Stray Light Analysis – Telescope System On-Axis – Ansys Optics

Zemax OpticStudio Speos Macroscopic optics A & D

The goal of this article is to show how to perform stray light analysis in Ansys Speos on an on-axis telescope system, understanding the impact of baffles and coatings can have on the prevention of stray light in optical systems. Following the stray light analysis workflow documented in Stray Light Analysis – Smartphone Camera – Ansys Optics , a Ritchey-Chretien telescope will be exported to Speos from Zemax OpticStudio using the Optical Design Exchange feature. In Speos, optomechanical components will be integrated into the optical system before starting the stray light analysis.

[[NOTES:]] Software Prerequisites

To follow this example, the following tools need to be installed:

Ansys Zemax OpticStudio 2025 R1 or more recent releases (optional for exporting the .odx file)
Ansys Speos 2025 R1 or more recent releases
CAD Editor - Speos is compatible with most native CAD files through SpaceClaim. See Additional Resources section for a detailed list of supported CAD software and format.

Authored by Laura Martin Jimenez, Flurin Herren, Noah Hamstra

download example

Overview

Understand the simulation workflow and key results

Stray light analysis is one of the most common problems that impacts an optical system’s performance. Unwanted light within the system can degrade optical performance by reducing image quality, increasing background noise, and lowering overall system efficiency.

There are several components and parameters that play a crucial role in preventing stray light in optical systems. The article Stray Light Analysis – Mechanical Geometry Modification – examines how changes to mechanical components can influence stray light suppression in a double Gauss optical design. In this example, we will go one step forward and analyze how optomechanical components, like baffles and different surface coatings can reduce the unwanted light in a Ritchey-Chretien telescope.

The workflow above outlines the process for conducting system-level stray light analysis in a Ritchey-Chrétien type Cassegrain telescope.

[Optional] Transfer Zemax Optical Design to Speos (ODX): telescope designed in OpticStudio will be exported to Speos using Optical Design Exchange (ODX) export. A Ritchey-Chretien telescope is used in this article.
Importing mechanical components into Speos: In the CAD environment of choice, mechanical components will be designed, modified, and used to design the optomechanical parts of the system. These components will then be imported into Speos for optical simulation and analysis.
Speos simulation set up: Stray Light Analysis. Prepare the simulation for the stray light analysis, including sources, sensors and all the geometries.
Comparison Parameters for Stray Light Analysis: Baffle and/or Materials/Coatings
1. Analysis of the impact of the baffle on stray light in the telescope: studying the optical system with and without the designed baffle.
2. Applying different optomechanical coatings: we will use Speos material library to select an additional finish to compare to different coatings: Vanta Black and Mescalito Black.

Run and results

Instructions for running the model and discussion of key results

Step 1: Transfer Zemax Optical Design to Speos (ODX) [Optional]

The example of this article is based on Ritchey-Chretien telescope type, which can be found in the downloadable data of the article.

To export the telescope system from Zemax OpticStudio to Speos we will use Optical Design Exchange Export, generating an *.odx file format, that can be imported directly into Speos. However, before doing the export, there are some modifications that need to be made to adjust the design to the optical components:

In Zemax OpticStudio, open RCTelescopeZMX_FINAL_200mmDiameter.zmx , which can be found in the downloadable data of this article, inside the folder END > Zemax files.
Draw the mirror substrate for the mirror surfaces in the design. The mirror M1 has 4mm thickness and mirror M2 has 2mm thickness. You can do that in the Lens Editor > Select the mirror surface > Surface Properties > Draw > Thickness. This will update the thickness for the mirror substrate produced with the optical design exchange file.
Finally, on the File tab, under Export options, click on Export Optical Design to Speos and wait for the generation of the *.odx file.

Both tools are designed to seamlessly integrate for this purpose, ensuring an efficient and smooth stray light analysis workflow. The .odx file serves as a container storing information about the lens design, material and coating properties, stop surface, and sensor. By using .odx, both sequential and non-sequential components can be exported.

