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Comment by mabbo | Hacker News Reader

I think the real challenge here is that you'd need to stay in place while you observed anything. The line from the target object under observation, the lens, and the viewer must remain as close to perfect straight as possible.

For earth or space based telescopes, we just keep the observer and lens rigidly attached and then move the whole apparatus slowly and carefully (computer controlled) to maintain a straight line.

For an earth-atmosphere lens, you don't have the rigid stability between the observer and the lens. You can't move the target or the earth, so you'd need to keep moving the observer to keep it perfectly in place.

That sounds like a lot of delta-V expended for each observation. It's probably easier to just build a 150m telescope on the moon or in space.

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You're certainly right about the need to move the observer, but I suspect you're wrong about how hard it is relative to building a 150m optical telescope (the largest to-date is ~10m).

Orbital mechanics is a pretty well understood thing, so figuring out how the earth will move over the next 20 hours isn't that tough.

The rest is just making sure you're able to position the 1m receiver very precisely and move it around in a very controlled way. I'm no expert, but I suspect we've demonstrated both in one or more NASA missions already.

Probably super-expensive, but less of a technical challenge than building a proper 150m optical telescope.

The math is easy, the delta-V is hard.

There is a precise point the craft would need to stay at relative to the earth and the target. There aren't any stable orbits that keep a craft at precisely that position, which means you are firing engines for 20 hours straight.

Remember, you don't stay in space unless you are either in a stable orbit, moving at many km/s, or you're firing engines to resist gravity's pull.

A spacecraft to stay at this point for 20 hours would be massive, and 99.9% fuel tanks.

Could you get anywhere by taking the image over multiple passes?

Even though the first image wouldn't be ready for a long time, if you have a lot of targets then you could increase efficiency to something like a tolerable level.

A LaGrange point would do, and Professor Kipping suggested putting a telescope near the JWST at L2.

At that distance you would be sampling light that has passed through the Earth's atmosphere at an altitude of 14km. About 8% of the light would be lost, but at that altitude there's little weather to disturb the image.

They wouldn't though.

The Lagrange points are stable positions relative to two bodies, like the earth and the moon or the earth and the sun. But those things are moving, orbiting. The points move with them. The line from the earth to any Lagrange point won't stay stably pointing at any star in the sky.

And even then, the focal point of this atmospheric lens is at a specific distance that is not, afaik, the distance to any Lagrange point.

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