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The Hubble Space Telescope encloses its primary and secondary mirrors in a tube. The James Webb Space Telescope has no tube around its optics, which obviously leads to significant mass savings. Does JWST not need a tube because

(a) it will be observing a different part of the spectrum to HST;

(b) it will be in a different location, L2 rather than LEO;

(c) its optics will be shielded by a sun shield;

(d) some other factor ?

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(a) it will be observing a different part of the spectrum to HST;

(b) it will be in a different location, L2 rather than LEO;

(c) its optics will be shielded by a sun shield;

(d) some other factor ?

It's mostly (c) but that only works if (b) is true.

By being in a halo orbit around the Sun-Earth L2 point, the Sun, Earth, and Moon are all in roughly the same direction, and so can all simultaneously be shielded by a single shield.

In this answer I talk about JWST's planned orbit and include the diagrams below taken from NASA's Stationkeeping Monte Carlo Simulation for the James Webb Space Telescope

In this comment @AnthonyX says:

I think the answer to this question is in the NASA FAQ linked in the question. The telescope needs to be kept very cold to observe faint objects in the infrared. It has to keep Sun, Earth, and Moon constantly on the other side of its heat shield to stay cold enough. The closest place that meets those requirements which is also an orbit (minimal station-keeping fuel) is Sun-Earth L2.

I've added links there to put the quoted comment in context.

But JWST needs more than optical shielding from stray light! All of JWST needs to be kept very cold because it is an infrared telescope. Luckily, as long as the sunshield protects it from being warmed by strong sunlight and thermal infrared radiation from the Earth and Moon, JWST telescope itself (mirrors) can keep cold by radiating its heat directly into space.

There may be some additional refrigeration for the infrared sensor array, but the optical surfaces cool themselves by radiation.

I also think that JWST could have a tube if it needed it. It doesn't matter that it's huge and folded, if a tube was necessary, an "origami" tube could have been invented that would fold out somehow. Need of a tube wouldn't have been a show-stopper for the JWST, look at the complexity it embraces by rolling out all of those layers of thermal insulation!

enter image description here

enter image description here

above 1: Source, above 2: Source: Status of the JWST Sunshield and Spacecraft , found here. below: Source: Stationkeeping Monte Carlo Simulation for the James Webb Space Telescope

enter image description here

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The JWST is not enclosed in a tube because it would be too big for launch otherwise.

The primary mirror is too large for existing launch vehicles, so the mirror is composed of 18 hexagonal segments, which will unfold after the telescope is launched.

If the primary mirror is too large for launch, the enclosing tube would be too large also.

The JWST does not need a tube because its optics will be shielded by a sun shield. A tube as shield is not needed if the sun shield is oriented towards the sun. The sun shield needs folding for launch too.

There are and will be mirror telescopes on Earth without a tube too, see this image

The Herschel space telescope was build without a tube around the mirror and a sun shade too: enter image description here

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It’s really hard to stop IR radiation. Any surface that the JWST optics can “see” has to be kept very cold to prevent IR from that surface reaching the optics. If you had a tube, you’d have to work hard to keep it cold. Or you could not have a tube at all...

Usually tubes are structural (not needed in the JWST case) or to block light. Hubble, as a visible light telescope, needed starlight blocks. But space is quite cold and there’s not a lot of ambient IR to block once you’ve shielded Sun and Moon.

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  • $\begingroup$ IIRC on IRAS, the dewar with coolant was wrapped around the telescope, acting as the telescope tube. $\endgroup$ – Hobbes Aug 30 at 7:57
  • $\begingroup$ @hobbes. Yes, that’s a good example of the trade-offs. IRAS was in a sun synchronous orbit, often surrounded by the IR from Earth and Sun. A simple passive Sun-shield wouldn’t have worked. It needed a lot of coolant, so it made sense to combine those into a cold tube. $\endgroup$ – Bob Jacobsen Aug 30 at 13:27
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Because it is launched folded, and has to unfold after launch. JWST is big, too large to launch unfolded in a rocket. I guess @Uwe beat me to it, but here's a source explaining it: >"“Due to its origami unfolding architecture, the Webb telescope does not have a cylindrical light baffle, like is seen with Hubble or even your home telescope which is used to block unwanted light."

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