th to allow for discussion with the speakers.
Where to find the recording
The webinar may be found on-demand at this link: From Concept to CubeSat: Design and Iterate Faster with Zemax Software.
During the webinar, there were some technical issues with the slides. Please note that those issues were resolved in the on-demand recording!
Q: Can shock loads from the rocket launch be simulated to assure your mounts will not deform the optics?
A: Yes, of course. This was not done here because the purpose was to show the workflow using a CubeSat as an example. For this, only the deformations during the operation phase were of interest. The existing FEA analysis could easily be modified to simulate the loads encountered during the launch. It might be necessary to reinsert the outer panels to simulate the interactions and the mesh density on the mirrors should be lowered as the deformations are not needed for STAR.
Q: How to relate the number of Nodes to the quality of the mirror (in RMS)?
A: I’m assuming you were referring to the fit quality of the mirror within STAR. There are many different ways to influence the fit quality. Among other things, there are two adjustment screws in the STAR Fit Assesment tool, the Max Level and the Grid parameters. Internal research has shown that the default values for these work best with a mesh of roughly 10 thousand nodes. The exact behavior varies and differs from case to case, that is why the Fit Assessment tool is very helpful to assess the error.
Q: How would you model thermal gradients such as would be experienced in orbit when one side of the cubesat is illuminated by the sun and the other is not?
A: If you already know the temperature gradient, mapping it to the mesh of the structural FEA is a simple step. The next steps are similar to those presented in webinar: the thermal expansion will cause a certain deformation, that can be exported and used in STAR.
If you do not know the gradient, you may need to perform a thermal analysis first, the results of which will be used in the structural analysis. There are several options for thermal analysis: You can study the transient behaviour or consider a steady state. It would even be possible to use the absorbed flux of a non-sequential analysis (simulating the illumination of the sun) as a boundary condition for the thermal analysis.
Q: Why for step #3 (Mechanical design) do you use CREO not Speos (Ansys)?
A: In order to show-case the workflow with mainly Zemax products, Zemax OpticsBuilder was used and as OpticsBuilder is an Add-in for Creo Parametric 4,5,6 and 7, the CAD Environment had to be Creo. However, the mechanical design could have also been done within Speos. In this case you would want to export a STEP file of the optical system out of Zemax OpticStudio and import it into Speos to assemble the optomechanical system. But please keep in mind that other than the .ZBD file, the STEP does not hold any optical data, but only the geometry of the optical components.
Q: How do you verify the design simulations since so many tools used?
A: After the initial design within sequential mode of Zemax OpticStudio, the Convert to NSC Group tool can be used in order to automatically convert it to the Non-Sequential mode (To subsequently perform the boolean operation). Within that tool the Critical Rayset Generator can be used to verify the performance after the conversion.
Once the optical designed is finished within OpticStudio and the optical system was passed on via .ZBD file. The Optomechanical Engineer can verify the optical performance by running a simulation, within the results of that simulation OpticsBuilder will differentiate between the OS (OpticStudio) Baseline and the OB (OpticsBuilder) Baseline, so the optical performance without and with the influence of the mechanical components.
Q: In the Opticsbuilder/creo, is it possible to import CAD files for structural parts and integrate with the lens design?
A: The usual workflow for a Optomechanical system working with both Zemax OpticStudio (OS) and Zemax OpticsBuilder (OB) would be the following: A optical system is designed, analysed and optimized within OS, after that it is passed on to the CAD Environment into OB with via a .ZBD file. Once the optical components are imported into the CAD Environment the mechanical components are added to the assembly making it a optomechanical assembly. This can be all sorts of CAD files (E.g. STEP files), these mechanical components can then be assembled to the optical parts with the native CAD tools such as assembly constraints.
Q: After adding the mechanical supports and structure, what is the final usable aperture of the optical system?
A: As the mechanical components do not limit the optical aperture, the aperture is still same as set in the initial sequential mode in OpticStudio, which is defined by the Entrance Pupil diameter and limited by the STOP surface on the first surface. So in this case this would be 55mm.
Q1: Was this system in space?
A1: The CubeSat system discussed in this webinar is a completely simulated system that was built to illustrate the OpticStudio → OpticsBuilder → FEA → STAR Module workflow. While this system was not manufactured and put into space, this workflow demonstrates how such a system could be simulated with Ansys Zemax software.
