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To avoid placing a secondary mirror in front of the objective mirror, why not tilt the objective and design the eyepiece in such a way that comatic aberration is minimized?

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  • $\begingroup$ Seems that "tilting" the objective (i.e., letting the light come in and reflect off the objective mirror from some off-axis angle) would result in the light reflected off the objective mirror having some complicated, non-radially symmetric distortion, wouldn't it? $\endgroup$
    – user93237
    May 27, 2016 at 19:42
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    $\begingroup$ There are telescopes that have tilted mirrors and they give very good results, however they are more expensive to produce, telescope-optics.net/tilted2.htm $\endgroup$
    – Peter R
    May 27, 2016 at 20:01
  • $\begingroup$ The brownsnout spookfish has four eyes. One set looks upward and has lenses. The other set look down, and use mirrors to focus the image. I couldn't find references to how clearly this arrangement works, but I suspect that since this is a deep water species, the substitution of a mirror for a lens is to increase the eye's sensitivity to light, since objects below it will have even less light to reflect. The retina is approximately 90 degrees to the incoming light and the reflector is shaped to reflect the light accordingly. $\endgroup$
    – Howard Miller
    May 28, 2016 at 1:18

2 Answers 2

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Aberrations. A parabolic mirror perfectly focuses rays along the optic-axis, while off axis rays are blurred--tilt the mirror and you get more light, but it's out of focus. I would guess that the non-tilted mirror has 2nd order aberrations, while tilted one is 1st order: it's a lot worse.

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You can't just tilt the mirror (for the reasons described in JEB's answer).

You can have off-axis parabolic mirrors but they are tricky to make. You either have to machine the surface with a diamond tip CNC machine, or saw an off axis region out of a larger mirror

At radio frequencies they are a lot more common. Where you can make the shape from metal plates.

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