# Advantages of the new nonsequential Off-Axis Mirror object

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Userlevel 3
+1
• Zemax Staff
• 6 replies

We’ve introduced an Off-Axis Mirror object in nonsequential mode.  It has some advantages for users:

• The object has a coordinate system that is at the center of the off-axis part.  (See sketch below.)
• This makes tolerancing and moving the part easier, because there’s no need to define motions with respect to the parent vertex.
• It also means that the edges of the part’s substrate are perpendicular to the surface slope at the vertex.  This is more realistic for parts that are polished directly onto a blank (versus small parts that can be cut out of a parent part).
• It is simpler to define the off-axis apertures.  The part supports native elliptical and rectangular apertures (including circular and square).
• The object simplifies the nonsequential editor, since a Boolean is not required to define the aperture.

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Userlevel 6
+3

Will there be a sequential equivalent?

Userlevel 3
+1

The sequential equivalent is the Off-Axis Conic Freeform.   This is an off-axis part with the vertex at the center of the geometric aperture, and sides parallel to the vertex.  The user just specifies the conic constant and the off-axis distance.  It’s handy!

Jose Sasian at the University of Arizona did this work and helped with implementation:

Dmitry Reshidko, Jose Sasian, "A method for the design of unsymmetrical optical systems using freeform surfaces," Proc. SPIE 10590, International Optical Design Conference 2017, 105900V (27 November 2017); https://doi.org/10.1117/12.2285134

We’ve gotten a lot of enthusiasm from users about the nonsequential Off-Axis Mirror so far.  They especially appreciate that we have implemented User-Defined Apertures (UDAs) for the part.  In the telescope world, there’s no such thing as a standard mirror shape, because you have to trim the part around the beam to reduce mass and size.

Userlevel 6
+3

Is that in the code now? Wow, sorry I missed it!

I had the same question as Mark.  It would be useful to have the off-axis conic freeform extended to include the even radial asphere terms (r^4,r^6,r^8, etc.)--for the same ease of use in tolerancing as with conics.  Or potentially have a way to use the convert asphere tool to include converting to off-axis conic freeform.  I was hoping that the user-defined tolerance operands would do this, but that is not how they are constructed.

Assuming that I haven’t missed this capability somewhere, is there a graceful way of implementing a tilt tolerance on an off-axis even asphere?  I’m hoping not to have to figure out a brute-force method.