In Ansys Speos, in the Light Simulation tab > Components, click on Optical Design Exchange.
In the Simulation panel, click on the new Optical Design Exchange component to open the definition panel. Then, in Optical Component > File browse and import the *.odx file you have just exported from Zemax OpticStudio.
Finally, click on compute and wait to see the Optical Design directly imported in Speos.

This first step is optional for following the article, as the ODX file is already included in the SPEOS input files folder within the downloadable data.

For more information about the content, supported surface, and object types, refer to the Optical Design Exchange Overview .

Step 2: Importing mechanical components into Speos

Next step in this example is designing the optomechanical housing and mounting around the mirror components. The housing for the telescope in this example has already been created using Creo Parametric 9.0, but a wide range of software formats are natively supported by Speos.

In CAD, the STEP geometry file of the optical component, which can be generated in OpticStudio, is imported as an assembly file. The mirror size and location are then used as a base reference to design and assemble the mechanical housing and baffles in a CAD platform. To ensure the housing was accurately fitted to the optical system, the telescope mirrors were imported into Creo as a STEP file. This allows for design of the housing precisely around the mirror geometry.

Once the optics are imported via the ODX into Speos you can then import the mechanics previously designed. Note that Speos can import native files from Creo. See additional native file compatibility at the end of this article

Open Ansys Speos.
In the Assembly tab > Part, click on the File icon, then select the parts saved for the mechanics of the design. You can find the Creo files for the mechanics of this article’s example in the downloadable data, inside the folder END > Mechanical files.

Once you have done this, you will have the mirror geometry twice (the ones corresponding to the ODX import and the ones corresponding to the STEP file imported in Creo). The mirrors of the imported geometry can be deleted as they will be replaced by the ODX mirrors. And as it is the exact same geometry, the mechanics will perfectly match the ODX mirrors.

Like previous steps, the final optomechanical design for the telescope is already imported into the Speos project, under the name rctelescope_optomech.asm.

Step 3: Speos Simulation Setup: Stray Light Analysis.

With optomechanics and the optical system already imported into Speos, the next step is to prepare simulation for an initial stray light analysis. You can find the Speos project with the optomechanics and the optical design exchange already integrated in END > Speos files in the downloadable data.

Start Ansys Speos 2025R1 and open “Stray Light Analysis – Ritchey Chretien Telescope.scdocx” from the downloadable data of this example. The project includes several predefined Speos objects to speed up the simulation setup. These can be found in the simulation panel (see image attached below).
Under Materials , different materials and coatings for the optical system are defined and applied. Mescalito Black and Vanta Black will be the coatings used for stray light analysis comparison.
Light Simulation tab > Components icon includes the Optical Design Exchange component with the *.ODX file imported from OpticStudio. Under this icon, the three optical elements that make up the system are specified: MirrorLens_2-2, stop, and MirrorLens_1-1. In addition, when importing the ODX file into Speos, additional Speos objects are created. Since 2025R2 release, ODX includes the generation of materials applied to the optical system, in addition to a local meshing component (more information about meshing best practices can be found in the Important Model settings of this article). An irradiance sensor has also been created. This sensor will be used for GPU layer by source simulations during the analysis.
For this example, several surface sources are created representing the sun, appearing under the Sources component.
Finally, the Sensors component includes the rest of the sensors used in the project. For this example, three 3D Irradiance sensors are defined for different system’s geometries: housing, primary baffle and secondary baffle (see image below). Additionally, and Irradiance sensor defined with layer by sequence has been created for CPU simulation.

In addition to the different Speos objects already described, inside the Simulation panel you can also find several pre-defined simulations.

For each coating (Mescalito Black and Vanta Black) there are four simulations defined: two for GPU computation and two for CPU. GPU simulations will provide results with separate layers for the sources of our system, while CPU simulation results will include sequence analysis with Light Expert activated to visualize the ray path inside the telescope. Finally, a simulation for detailed baffle stray light analysis has been created, where 3D sensors for telescope barrel, primary, and secondary baffles will be used.

