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Apparently you can't collimate sunlight because it's not a point source.

But I've been unable to find an explanation as to exactly why which I can understand, because this aspect of optics is deeply unintuitive, but the magic phrase to search for is 'conservation of éntendue'. If you find one, let me know.

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Have you seen the XKCD What If on this topic? [1] It doesn't get into the details of éntendue, and even makes a joke about the topic's complexity, so it probably won't satisfy your technical demands. But I've found the arguments about reversibility (light paths through lenses are reversible, so how could one point map back to multiple points?) and area increase (lenses do not intensify image values but can change the overall image size) intuitive enough to help quell myself and my friend's arguments about the matter.

[1] - https://what-if.xkcd.com/145/

Also, it's easier to make mirrors to reflect monochromatic laser light than broad-spectrum sunlight. You don't want to vaporise your probe.

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