Frequently Asked Questions

Features and Applicability of SigFit

Q: SigFit seems to be commonly used on relatively large systems like telescopes. Can SigFit be used on smaller optical systems such as cell phone cameras?

A: The size of an optical system does not limit SigFit’s applicability. SigFit is designed to be used on any optical system that can be modeled in an FEA tool.

Q: My optical software has an environmental analysis feature. Why do I need finite element analysis?

A: The environmental analysis feature in optical analysis software has significant limitations in representing mechanical effects of actual environments. The most significant limiting assumptions are listed below.

  • The deformation of the optical surfaces assumes free thermal growth neglecting the effects of CTE mismatch and the elastic effects that dominate the actual behavior. This assumption is especially limiting in the case of cemented optics.
  • The assumptions made for metering optical elements are often inadequate for useful results.
  • An optical analysis tool has no way to predict temperature gradients, which can significantly impact wavefront error.

Q: Variational analysis can be conducted in my optical analysis software using the tolerance feature. Why do I need SigFit’s variational analysis capability?

A: An optical analysis tool can only consider certain simple variations in a Monte Carlo analysis. These variations might be tilt and decenters, change in radius-of-curvature, or polynomial coefficients. With SigFit’s variational analysis capability variations may be any behavior that can be modeled by a finite element analysis. This offers significantly greater capability than that offered in optical analysis tools.

Q: Can SigFit perform finite element analysis?

A: No. SigFit uses finite element analysis results from other software in order to conduct its analyses.

Q: Can SigFit perform optical analysis and lens design?

A: SigFit does not perform and is not intended to perform detailed ray tracing analysis or lens design. SigFit does have the capability to predict line of sight error due to rigid body motion of optical surfaces and as predicted by a single user definable ray. SigFit also has the capability to predict the change in optical modulation transfer function due to the effect of line-of-sight jitter.

Q: Can I use SigFit to improve my optical performance?

A: Yes, but the answer needs explaination. The equation generation option is used to support design optimization in finite element analysis. The equation generation feature allows users to compute optically meaningful quantities from within the design optimization features of their finite element analysis so that optical performance metrics may be optimized or constrained. In addition a design optimization feature is available in adaptive control simulation that will optimize the actuator layout for an adptively controlled optic. All other featSigFit is primarily used as an analysis tool. This means that SigFit is used in the process to make optical performance predictions for a single point design subjected to mechanical disturbances. Such a tool is useful for performing manual trade studies to understand how to improve a design. However, SigFit also offers some diagnostic features which will more directly indicate how to improve performance. For example, in random response analysis the modal contribution output will direct the user to the modes that most impact optical performance.

Q: Who is best suited to use SigFit: mechanical engineers or optical engineers?

A: Generally, the person best suited to use SigFit is the person who is most familiar with the finite element model. However, many optical engineers successfully use SigFit in combination with good communication with the finite element analyst. In addition, mechanical engineers using SigFit will benefit from good communication with the optical engineer.

Q: What finite element software is supported by SigFit?

A: SigFit supports MSC Nastran, NX Nastran, NEi Nastran, ANSYS Mechanical APDL, ANSYS Workbench, SIMULIA Abaqus, and SolidWorks Simulation. Not all features are supported for all finite element tools but basic surface deformation and thermo-optic analysis are supported for all.

Licensing Issues

Q: SigFit cannot access the license. What should I check?

A: Resolving license access issues generally involves checking three things: the licensing service running on the license server, the reference to the license server on the client machine, and the network that connects them.

