Eye of Sauron lens (Canon EF 200 mm F/1.8) or "equivalent"?

[SIGSEGV]

Asking here since I know there are a number of photography gurus...

For one of my experiments, we're trying to obtain several of the famous, yet sadly discontinued Canon 200 mm F/1.8 lenses ("Eye of Sauron"). They appear regularly on eBay but it seems a bit tricky to buy off eBay with NASA grant money (or at least the university purchasing department doesn't like it, though maybe this can be pushed back on). Does anybody have any idea of other places to get one, or a good sort of equivalent lens?

I also can't find the t-stop of the lens measured anywhere, which would be useful for any comparison. Obviously the 200 mm F/2 lens would be a reasonable replacement, but we have some leeway on focal length because there are several sensors we are considering and we have some leeway on FoV anyway. The mount doesn't matter too much since we're using an astronomical camera (not determined yet) and can probably make most mounts work (maybe not some mirrorless ones?).

The lens would be used on a high-altitude balloon for a daylight star tracker (i.e. by seeing which stars are in the field of view, you get a very good measurement of the balloon orientation).

[IvanMaype]

Hello all,

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Please help