In the definition panel for each simulation you can find the geometries, sources and sensors included, as well as general and stop conditions parameters.

The workflow for performing a complete stray light analysis with the telescope system follows the step-by-step description in Stray Light Analysis – Smartphone Camera – Ansys Optics , and therefore, the details of it won’t be covered in this example. Simulation results can be found in the next section of this article and in the downloadable data provided.

Step 4: Comparison of Parameters for Stray Light Analysis. Baffle and Material Coating Analysis.

The following section explores two techniques for mitigating stray light in an optical system. The first approach involves strategically placing baffles to obstruct unwanted light paths to reduce sensor noise. The second method evaluates the effectiveness of two different surface coatings, namely, Vanta Black and Mescalito Black, analyzing their impact on minimizing stray light contributions from reflections and scattering due to the coatings.

Baffle Impact on Stray Light Analysis

In this section, we will evaluate the impact of the designed baffle on the results. To do this, we will compare two simulations: one with the baffle included and one without it. Baffles are essential in telescope systems because they block stray light from entering the optical path to enhance image contrast and clarity. They help reduce internal reflections and background noise, leading to sharper and more accurate observations. By comparing this case with and without a baffle, the signal (imaging path) can be distinguished.

For the following cases, this is the model setup for both coatings without baffles.

[[NOTES:]] No baffles are included in this simulation.

In the following images we can see the results for Vanta Black coating applied to the housing and baffles without (left) and with (right) baffles. The results are recorded with a logarithmic scale, that will accentuate the results showing the stray light more clearly, but it is clear that using well designed baffles decreases the amount of incident irradiance on the sensor that is not the imaging path.

From the region highlighted in the previous image results, Light Expert can be used to find where the stray light is coming from by drawing a region of interest to overlay the rays on top of the 3D model. Here, it is clear that the lack of baffles are causing this large hot spot of additional stray light due to the light path travelling directly from the source, reflecting off the back housing lid, and converging on the sensor.

As a conclusion, using well designed baffles decreases the amount of incident irradiance on the sensor that is not the imaging path.

Given this study which includes various material and geometry cases, the simulations have been preconfigured to give results without any needed modification. However, to manually change which geometry is included in the simulation, these steps must be followed.

Go to Simulation tab > Simulations > GPU_VantaBlack_WithBaffles > Definition > Geometry, to remove the baffles from this run or compute the simulation named “GPU_VantaBlack_WithBaffles”. This case includes the baffles, but other simulations like “GPU_VantaBlack_WithoutBaffles” do not include the baffles.

Additionally, some “NS” groups have been created to easily keep track of which geometry has been applied in each simulation. These can be found under the Groups tab. Once the “NS” is selected, the geometry included is highlighted in the structure tree and the 3D editor window as shown below.

Analysis and Comparison of Results: Reduction in Signal to Noise

SNR (Signal-to-Noise Ratio) quantifies the clarity of an optical signal by comparing the amount of signal light (imaging path) to the unwanted variations (stray light). A higher optical SNR indicates better image or measurement quality, making it crucial for all applications.

Here, SNR is defined as: SNR = Signal/Noise.

Signal Flux

(Imaging Path) mW

Noise Flux (Stray Light) mW

SNR

Without Baffle (Mescalito)

224.77

0.434

516.5

With Baffle (Mescalito)

207.66

0.063

3296.2

The results show a 84.4% reduction in stray light , corresponding to a significant increase in signal-to-noise ratio from 516.5 to 3296.2. This highlights the importance of using baffles to suppress stray light and improve optical system performance. This result includes the total amount of stray light from all the source’s contributions.

Applying Different Coatings

Next, we will explore different coatings for the optomechanical components of the telescope, focusing on their influence on stray light. In this example, we consider two coatings available in the Ansys Speos Optical Libraries.

By default, the coating applied to the simulation is Mescalito Black. One quick and easy way to tell if a material is applied is to look at the cube directly left of the material. In the image below, the cube next to Mescalito Black is colored purple whereas the cube next to Vanta Black is transparent meaning that there is no geometry applied to that material.