Verify that the Licensing Service is Running on the License Server

  • Ensure that you log on to the license server with adminsitrator privledges.
  • On the license server machine open Programs/SigmadyneFNLicensing/lmtools. On Windows 7 machines right-click and pick Run as Administrator.
  • Under the Service/License File tab select the “Configuration using Services” radio button, select “Sigmadyne License Manager” from the list, and be sure “LMTOOLS ignores license file path environment variables” is checked.
  • Under the Server Status tab pick Server Status Enquiry. Scroll down to see if licenses are issued for the features being requested by the analysis. If the licenses are available then the license server is functioning. Be sure to note the port number over which the license is shared.
  • If the licenses are not shown then go to the Administrative Tools in the control panel and open Services. On Windows 7 machines right-click and pick Run as Administrator.
  • Scroll down to Sigmadyne License Manager and check if the service is started and is set to start automatically. If it is not started then try to start it by selecting the service listing and picking Start under the Action menu. If the service does not start and the operating system is Windows 7, then be sure you started the Services application by right-clicking and picking Run as Administrator.
  • On some Windows 7 machines nothing but a restart will get the licensing service working. If this fails then please contact technical support.

Verify that the Client Machine Points to the License Server

  • On the client machine, the machine running SigFit, the license location is found with two system environment variables. Right-click on My Computer or Computer and pick Properties.
  • Go to Advanced or Advanced System Settings and pick Environment Variables.
  • In the list of system variables scroll down to find SIGFIT_LICENSE_TYPE and SIGMADYN_LICENSE_FILE. SIGFIT_LICENSE_TYPE should be set to FLEXNET and SIGMADYN_LICENSE_FILE should be set to port_number@servername.

Verify that the Communication Between the Client Machine and the Server is Not Being Blocked

  • Communication between a client machine and a license server is most commonly blocked by a firewall. As a test disable all firewalls between the machines and try to run SigFit.
  • If the license is obtained after disabling the firewall then the port used by the licensing system must be opened on all firewalls that block communication of lmgrd.exe and sigmadyn.exe located in the licensing installation folder on the server machine. Please see the installation instructions.

Q: We would like to use a different TCP/IP port than the default value of 27001. How do we change the port?

A: The port number may be changed to any open port by editing the license file. Changing the port number does not require reencryption so the customer may make this change themselves

Usage of SigFit

Q: Why would I use interpolation instead of polynomial fitting?

A: In some cases surface deformations cannot be well represented by polynomial fits. One such example is the quilting deformation of a lightweighted mirror. In such cases an interpolated representation is more appropriate.

Q: How do I decide wether to use sag deformation or normal deformation?

A: If surface deformation results will be used in ZEMAX then there is no choice; sag deformations must be used. Only sag deformations are transferrable to ZEMAX. However, when transfering surface deformations to CODEV or OSLO then normal or sag deformations may be used. If normal deformations are used then surface deformations will be represented by INT files. These INT files will be used by the optical analysis software to compute a change in OPD to be applied to the wavefront. This allows ray tracing on the nominal prescription in the optical analysis software without the need for root finding. If sag deformation is used then the optical surfaces in the optical analysis are actually changed in shape and root finding is required. So normal deformation will give faster ray traces at some expense of accuracy while sag deformation will take more computational time during ray tracing but give more accurate answers. The level of accuracy in using normal deformations will vary greatly depending on the application.

Errors in SigFit

Q: I am getting an error that says my polynomial fit is singular. What should I check?

A: : A singular polynomial fit in SigFit can be caused by many reasons. Some items to check are:

  • Is SigFit finding the nodes on the optical surface? If there are too few nodes found then there is insufficient data for a well-posed fit.
  • Is the mesh of the optical surface offset from the parent vertex? If the aperture of the optical surface is offset from the parent vertex then the polynomial fit must be performed in an offset coordinate system by using the surface offset feature.
  • Are the units used for the finite element model and the SigFit analysis consistent? FEA units are specified in SigFit. Some FEA tools such as ANSYS Workbench allow modification of the analysis units and can be reset if the model is archived and opened by someone else.
  • Are there enough nodes on the optical surface mesh for the number of polynomial terms being requested? For SigFit’s default fitting setting a mesh with at least 10 increments in the radius and 36 increments in the azimuth will yield well-posed results.