Q2: How was the temperature estimated at orbit?
A2: For this CubeSat example, we assumed that the system would operate in Low Earth Orbit at 700km. Originally, the operating temperature of the optics in the Hubble Telescope was used as a comparison point. For the Hubble, the operating temperature for the optics was at 21C, or room temperature. However, if 21C was used for the operating temperature of the CubeSat optics, the results using STAR would not have been very interesting. This is because the Sequential model as “built on Earth” was also assumed to be soaked at 21C. To showcase the advantages of the STAR module, an assumption was made to lower the operating temperature of the optics by a few degrees. By choosing an operating temperature of 15C +/- 3C, interesting results with STAR could be obtained. This is because the temperature condition in-orbit is now different to the temperature condition on Earth. While the external frame of the CubeSat will experience larger fluctuations in temperature, space-based optical systems will generally use a thermal control system to prevent the optics from experiencing those fluctuations.
Q1: How does Convert to NSC know where to place the detectors in NSC? Is it based on the position where the chief ray strikes the detector in sequential mode?
A1: When the Convert to NSC tool is used, all sequential fields defined in the Field Data Editor are first converted into equivalent Non-Sequential Source Objects. Each generated Source Object will be paired with its own detector. All detectors are positioned at the same global Z position in NSC which is determined by the position of the image plane in Sequential mode. Each detector is then properly positioned in X and Y by being centered about that field’s centroid at the image plane.
Q2: Could you go over how the Invar keeps the mirrors at the same separation, even though the frame is expanding/contracting?
A2: The mechanical design allows the frame to expand independently from the invar rods which have a very low coefficient of thermal expansion. This is being realized by allowing one side of the rod to slide in and out of the frame while the other side is firmly connected. The mirror retainers are only connected to the invar rods and are therefore unaffected by the expansion of the frame.
Q: What is the meaning of a spot smaller than the Airy Disc? How should we interpret and use this information?
A: In this example, the spot size was used as one of the metrics to judge the optical performance of the system. An optical system will have a performance limit based on the size of the Airy Disc, or the diffraction limit. The effects of diffraction are important to consider as the size of the Airy Disc represents the smallest size to which a spot can be focused down to. By achieving a spot size that is smaller than the Airy Disc, we determined that the CubeSat optical performance was limited by diffraction. No matter how small the spot is focused down to in the Spot Diagram, if the spot size is smaller than the Airy Disc, the Airy Disc size will remain the smallest achievable spot size for a given system. This means that for the CubeSat example, the smallest achievable spot size has a radius of 8.905um (the radius of the Airy Disc). The spot size should be interpreted as an example of a metric that can help determine a system’s optical performance. However, other metrics can and should be considered. Wavefront error, MTF, and detector resolution are all metrics that can be important when analyzing system level performance. For this webinar, the Standard Spot Diagram was used as an example to highlight how the STAR module can interact with OpticStudio’s Sequential analysis features.
Q: Is sequential → non-sequential transfer replaceable with a user defined aperture in sequential?
A: Yes, the Sequential → Non-Sequential transfer can be replaced with a workflow that uses a user defined aperture to generate the cut-out at the bottom of the primary mirror. In our example, a Rectangular Aperture was already being used for the primary mirror in Sequential mode to model the rectangular shape of the optic. Because of this, it seemed more streamlined to define a Rectangular Aperture for the primary mirror in Sequential and then move the design to Non-Sequential for implementation of the primary mirror cut-out via Boolean logic. If the desired primary mirror aperture can be generated with a user defined aperture file, then that workflow can be used!
Q1: What is the reference for the mirror design?
A1: The optical design for this CubeSat example was referenced from the following paper: Optical Design of a Reflecting Telescope for CubeSat (Jin, Lim, Kim and Kim, 2013). This paper was published in the Journal of the Optical Society of Korea in 2013.
Q2: How many rays were you tracing during the stop analysis?