To apply the materials, double select the material to be applied.
Then, use the geometry apply button in the upper left of the 3d editor to select which geometry to apply that material.
Lastly, validate the selection with the green check mark to apply the coating. To check the material was applied, double select the material of interest and look for the corresponding geometry is included in the “Geometry” heading shown below.

[[NOTE:]] Before assigning a new material to a geometry, ensure that the geometry does not already have a material applied. If it does, you must remove the existing material before applying the new one

Verify the correct coating is applied to the geometry and run with CPU or GPU respectively (image below).

These materials can also be found in the Speos Inputs folder included in the downloadable data for this example:

Vanta Black: This ultra black absorptive coating is a widely used black finish for optical systems due to its high absorption across a broad spectral range. It is commonly applied in stray light suppression and beam dumps, thanks to its excellent absorptive properties for both light and thermal energy.
Mescalito Black: Typically used in the automotive industry, this coating also offers effective light absorption characteristics, making it a candidate for optical applications where stray light control is important. Already used in previous simulations for this example.

The results for both coatings and baffles are included in the folder “Speos Output Files”.

Analyzing the Results with Light Expert

Auto-sequencing using the Light Expert tool plays a vital role in stray light analysis by identifying and filtering ray path sequences that contribute most significantly to unwanted light, particularly from optomechanical components. This process streamlines the detection of critical surfaces and interactions, enabling targeted design changes such as adding baffles or modifying materials. Sequences are ordered by relative energy, considering both the area covered and the amount of incident light, which helps prioritize effective mitigation strategies. In comprehensive analyses, the tool demonstrates that while coatings can reduce stray light, baffling has a greater impact on improving signal-to-noise ratios and overall image quality.

Here are the results for each of the cases: with and without baffles and with the 2 different coatings:

[[NOTES:]] SNR values have been calculated using the same method described earlier in this section, where signal corresponds to the imaging path, and noise flux to stray light.

The images below illustrate a sequential stray light analysis of a telescope system incorporating both primary and secondary baffles, coated with Mescalito black. Each sequence case examines the interaction of on-axis and off-axis rays with various system components, with emphasis on stray light paths and mitigation strategies.

Image 1: This case represents the nominal optical path for an on-axis source. Rays undergo specular reflection off the primary and secondary mirrors. Both baffles function effectively to suppress stray light by physically occluding undesired paths, resulting in minimal contamination at the image plane and producing a bright image.

Image 2: On-axis rays reflect from the primary and secondary mirrors, but some subsequently interact with the interior surface of the primary baffle. This interaction produces a residual halo due to forward scattering or partial reflection. Mitigation strategies include implementing internal vanes to break up and trap incident light or adopting materials with even lower reflectivity or higher absorptance as we have done earlier with the Vanta Black coating.

Image 3: On-axis rays reflect off the primary mirror and then scatter off the front face of the secondary baffle, leading to widespread low-level irradiance across the sensor. This indicates a forward-scattering or grazing-incidence reflection phenomenon. Solutions may involve beveling the baffle edge, changing surface geometry, or increasing the absorption of the front baffle face.

Image 4: Off-axis illumination introduces a specular reflection off the eyepiece housing or mounting structure, resulting in a localized bright stray light artifact. This suggests sensitivity to mechanical features not contributing to the optical path. Applying high absorption coatings or redesigning the eyepiece barrel geometry may help suppress this effect.

Image 5: Off-axis rays directly reach the image sensor, forming an intense hot spot near the bottom of the image plane. These rays bypass both baffles, indicating a failure in geometric occlusion. This represents a worst-case scenario for baffle performance and implies that additional aperture stops or longer baffles with tighter angular constraints may be necessary. In a full-field implementation, such rays can also produce a diffuse halo artifact.

Image 6: Off-axis rays reflect off the interior surface of the primary baffle and then strike the sensor. This reflection is strong enough to degrade image MTF. Effective countermeasures include using baffle vanes to trap multi-bounce reflections or incorporating high absorption coatings with higher broadband attenuation.

