Best fit (minimum material removal) sphere for an off-axis asphere (parabola)

  • 17 June 2020
  • 8 replies
  • 209 views
J
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Sag analysis for aff-axis aspherics was added recently (in 20.1, I think)- a feature I had asked for. I have been trying to use it to get the best fit sphere  (with minimum material removal) for a large off-axis parabola which we will be polishing- very useful information when specifying a blank from the glass manufacturer . The analysis does not appear to give sensible results, reporting and ROC that is smaller than the base ROC of the parent parabola, with huge residual sags (100s mm). I suspect that this is because the sag calculations are done with respect to the plane tangential to the parent vertex rather than tangential to the local vertex (centre of the off-axis aperture).


I have, of course, been ticking the 'off-axis coordinates' box in the analysis settings window .


I have been trying to solve this via support@zemax.com but have just received an email saying that this forum is now the preferred method and quicker...


Lens and session file attached


Thanks for any help,


John


 


 

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8 replies

Hui Chen
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Userlevel 4
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Hi John,


Thank you for contacting Zemax support!


Yes, we are shifting our support focus to Forum and MyZemax so our customers can get a quicker reply to their inquiries. I noticed you have an on-going discussion with Haosheng regarding how the off-axis Best Fit Sphere radius is computed in the Surface Sag analysis with the 'Off-axis coordinate' box checked. 


Running this analysis in your file i can see the analysis returns a Remove BFS Minimum Volume radius of -337 mm, much smaller than the base radius of curvature 2560 mm. I don't have a good explanation on why this is the case. But i don't think what you suspected, 'the sag calculations are done with respect to the plane tangential to the parent vertex rather than tangential to the local vertex (centre of the off-axis aperture)', is the case. Because if it's the case, I would expect to see a strong 'tilt' in the reported sag which is the angle between the two tangential planes, but i don't see it in the sag plot, it appears symmetric to the local OAP vertex.


That said, I don't have a good explanation on how this Remove BFS minimum volume is being computed when the  'off-axis coordinate' is checked. In the Help File it says 'The coordinate system for the sag computation will have an origin at the vertex of the off-axis part'. This confirms the origin is at local vertex but doesn't specify how the reference plane is chosen to oriente for off-axis sag computation. 


I'll reach out to our Development team for some clarification and will let you know as soon as i hear back from them


Best regards,


Hui

J
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Thanks, Hui. I look forward to hearing what they say.


John

J
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Hi,


I have also modelled an OAP with 90 degree deviation (files attached). Again, the sag  analysis seems to make no sense- the best fit ROC= base ROC and the residual appears perfectly flat with a very large tilt !


John


 

2 Attachments
Hui Chen
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Userlevel 4
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Hi John,


Thank you for providing further information on this scenario.


Yesterday I reached out to our development team for clarification. They got back to me and confirmed following two things:


1. In the Surface Sag analysis with the 'Off-Axis Coorindates' checked, the coordinate system for the sag computation will have its origin at the local vertex of the off-axis part.


2. The reference plane for the sag computation is in fact tangential to the local vertex of the off-axis part.


That said, I forwarded the OAP 2560 file you provided to our developers. They ran some priliminary tests which seems to suggest the algorithm used for fitting the off-axis sag data is experiencing difficulty, it causes very large fitting error, which results in the small BFS Minimum Volume of -337 mm you saw. They are going to investigate this further.


I also looked at the new file you attached. This time i saw the fitting seems not working at all. The BFS radius is identical to the base radius of the parent parabola, and the residue sag is huge. 



At this point, I think our developers will need more time to investigate this issue and possibly improve the algorithm and include fitting error in the analysis. I'll file a bug report for them to look at this, and keep you posted as soon as I hear anything back. Meanwhile, if you have any other questions, please do feel free to let us know.


Best regards,


Hui

J
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Many thanks, Hui. That sounds like progress.


John

J
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Hi Hui,


I wonder whether any progress has been made in fixing this bug?


John


 


 

Hui Chen
Rank
Userlevel 4
Badge +1

Hi John,


Thank you for your message!


I just checked and it appears the bug as been assigned to a developer for investigating. It's still marked as In Progress and hasn't been resolved yet. I'll check with the developer to see if there is any update. 


I'll keep you posted once I hear back from him.


Sorry for the inconvenience and thank you for your patience!


Best regards,


Hui

T
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Hi, i still have this problem, any solution?

 

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