You can make really really really big mirrors,. Space telescopes mirrors are limited by Fairing size, so barr folding mirrors ala James Webb, there is a fairly hard limit.
I'm as much a fan of astronomy as anyone, but I'm not willing to let it block the best chance we currently have of becoming a space-faring species. The comm satellites aren't important, but the launch capacity scaling is.
And astronomy is still an extremely important component for becoming space-faring. No use in going somewhere blindly when you can have a look first. But if there is no-one looking because nothing to see, funding dried up, scientists demotivated,...
Never mind the problem of us being one asteroid strike or supervolcano away from a cataclysm, and that's not an if but a when.
I have a feeling that for humans to transcend the proverbial great filter, we have to tap into the vast quantities of resources and energy in the solar system, but more importantly rekindle the pioneering era that last ended with the industrial revolution.
The expensive part of the space industry is lifting infrastructure from ground to space. Moving within the solar system is comparatively cheap if we avoid descending into the gravity well of other planets. Luckily, this is unnecessary for most asteroid mining.
Humankind experienced incredible advances with the toppling of every transportational frontier. The wheel, seafaring, motorized transport and flight all resulted in expansions lasting a hundred years each.
The next frontier is the solar system. We don't know if we'll be able to ever leave it, but that's irrelevant because it can be our home for the next billion years. Our best shot at actually preserving the habitability of Earth is exploiting resources out in space.
In fact, I'm pretty sure both "sides" would benefit greatly from cooperating. It might even be that these aren't "sides" but just different cousins of the family with different outlooks on their own life, and we need a little bit of everything, and everyone, to make a world.
That might close the window of opportunity and not give us a second chance. So better not risk it. :-)
Many kilometers in diameter in circumference should be possible and likely much more. The whole thing would at the same time be likely really really light, just thin stabilized foil, as it does not need to fight gravity or survive launch loads. Could be quite a sight. :)
>Many kilometers in diameter in circumference should be possible and likely much more.
This is a misconception. There is plenty of "gravity". The mirror has to keep a very precise shape and attitude, which severely limits the possible size, considering it has to be light and is a subject to gravitational perturbations. Large and thin constructions in space (solar panels, antennas etc) are mechanically non-trivial on their own, and for telescope-quality mirrors it seems downright impossible.
But in the short-medium term, the cheapest course that delivers is to use normal telescopes and interferometry (say on some orbit between Venus and Mars). I'm pretty sure it's also a domain where narrow AI may help because finding "anomalies" in space is a lot like finding anomalies on X-rays to find malignant tumors — something AI apparently can do well. Both problems fit incredibly large datasets + ultra low resolution of said anomalous blobs, and discrepancy with normal ones barely statistically significant (well below what human eyes may spot).
This is how I see the immediate future of space-based observation: lots of small things that cooperate extremely (increasingly) well with each other, "networks" more than "giants", much like down here on the ground.
It's just easier, cheaper, and lets you grab a lot of low-hanging fruits. Meanwhile, space-based fabrication can kick off and take the time to reach 'self-sustaining' velocity.
I would assume a micro gravity only mirror could be much thinner & thus easier to cool down. Or possible alternative techniques could be used to get the needed reflective surface geometry if it does not need to take gravity and atmosphere into account.
edit: book -> novel
Assembling from pieces on the other hand, is more doable.
A hollow toroid could be spun to provide force and the liquid would naturally form a parabola.
There've been a few experiments on earth and it could have huge cost saving potential in space. Instead of having to launch a 8m+ solid mirror machined to perfection, you could launch a lightweight toroidal substructure with a vat of reflective liquid at a fraction of the cost.
[1]https://www.space.com/22505-worlds-largest-telescopes-explai...
Shipping complicated physical hardware to space requires that it absolutely must work on the first try, and never require maintenance for the lifetime of the instrument. In contrast, the large earth-based telescopes can be regularly updated, maintained, and debugged. Access for such activities costs as much as a plane ticket to Chile, instead of a dedicated mission to space.