Is it possible to arrange two off axis parabolic mirrors (OPAs) to transform the emitted rays of a point source into a collimated beam, as in the figure below?

enter image description here

That is, similar to a Keplerian telescope, but achieved with mirrors and not lenses.

EDIT: I want to desing an optical system that takes a point souce and collimates it to a beam of specific width, given that optics manufacturers (ThorLabs, Edmund, Newport) have OPA mirrors with foci at only discrete, large steps. Of course, lenses can do that as well but I want to avoid chromatic abberations, therefore the use of mirrors.

As is well known, a parabolic mirror can focus a collimated beam to a point at the parabola's focus. I came up with the following two configurations, which are somewhat similar, but not identical to the one I mentioned above:

enter image description here

Figures created with the help of ricktu288's simulator.

It seems to me that to achieve the "Keplerian telescope", I need to position the point source not at the focus of the parabolic mirror. But I would like to ask for help with finding this point.

Any help is much appreciated!

  • 1
    $\begingroup$ The name is Kepler. $\endgroup$
    – my2cts
    Commented Feb 13 at 22:03
  • $\begingroup$ 2 parabolic mirrors are not a great choice for that configuration. A spherical mirror followed by a parabolic mirror would do better, but then a 45° angle is not a great choice on a spherical mirror, as you would need to adjust the angle of the parabolic mirror to compensate the astigmatism. But even better than that, simply a flat mirror followed by the parabolic mirror, with no intermediate image of the point source. This way you just need to adjust the parabolic mirror to the same focal distance. $\endgroup$ Commented Feb 13 at 23:56
  • $\begingroup$ This is an off-axis Gregorian telescope. $\endgroup$
    – John Doty
    Commented Feb 14 at 0:41
  • $\begingroup$ @JoséAndrade Quote "simply a flat mirror followed by the parabolic mirror, ... adjust the parabolic mirror to the same focal distance". I don't see how an off axis parabolic mirror and a flat mirror would do that. Can you draw a schematic? $\endgroup$
    – tush
    Commented Feb 14 at 14:21
  • $\begingroup$ Are you OK with using a hyperbolic mirror along with a parabolic mirror, instead of two parabolic mirrors? You have one focus of the hyperbolic mirror co-located with the focus of the parabolic mirror, and put your point source at the second focus of the hyperbolic mirror. $\endgroup$
    – BaddDadd
    Commented Feb 14 at 21:36

1 Answer 1


This is what I meant in the comment, sorry for poor drawing skills:

enter image description here


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