• Question: Why not use an internal coronagraph?

    Internal ("Classical") Coronagraph External ("Paleolithic") Occulter
    Pros: Cons: Pros: Cons:
    Internally Scattered Light. Technically challenging for higher star light suppression. None! No scattered light!
    Control of occulting spot position? Has been an issue in previous coronagraphs. Control is the name of the game!
    Usable with any telescope science instrument? No. Yes.
    Place target anywhere in image plane? No. Yes.
    Variable spot size (target optimized)? No. Yes.
    Optimum PSF redistribution for planet search? Some designs can optimize. All used to date are not optimally shaped. Better than previous classical coronagraphs.
    Occulting Spot Shape Can use arbitrary shape. Design fixes shape. Some shape change capability with occulter tilts. Can't make occulter into some shapes.
    Light Suppression factor Some designs with higher theoretical contrast, but not demonstrated. Better than previous generation of classical coronagraphs. Highest suppression requires distant, large, slower occulters.
    Exposure length limits. Detector/background limited. Limited near higher gravity gradients.
    Separate Spacecraft? Unnecessary. At least telescope + occulter.
    Operations Well understood & simple. More complex. Infeasible near Earth.
    Cost Coronagraph within telescope. Microengineering costly for higher performance Competitive for smaller occulters. Usually always more for larger occulters.
    Observation rate limits Just slew the telescope around. Must move hundreds to thousands of kilometres.
    Lifetime Life potentially unlimited. Limited by s/c fuel and operations.

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