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The James Webb Space Telescope has some very specific positioning requirements; the second Lagrange point and the heat shield positioning being at the top of my mind.

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Do these constraints eliminate the possibility of pointing the telescope at any general region of space? For example, a telescope positioned at the south pole can never image Polaris because Earth is always obstructing the view.

For the JWST at first I thought you could analogize this to "the belt of space always hidden behind the earth and the sun", but then I realized that part of space isn't always hidden, but it instead would simply be unavailable on a "rotating" basis depending on the time of year; after all what is hidden behind the sun in January is out the opposite direction during summer. But my astronomy foo is weak and I wonder if there are other aspects of the deployment and operation that could affect things that I'm unaware of.

Are there any portions of the sky that the JWST will never be able to image because of constraints on it's positioning?

Cory Klein
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Here's a really great article with pictures that explains it better than I can. But to summarize:

JWST Observatory Coordinates diagram

If you draw a line between the JWST and the Sun, it can point up to 5° towards the sun or 45° away from it while keeping all the sensitive parts shaded by the heat shield. In addition, it can rotate in any direction around that line. And (not really relevant to this question) it can roll up to about 5° side-to-side.

The net result of this is that

  • At any given time, the telescope can see a lot of the sky, but not all of it. There's a 45° cone pointing away from the sun and an 85° cone pointing towards it that the telescope cannot image.
  • Although the telescope cannot point directly towards or away from the sun, 3 months later the directions that were towards or away from the sun become perpendicular to the sun and can be imaged. As it completes its orbit around the sun, it's eventually able to see the entire sky (although it's never able to point at planets orbiting closer to the sun than it). How much of the year it's able to see an object depends on it's latitude (above/below the plane of the eccliptic); see this chart for exact numbers.
  • There are two small about-5° patches of sky straight "up" and "down" (aligned with the solar system's north and south poles) that it is able to see year-round; every other patch of sky is only visible for part of the year. These areas are called the "Continuous Viewing Zones"
Gavin S. Yancey
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    There’s a really good chart here, which is on the same site that you sourced a bunch of info from. It shows the number of observable days vs. viewing angle. There’s an even better plot I’ve seen somewhere, possibly even on this stack somewhere, that does the same thing but is a heat map of the full sky and shows how many days different zones are observable (and the fact that every area has at least some nonzero viewing time. – fyrepenguin Jan 11 '22 at 12:19
  • Thanks! So the shield does not "wrap around" in the sense that it is impossible to point the telescope away from the Sun. I do realize that this is not a serious handicap, because the astronomers only need to wait 3 months. – Jyrki Lahtonen Jan 11 '22 at 14:26
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    The continuous viewing zones are at the ecliptic poles, not quite the same as the Sun's north and south poles. The Sun's rotation is irrelevant to the geometry here: the Earth's orbit plane is the determining factor. – John Doty Jan 11 '22 at 15:21
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    @JyrkiLahtonen if the limited viewing area at any given time is a problem or not depends on what the observation target is. For many observations you're right that it won't matter much because they're not time sensitive; but anything that is (ie transient phenomena like supernovas, neutron star mergers, interstellar comets, etc) will have a significant chance of not being observable by Webb while they're occurring. – Dan Is Fiddling By Firelight Jan 11 '22 at 15:32
  • Very true, @Dan. It will be sad if we miss the chance to record something spectacular because of that. If I did the math right, a bit less than 40 per cent of the sky can be viewed at any given instant of time, so the risk is there. – Jyrki Lahtonen Jan 11 '22 at 16:11
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    @JyrkiLahtonen: To be fair, that's similar to the Hubble, which always has ~50% of the sky blocked by the Earth. In addition, it can't be pointed within 50° of the Sun, which blocks out an additional 0–18% of the sky depending on whether the Hubble is on the night side or the day side of the Earth. – Michael Seifert Jan 11 '22 at 20:35
  • A fair point @MichaelSeifert At least the Hubble would be on the other side of the Earth soon enough. It's late here, I may be getting a cold, and I cannot wrap my head around it. That won't help with targets near the Sun, but... – Jyrki Lahtonen Jan 11 '22 at 21:05
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    I suspect I'm being dense, but I'm having trouble understanding how the telescope can ever point directly in the anti-sun direction. Is it because the anti-sun direction in summer is 90deg from the sun (in/out of the page) in autumn/spring? – craq Jan 11 '22 at 22:02
  • @craq yeah its complex. The scope can aim 360 degrees tangentially to its orbit. After 3 months (25% of a solar orbit), it has gone another 90 degrees around the sun, and that 360 degree view has turned sideways 90 degrees. So within 90 days, the camera can see everything excluding the sun itself and near-sun objects and anything inside the solar orbit of Earth. So patience is needed. – Criggie Jan 12 '22 at 00:33
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    @criggie I believe it needs to travel about 170 degrees in the orbit to view all objects. The 85 deg limit in prograde direction needs to...process(?) to reach the retrograde 85 deg limit. – Prometheus2508 Jan 12 '22 at 02:21
  • @Prometheus2508 ahh good point. I looked at "360 degree rotation around Sun line" and didn't account for it not pointing 90 degrees to that line (like a wheel on an axle) – Criggie Jan 12 '22 at 02:59
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    @Criggie Well we waited 10 years for it to launch, what's another .25 years to see a section of sky? :) – corsiKa Jan 12 '22 at 23:13
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    @Criggie Also, I'd say it's not the orbit of the Earth, but the orbit of the Earth Sun L2 that is out of scope. – corsiKa Jan 12 '22 at 23:14
  • Why cannot it point outward? Why the 45° limit? It seems a weird restriction... – am70 Jan 18 '22 at 12:42
  • @am70 the telescope is fixed to the sunshield; they have to rotate the whole thing as a unit. If they rotated it to point straight away from the sun, then the sun would hit the back of the mirror and instruments, and that would be bad. The 45° limit is as far as they can rotate it while keeping all the telescope parts still in the shadow of the sunshield. – Gavin S. Yancey Jan 18 '22 at 18:29
  • Thanks. Still this is weird... they could have added a gimbal, so that the telescope azimuth could be changed wrt the sunshield. In any case they do have a gimbal to the earth-pointed gimbal – am70 Jan 19 '22 at 15:10
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    For a fixed interface between the telescope and sunshield, the current "side-pointing" orientation has two advantages I can see. First, it allows for more area to be viewed for the given sun shield size compared to an "outward" telescope; second, it allows for a larger mirror for the given launch vehicle (the mirror sits vertically in the fairing rather than across. Gimbling the telescope would provide yet another mode of failure and place additional loading on the attitude control to keep the sunshield "still" while the telescope moved. – Prometheus2508 Jan 20 '22 at 23:27
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There are restrictions not only in direction but also in distance.

