@Lisa.gambcorti 

Q1: Can you control the sampling of the surfaces in case of shorter wavelengths?

A1: The fit generated by STAR creates a continuous surface deformation from the discretized FEA result, so there is no actual sampling that you need to worry about in case of shorter wavelengths. However, if you want to capture deformation effects on a very small scale you might need to increase the number of nodes of the FEA mesh. The fit could also be adjusted to capture the FEA node deformation more tightly by increasing the Max Level or the Grid parameters in the STAR Fit Assessment tool.

Q2: Have you validated with environmental test your model results?

A2: The CubeSat example was not validated with any hardware tests as the intent was to demonstrate the workflow with Ansys Zemax software. The STAR module itself has been used and tested by several costumers where the results matched their measurements. In addition, we've compared to the OpticStudio "Make Thermal" tool, to Grid GRINs (using .GGD files), as well as other known results - like deformations with a known shape, where we can verify that the fit is returning the expected shape.

@Bruce.Cannon 

Q1: How does this CREO Ansys and FEA differ from the earlier Ansys product Sigfit?

A1: SigFit is not part of the Ansys portfolio, but an independent software product from Sigmadyne. The STAR Module is built directly into OpticStudio which enables you to use a majority of the OpticStudio tools and analyses and see the impact of the FEA data inside of the OpticStudio immediately after loading the FEA data. In addition, the STAR Module features visualizations and a UI that is easy to use and fit into a variety of workflows.
The CREO CAD package was used to create the mechanical geometry with the help of OpticsBuilder. Ansys Mechanical was used to perform the thermal structural FEA simulation.

Q2: Does this come in as different configurations, so like ambient vs colder?

A2: Currently STAR does not support the use of multiple configurations. Instead, each of the datasets is imported to a different OpticStudio file. You can swap the FEA data in one file, but you would have to do a fit each time you changed the FEA data file for a surface. A surface cannot be assigned more than one FEA dataset at a time.
If you are interested in the difference for STAR effects applied, there are several ways to disable the FEA data momentarily or evaluate the difference in a wavefront plot.

@Harvey.Spencer 

Q1: What is the FOV?

A1: The FOV of this example system is ~0.38 degrees. Per the paper that was used as a reference for the optical design, this design can theoretically “take images of a ~4km x ~2.3km area of the Earth’s surface at 700km altitude”. 

Q2: Since the mirrors in this design are aluminum wouldn't you want the housing to be the same aluminum so that the figure changes of the mirrors over temperature will be matched by the proper change in the airspace between the two mirrors?  If the housing has essentially zero expansion, I would think that the system would go out of focus over temperature.  Maybe it has something to do with the spring-loaded mounting bolts?

A2: This is a very interesting suggestion and our material choice was not impacted by the spring loaded mounting. There are designs where you would build the entire instrument from the same material, even machine it from one huge block. We have gone with a different material choice here to show the effect with the new capabilities within STAR. Furthermore, if you would look at a case with a temperature gradient across the assembly instead of the uniform temperature, aluminum would cause much larger deformations that don’t cancel out, so using the same material for all parts is not always the best choice.

Q: Can the Prepare for OpticsBuilder tool in OpticStudio also be used in Sequential mode?

A: Yes, if the optical system in OpticStudio is still in sequential mode when running the Prepare for OpticsBuilder tool, OpticStudio will automatically convert the sequential file to a non-sequential one. This includes: converting the sequential surfaces to matching non-sequential geometry, check to make sure rays are positioned and angled correctly, confirm that spot size change has remained below an allowable value and allow the user to open and inspect the non-sequential. 

Q: You mentioned that the telescope was simulated on-earth but will be utilized in low earth orbit. Are the effects of the vacuum of space considered when looking at performance in OpticStudio?

A: Yes, the vacuum of space can also be considered in an analysis like this. We did go through the process of analyzing the structural deformation effects that result from simulating the pressure environment of space. However, it was found that in comparison to the structural deformation experienced by the optics from the temperature change, the deformation that occurred from just changing the pressure was 2 orders of magnitude lower. Thus, pressure effects did not severely degrade optical performance. For a full analysis of a real space-based system, you will want to include pressure effects even if it has a small effect on performance, just for the sake of having a full engineering model. Simulating temperature and pressure effects also allows for a more accurate comparison when analyzing test results of a system soaked in a thermal vacuum chamber.

@KenC 

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.

• Methods for stray light analysis in OpticStudio - webinar – Knowledgebase (zemax.com)
• Introduction to stray light analysis - Part 1 – Knowledgebase (zemax.com)

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: 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.

@Patrick.Thompson 

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: 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: 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.

@ofir 

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.

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.