A2: When you refer to the “Stop Analysis”, I assume that you are referring to the analysis completed with the STAR module in OpticStudio’s Sequential mode. While Non-Sequential mode allows the user to define the number of analysis rays that will be traced through the system from a defined Source Object, Sequential mode (where the STAR module is used) works a bit differently. When FEA data is applied to the optics via the STAR module, no rays are traced during the fitting process. Once FEA data is loaded via STAR, the module can work in conjunction with Sequential analysis features. The loaded FEA data will now affect how rays behave when they interact with the optics. The number of rays traced through the system depends on the analysis feature being used. For example, the Standard Spot Diagram analysis has a setting called Ray Density which specifies the number of rays to be traced during the analysis. To provide one more example, the FFT MTF analysis has a setting called Sampling which refers to the size of the ray grid used to sample the pupil. For the CubeSat example, a Ray Density of 6 (which refers to 6 hexapolar rings that trace 126 rays overall) was used for the Standard Spot Diagram and a Sampling of 128 x 128 rays was used for the FFT MTF.
Q1: I'd like, if it’s possible, to iterate the last bit with STAR and how you figured out the correction for temperature.
A1: Hi Reuven! I believe that your question sounds like something I covered in the slides during the webinar presentation. I know that there was a technical error during the webinar that prevented the slides on STAR and the details surrounding the temperature correction from being shown. At the top of this Community post, there is a new link for the on-demand webinar where this issue has been fixed! If you open the on-demand link and go to the last section of the presentation, the slides on STAR will now be visible. If you still have questions on the STAR Module or the correction for temperature, please reply here and I’ll be happy to answer.
Q2: And why the 3 temperatures? 12C, 15C and 18C?
A2: On this post, Aleksander Makarov asked a similar question. I recommend looking at the answer that I wrote for them to see if it answers your question! Aleksander’s question was about how the temperature was estimated at orbit. To add a few more details, once 15C was chosen as the nominal temperature, a range of +/- 3C was chosen as the possible fluctuation range for the operating temperature. In a space-based optical system, it is unlikely for the optics to remain soaked at the exact same temperature. There will usually be some drift in temperature over time. Looking at FEA data for 12C, 15C, and 18C allows us to examine data at the boundaries of this range and at the nominal temperature. If desired, FEA analysis can also be run with additional temperature data points to achieve a finer sampling of the data.
Q: Can you deform a mirror in FEA and analyze the optics in the Optics Studio? What would the workflow be?
A: Yes, actually the FEA analysis done in this example was a structural analysis, only that the deformation was caused by the thermal expansion. You could always add structural loads to the mirror that cause deformations. The workflow would be exactly the same as the one presented, with the deformation results being exported via the ACT and loaded into OpticStudio with the STAR extension.
Q: Is it possible to extract information about the optical axis changes of each lens during FEA analysis?
A: There is a way to evaluate the rigid body motion (which is comparable to the optical axis change) with a remote point that moves based on the deformation of the nodes in Ansys. In the STAR module the rigid body motions (RBMs) are calculated and extracted before the fit and you can investigate them in the Structural Data Summary or the Fit Assessment tool.
Q: When importing surface deformations from FEA to the STAR module, how are rigid body transforms applied to ensure the correct importation of the deformations?
A: The rigid body motions of the surface are handled by the same algorithm that handles the fitted deformations. You can even choose to not extract the rigid body motions before the fit in the STAR Fit Assessment tool.
If you are worried about the coordinate transformation during the import, there are options as well. While loading the FEA data you can choose a user-defined coordinate system, but if your Global Coordinate Reference Surface in OpticStudio matches the coordinate system of your FEA data, that is not necessary as the STAR module will handle all of that.
Q: Which surface types are supported by STAR?
A: Deformation data can be applied to all sag-based surfaces. This means that the only surfaces not supported by STAR are idealized surfaces (e.g., Paraxial) and surfaces that do have some effect that is not related to the geometric shape (e.g., Grid Phase, Coordinate Break).
Q: What about Launch Loads (both quasi-static & dynamic) ? . . . micro-yield in optics & bench ?
A: We did not analyze launch loads as the optical performance will be only necessary on orbit. Furthermore it was just intended as a use case example to show the capabilities of our software. For the mechanical design you would probably have to do a lot more analyses and tests to ensure that all components survive the harsh conditions at launch. Within Ansys there are all kinds of analysis types that could help with that investigation but for the scope of this example we focused on a simplified on-orbit case only.
Q: Was a rough tolerance budget made during the initial design to get a rough idea if the optic tolerances can be maintained by the mechanical tolerances?
