1) Could a starshade in orbit be used by ground based telescopes? If so, what kind of orbit should it have?
2) Could the same starshade be used by two different space telescopes, or does the positioning make it impractical?
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1$\begingroup$ To what end would 1) be intended? If the purpose is to block all unwanted light, putting an atmosphere between the telescope and the shade when your signal is the light from an exoplanet, would the noise not overpower signal by several magnitudes? 2) sounds impossible given the proposed depth of the shade, but even if it were 2D, at some point in order to maintain alignment with the planet one of the telescopes would have to move between the shade and the second telescope. $\endgroup$– Mitch GoshornApr 2, 2015 at 15:36
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$\begingroup$ @MitchGoshorn Fantastically enough, they can already make direct imaging of exoplanets from the ground using coronagraphy inside the telescope. $\endgroup$– LocalFluffApr 2, 2015 at 15:52
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1$\begingroup$ I guess this signal to noise problem isn't an issue afterall, though as the linked article says, the observed planets are putting out more light than an 'Earth-like' planet would. But frankly coronagraphy seems like a lot more practical as the telescopes own tracking would maintain alignment. $\endgroup$– Mitch GoshornApr 2, 2015 at 16:00
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$\begingroup$ I was going to ask the same question, but with a high-altitude drone (or such) instead of a satellite. The satellite would need to avoid apparent motion relative to the stars, which would be difficult to impossible. The nearest I can imagine is a highly elliptical orbit with the apoapsis velocity equal to the ground velocity at the observatory. You'd need an array of such shade-satellites, and you'd have to move them all when switching to a new target. $\endgroup$– Blackbody BlacklightSep 20, 2015 at 8:14
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1 Answer
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If the starshade was to stay in place over the ground telescope, it would have to be in a geosynchronous orbit, which puts it almost 36,000 km from Earth's surface. To cover an angular distance of 1 arcsecond (1/3600 of a degree), it would have a size of around 10km. Certainly not impossible, but technologically impractical at the moment. Also, the starshade would only shade a small region of the sky, so it wouldn't be useful from other observing sites.
As for the space telescopes using a starshade in geosynchronous orbit; that would be impossible, at least for scopes orbiting Earth, like Hubble is. The starshade is orbiting Earth at a different distance, and therefore speed, than the telescopes. As far as I can see, it would be near impossible to keep them aligned for anything more than a few minutes. I haven't done any number-crunching on this, though.
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1$\begingroup$ Maybe one can find two synchronized halo orbits at the Moon? A telescope in GEO is of course another solution, which allows for smaller star shade. $\endgroup$ Aug 13, 2017 at 17:25