[[NOTES:]] further details of each sequence can be viewed in the Sequence Detection Viewer , which can be accessed in Tools>Sequence detection in the XMP Viewer when opening simulation results.

Important Model Settings

Description of important objects and settings used in this model

GPU and CPU computation

By comparing CPU and GPU performance, there is a dramatic reduction in computation time when using GPU acceleration. While both CPU and GPU simulations traced the same number of rays (10 billion), the CPU required 2 hours and 4 minutes , whereas the GPU completed the task in just 3 minutes and 37 seconds . This represents a ~96% decrease in simulation time. These results highlight the significant efficiency gains achievable with GPU-based ray tracing in high-fidelity optical simulations.

Computer specs:

CPU: 13th Gen Intel(R) Core(TM) i7-13850HX 2.10 GHz, 64Gb RAM
GPU: NVIDIA RTX 5000 Ada Generation Laptop GPU

Meshing best practices

The article Speos Meshing Best Practices outlines how meshing is essential for accurately representing 3D geometries in optical simulations. By using triangular surface meshing, simulation accuracy and performance are balanced. It is recommended to apply finer meshing to critical optical components while using coarser meshes for less impactful geometries to optimize simulation efficiency.

Optical Design Exchange component uses its own local meshing parameters, which are automatically generated when importing the ODX file into Speos. For more information, please refer to Speos User’s Guide Understanding What Is Imported to Speos.

Taking the model further

Information and tips for users that want to further customize the model

Geometry Update with Speos

Speos offers a solution for updating the geometries of the optomechanical components of the system, to avoid uploading the whole mechanical file with every single modification. In addition, you don’t need to reapply optical properties to the updated geometries. This feature can be accessed by right clicking on the geometry that needs to be updated. Alternatively, the Geometry Update icon can be also found in the assembly tab.

A detailed example of how to use this tool is described in Step 4 of Stray Light Analysis – Mechanical Geometry Modification – Ansys Optics.

[[NOTE:]] geometry update tool is compatible with CATIA V5 files (*.CATPart, *.CATProduct), Creo Parametric files (*.prt, *.xpr, *.asm, *.xas), NX files (*.prt) and SolidWorks files (*.sldprt, *.sldasm).

Baffle study

Thanks to the definition of 3D Irradiance sensor around the housing and baffles of the system, we can take the stray light analysis one step further and study its direct impact on the optomechanics. In the Simulation panel of the Speos project the folder Baffle study includes all simulation results for this type of analysis.

See Step 4 of Stray Light Analysis – Smartphone Camera – Ansys Optics for more detailed information about this study.

Speos Optical Material Library

Speos enables users to quickly evaluate the visual and optical impact of different materials using the Speos Sphere model and embedded material libraries. For stray light analysis, this means you can efficiently swap out materials—such as different coatings or surface finishes—by dragging and dropping them from the library onto your geometry, then rerunning the simulation or using Live Preview to instantly assess how each material affects light scattering and absorption behavior. Take a look at this article for more information: Material Simulation and Selection .

Reverse Raytracing for Critical Light Pathing

In the stray light analysis workflow for a Smartphone Camera , reverse ray tracing is used to detect all critical source positions—both inside and outside the field of view—and identify light leakage paths throughout the optical system. While this article does not cover reverse ray tracing, further analysis can be considered to look at these critical ray paths on-axis and outside the field of view for this telescope system.

STOP Analysis

The STOP (Structural-Thermal-Optical Performance) workflow in Ansys Speos for a telescope system involves coupling Ansys Mechanical, Zemax OpticStudio, and optiSLang to simulate how thermal and structural deformations in the housing affect optical performance. This automated process enables sensitivity analysis and metamodeling, allows exploration on how variations in housing materials, mounting stresses, and heat loads influence critical optical metrics like wavefront error and beam pointing accuracy. See the following article for more information: Automated Optomechanical STOP Analysis .