I know technology has come a long way, but it is amazing to me that they can fly two hunks of metal through space and align their positions down to a millimetre, while being a footy field and a half apart.

I wouldn't even know where to begin to tackle a problem like that.

Now and again the actual engineers at NASA or ESA or the like pull something off like this to remind the rest of us that we're just cosplaying

Greg Egan's story "Riding the Crocodile" features a fancier version of this trick, fire three objects from three origins at close to maximum speed, precisely timed to arrive at a point despite the enormous relativistic delays, the objects incorporate powerful magnets, and are designed such that they don't actually collide per se if timing is correct - but as they pass each other at the point where they would collide if shaped differently the magnetic force slows them all and so they eventually stop, together, at this focal point.

The Amalgam in that story have fantastically better technology than we do, but this is certainly at the edge of what's practical for them. It's a fun idea though, I wonder how close we could get with our technology.

Just in case anyone else late to this discussion is curious:

https://www.gregegan.net/INCANDESCENCE/00/Crocodile.html

If that sounds cool, check out the LISA mission.

https://en.wikipedia.org/wiki/Laser_Interferometer_Space_Ant...

Edit: and the already-flown Pathfinder : https://en.wikipedia.org/wiki/LISA_Pathfinder

Why aren't telescopes built this way?

1. They sometimes are. The imaging here uses a telescope. (Coronagraphs work on Earth-based telescopes too: https://en.wikipedia.org/wiki/Coronagraph.)

2. They don't need to be. Most telescopes aren't trying to image the corona of the sun or another star.

3. Space-based telescopes are really expensive and have significant limitations on size, shape, mass, etc.

Sorry I meant instead of having one big piece of metal which would have restriction in terms of length, having 2 pieces of lenses seperated by just vaccum? You would have lesser material, easier launch and higher magnification because of larger focal length etc?

Mostly because they don’t need to be, i would guess.