Are the profiles for reflecting/refracting spherical?

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Discussion Overview

The discussion revolves around the geometric profiles of reflecting and refracting surfaces used in advanced astronomical instruments, particularly focusing on whether these surfaces are spherical or designed differently to mitigate spherical aberration. The scope includes theoretical considerations, practical applications in telescope design, and historical context regarding the Hubble Space Telescope.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant questions whether the surfaces of reflecting and refracting elements in astronomical instruments are spherical, suggesting that while spherical surfaces allow for a clean focus under the paraxial approximation, they still introduce spherical aberration.
  • Another participant references the Ritchey–Chrétien telescope design, implying that it may be a solution to the issues raised regarding spherical aberration.
  • Multiple participants express interest in the historical design and errors associated with the Hubble Space Telescope, questioning if the original design was based on existing technology for Earth surveillance.
  • There is a suggestion that the surfaces of telescopes are designed to minimize the effects of spherical aberration, potentially through the use of paraboloid shapes.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether the surfaces are strictly spherical or if they are designed differently to address spherical aberration. There are competing views regarding the design considerations for astronomical instruments.

Contextual Notes

Participants express uncertainty regarding the specific design calculations and historical context of the Hubble Space Telescope, indicating that there may be limitations in available documentation or understanding of the design process.

Who May Find This Useful

Individuals interested in telescope design, optical engineering, and the historical development of astronomical instruments may find this discussion relevant.

swampwiz
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Are the profiles of the reflecting/refracting surfaces spherical for the bleeding-edge astronomical instruments?

I realize that because of the paraxial approximation, a "small angle" for a "ray" of light on spherical reflecting & refracting surfaces allows for a clean focus to take place. However, even a very small angle yields some spherical aberration, so I wonder that since astronomical instruments are always focused on infinity (or something so far that the error in presuming infinity is orders of magnitude less than an spherical aberration), the surfaces are tightly designed as a paraboloid - or at least all the surfaces are carefully designed to have the net effect of true focus that is not dependent on the spherical aberration to avoid blur.
 
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PAllen said:
https://en.m.wikipedia.org/wiki/Ritchey–Chrétien_telescope

I see that is the type used for the Hubble. I wonder if the exact calculations of the original, correct design, the error, and the fix to the error are documented somewhere online. I'd be interested in taking at a look at that.
 
swampwiz said:
I see that is the type used for the Hubble. I wonder if the exact calculations of the original, correct design, the error, and the fix to the error are documented somewhere online. I'd be interested in taking at a look at that.
At a purely 'gossip' level, I seem to have read reports that the Hubble mirrors were based on an existing design for Earth surveillance applications. Is that a possibility? Or will you have to kill me once you have told me?
 

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