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Off axis parabola telescope: why coma?

  • 16 January 2023
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Dear All,

I am just trying to design an off axis afocal parabolic telescope. The design is in principle quite simple: two off axis parabolic mirror with different focal length and the focal points coinciding.

Sending a collimated bundle of rays to the telescope and collecting the rays behind the telescope with a paraxial lens I get a coma aberration! No way to get rid of it even turning or shifting the mirrors. Or better, moving the mirrors I can get a round spot but much larger then the Airy disk.

My question is simple:Why? It should image a perfect point.

I have tried in both Afocal and focal mode. In Afocal mode (which should be the wrong mode) i get a round spot very much larger than the Airy disk and insensitive to the movement of the mirrors.

 

I already had a look to the Knowledgebase about, it doesn’t help me a lot.

I attach a Snapshot of the design to show it.

Thanks to everyone who could help me

Bests

Gabriele

 

 

 

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Best answer by Jeff.Wilde 16 January 2023, 20:57

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Hi Gabriele,

I think you must have some sort of alignment error.  Coma is an off-axis aberration (in this case, meaning angularly off-axis, i.e., an input beam with a relative tilt compared to the optical system) -- and OAPs are notorious for being sensitive to alignment and/or off-axis use.  I put together a replica of your system and don’t see any aberration.

 

Here an off-axis aperture is placed on surface 1, and vignetting factors define the corresponding apparent entrance pupil:

 

If you can attach a version of your model, that would help with debug.

Regards,

Jeff

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Hello Jeff,

 

Thanks again to help me.

I Implemented the solution as from your Lens Editor and it works.

I wondeer anyway why my version doesn’t work. I cannot share the lens editor unfortunately because of restriction policy but fundamentally I do not implement a stop aperture but I declare immediately the aperture of the beam in the system explorer.

Then I use the Tilt decenter of the OAP 1 to shift the parabola vertex down and then I place the OAP2 in a coordinate breake to shift alogn y  it and turn to get the two focal point coincident using RAID=0 degrees  as opimization criterion.

Once finished it seems  all ok but as you saw the system looked misaligned. Have you got a clue why this way does not bring to the desired result?

 

Thanks again

 

Gabriele

Userlevel 7
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Hi Gabriele,

I’m not sure if I completely understand your approach.  Without seeing your model, it’s difficult for me to say what might be happening, other than there is some sort of alignment problem.

If you don’t want to use vignetting factors, then you can simply specify the desired input beam size (using system aperture settings) and shift the entire optical path down in the y-direction with a coordinate break:

 

Regards,

Jeff

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Hello Jeff,

You understood very well my point and it seems that for you is not a problem to make it working.

I have to see what is the bug in my model. Unfortunately as I said I cannot share too many details.

Thanks anyway I can go further now.

Bests

Gabriele

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