A: For this example, there was not a defined tolerance budget used for determining whether optical tolerances could be maintained by mechanical tolerances. When going through the process of designing such a system, this is definitely an important area of consideration. If mechanical tolerances aren’t sufficiently tight enough to control the optical tolerances needed for best performance, then a different design issue would present itself. The primary purpose of this design example was to illustrate the OpticStudio → OpticsBuilder → FEA → STAR Module workflow. Because of this, we did not go in depth on all design considerations that would be made when designing a real system. This would be a great topic to investigate in the future for further validation of the opto-mechanical design.
Q: Hello, thanks for this webinar. I am new to this area. I have a question about the optical design of the optical system. In the overall payload specifications was shown a Ritchy-Chretian design using two mirrors for a 3U volume, resulting in a 12.4 f-number. It is hard to think about a design fitting a 1.5U volume maybe increasing the number of mirrors to obtain a ~3.0 f-number and good quality image, or it is necessary to implement a more complex system just as free-forms or catadioptric?
A: As you mentioned, this specific implementation of the Ritchy-Chretian design form was designed to have a f-number of 12.4, which is a slower system. If you are looking to try and design a reflective CubeSat system with a f-number of 3.0 where the optical train fits 1.5U-2U’s of space, different design considerations need to be made. While I can’t speak to if an F/3 design is feasible in 1.5-2U’s of space, developing a catadioptric system or using more complex optics like freeforms could help with achieving this design goal. If you wanted to increase the number of mirrors to improve the design, a bigger CubeSat may be needed. As I mentioned in the presentation, CubeSat sizes can range from 1U to 12U.
Q: Will redesign be necessary when considering the heat generation in the allover payload adjacent to the mirrors, and what about recommended margins?
A: I assume by “allover payload”, you are referring to if heat generation from the electronics will affect the optical design. While we did not consider the effect of heat generation from the electronics adjacent to the mirrors in this example, this would be an important consideration to make when evaluating a real system. It is possible that a thermal control system could be implemented for the CubeSat that limits the amount of heat the mirrors experience from the electronics. It could also make sense to consider heat generation from the electronics when evaluating the operating temperature of the mirrors. If the heat generation affects the operating temperature to a large enough degree, the operating temperature range could be adjusted to account for this. FEA analysis could then be run with this new operating range to see how the optical system performs. Whether a redesign is necessary depends on the amount of heat the electronics generate and if the optical performance is severely affected.
Q1: Can you also analyze the image contrast from out of field light (clouds, ice, etc.)?
A1: While we did not consider stray light in this example, analyzing stray light effects can be done with OpticStudio’s Non-Sequential mode. In Non-Sequential, you can define a Source Object and modify it such that it represents the properties of stray light that are expected to interact with an optical system. To provide one example, let’s consider we wanted to model stray light that enters the CubeSat after reflecting off a cloud. A Source Object like a Source Ellipse could be placed at a specific out of field position to model the stray light source. A second object could then be placed in front of the Source Ellipse. Next, a scatter profile representing the scatter profile of light after interacting with a cloud could be applied to this object. Using Non-Sequential ray tracing, rays from the off-axis source can be traced and a Detector Viewer can be used to examine how much energy makes it to the system’s detector. The incident energy from this stray light source can now be compared to the amount of energy that would normally make it to the detector to assess the impact of stray light. On the Zemax Knowledgebase, we have a webinar on stray light analysis and a 3 part article series that you may be interested in! I have linked to the webinar and part 1 of this article series below.
Q2: Were the aluminum mirrors made of a composite to have a lower expansion coefficient? If not, why not make the Invar rods out of aluminum?
A2: The material choice here was not done by evaluating all the possible options against each other and choosing the best one, instead we just tried to come up with a reasonable assumption about the materials being used in such a CubeSat. The main purpose was to show the capabilities of OpticsBuilder and to generate FEA data that could be used in the STAR analysis.
Q: What was the main motivation for using Boolean logic to create the cut-out at the bottom of the primary mirror instead of, for example, an imported CAD file?
A: The main motivation for using Boolean logic to create the cut-out at the bottom of the primary mirror comes down to having a more streamlined workflow. When you use a CAD component in Sequential, this changes how you need to treat the object for optimization purposes. For CAD components, if you want to optimize parameters like the radius of curvature or the conic, you will need to utilize the Dynamic CAD link which adds an extra step to the process. Using native OpticStudio objects also allows for easy adjustment of other features that were important to this example such as the aperture size. Generally, if a part isn’t too complex, I prefer to use native OpticStudio surfaces to streamline the workflow.