Hacker News Reader

We're talking about "see the faintest light from stars millions of lightyears away"-dark (for that reason you also can't shine a light on it), with our sun in the background. To see something on that side, you'd basically need the vision capabilities the telescope itself has and that's obviously not going to happen.

View on HN

It obviously needs a selfie mirror.

It has a selfie 'lens': NIRcam has extra optics that can be swung in that allows the camera to focus on the primary mirror instead of out in the great infinity.

So in this sense webb is already a camera which can photograph itself!

This capability exists for mirror alignment: They'll point webb at an isolated star, switch to focusing on the primary mirror, swap in optics that cause small phase differences to result in diffraction patterns, and then they can use the resulting images to fine tune the positioning of the mirror segments to a small fraction of the wavelength of light that they're using.

That's amazing. I was wondering how they did that. Every new thing I learn about this machine blows me away.

> to see something on that side, you'd basically need the vision capabilities the telescope itself

I'm not sure if that's correct - there's a lot of photons hitting the telescope (the night sky), and night vision systems can work off of a relatively small number of photons

Reply on news.ycombinator.com