The STOP surface in Composite surfaces

  • 18 January 2023
  • 3 replies

Userlevel 4
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While looking at Composite surfaces, I have noticed that if the surface is the system’s STOP surface, I cannot use the surface as an Add-On. The box for it is greyed out. I can use the STOP for the Base surface. What’s happening here?


Best answer by Kevin Scales 18 January 2023, 19:21

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Userlevel 4
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Add-Ons interact with the rest of the system slightly differently than other surfaces do. They don’t have actual apertures, nor do rays directly interact with an Add-On surface. The Sag Map tool will show any apertures, but the actual ray trace does not include their effects. Since an aperture is needed for a stop, the stop cannot be an Add-On. If you have a composite surface that serves as the stop, just make sure that the base surface has the System Stop property rather than one of the Add-Ons.

Note that at the time of this writing, the help file does not clarify this point, but should do so in later releases.

Userlevel 7
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Hey Kevin,

Does this mean that you can only have an aperture on the Base surface of a composite surface? That seems reasonable, but I just wanted to check.

  • Mark
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Mark, you raise a great point and I wish I’d been more expansive in my original statement. To be clear, no apertures in the Add-On surfaces are applied to the ray traces. They show up in the Sag Map for reference, and the Aperture properties are still seen in the LDE if you look at the properties, but they won’t apply to the actual real surface. Base surface apertures will apply for everything.

In the case of the STOP, the user is prevented from making it an Add-On, but other apertures will simply not be applied, so it may not be immediately obvious that the apertures are no longer in use.