Hacker News Reader

Comment by terramex | Hacker News Reader

It matters a lot, L2 is unstable point so if it ventures beyond L2 there would be no way to bring it back and it would enter a heliocentric orbit. Communication between Earth and telescope would become impossible after some time as gimballed antenna can only rotate so far and stray light reflected from Earth would limit its field of view.

Due to that JWST will always be on 'close side of L2' and technically in slow freefall back to Earth and boosted up periodically, but always a bit short of passing to the other side.

View on HN

> as gimballed antenna can only rotate so far

That's a fixable design problem. Or you can rotate the telescope.

> stray light reflected from Earth would limit its field of view

What? The earth and the moon are nearby anyway. What about that reflected light? And heck, what about the sun limiting its field of view?

> That's a fixable design problem.

You are right, it is fixable. It was fixed by adding active station keeping to the telescope.

> Or you can rotate the telescope.

There are limits on its rotation with respect to the Sun, dark side must be kept away from sunlight at all times. It can rotate 5 degrees "pitch down" toward the Sun and 45 degrees "pitch up". Gimballed antenna has enough authority so that it can communicate with Earth at whatever valid rotation telescope is so that science operations are not interrupted for transmitting data.

> The earth and the moon are nearby anyway. What about that reflected light? And heck, what about the sun limiting its field of view?

That light is reflected back by sunshield as all 3 bodies are behind it. Sun is limiting field of view but area of exclusion changes as telescope orbits and it can image every point in the sky at least every 6 months and 39% of the sky at any given moment.

Actually you can't rotate the telescope; they managed orientation even through the launch because if some of the science instruments get pointed towards the sun for too long they'll be destroyed. Now that it's deployed it's even more important that the cold side stays cold.

Could they put up some relay sats?

Reply on news.ycombinator.com