The JWST is sensitive to wavelengths from 0.6 to 28 µm. Any very early object very far away with a red shift to longer wavelengths above 28 µm could not be observed by the JWST.

Uwe
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The JWST can aim from 85 degrees to 135 degrees along the orbital plane. 0 degrees is toward the sun and 90 degrees is tangent to the orbital ellipse. This keeps the Sun and Earth in the angle presented by the sunshield no matter the pointing angle. At any point in time, the JWST can only image something that lies within this field - of course, things outside that must wait until the telescope's orbit brings it into view.

The telescope would need to rotate on the axis defined by a vector from the Sun through the Earth to view anything "above" or "below" the ecliptic. I am assuming there is no problem to accomplishing this.

Prometheus2508
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  • How does the field-of-view play into this? If I understand correctly, I think some area more-or-less shaped like an hourglass, centered on the Sun and aligned with the axis out of the ecliptic plane, is out-of-view. If only I could draw well... – Ludo Jan 10 '22 at 21:10
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    @Ludo I think I understand what you're describing, but believe it's wrong. JWST doesn't aim outward, but sideways from the a line drawn from the sun to L2.

    https://jwst-docs.stsci.edu/files/97976947/97976951/1/1596073033309/Fig3_JWST-side-view_coordinates_x1200px.png

    It rotates fully on the sun line, making a ring of observable angles at any point in the orbit. Swinging that ring around the orbit, it can look at just about anything. It can't see a cone-shaped region defined by the Earth-Sun L2 orbit and forming two symmetric cones with a 5 degree pitch above and below the ecliptic.

     – Prometheus2508 Jan 10 '22 at 22:15
  • @ Prometheus ---- can you clarify " It can't see a cone-shaped region defined by the Earth-Sun L2 orbit and forming two symmetric cones with a 5 degree pitch above and below the ecliptic." ? I understand JWST can aim at any point on the celestial sphere over a 6 month half-orbit – Woody Jan 10 '22 at 23:40
  • @Prometheus2508 Yes, you're right; thinking more about it it's indeed not an hourglass-like thing. Not sure what the shape of the unobservable space does look like though. I don't think it's a sphere centered on the sun, but not certain. – Ludo Jan 11 '22 at 07:53
  • @Woody It's the volume of space the telescope can't look at (not accounting for any minimum focusing distance). Draw the Earth-Sun L2 orbit, draw a line tangent to that ellipse at some arbitrary point. That line is the 90 deg look angle for JWST frame of reference. Its sunshield is designed to support a minimum look angle of 85 deg, so draw that line, then imagine rotating it about the L2 point. Anything sun-side of that surface it can't see from that point. Consider how that surface moves across all orbital points. Any point that never falls anti-sun of that surface is unobservable. – Prometheus2508 Jan 12 '22 at 00:18
  • @Prometheus2508 --- You're right. As well, the telescope is not articulated with the shield, so it can't look anti-sunward more than 45* or the Sun would fry its butt. So, at any given time JWST can only view about a third of the celestial sphere. But 3 months later, it is 90* around in its orbit and has a whole new piece of sky to look at. – Woody Jan 12 '22 at 01:58
  • "The telescope would need to rotate on the axis defined by a vector from the Sun through the Earth to view anything "above" or "below" the ecliptic." I would think the axis would be tangent to the orbit. – Acccumulation Jan 12 '22 at 04:06
  • @Accumulation The axis of rotation is the axis that stays fixed. If it rotated about an axis tangent to the orbit, it would rotate the sunshield out of solar coverage. – Prometheus2508 Jan 12 '22 at 04:14