The JWST’s field of regard excludes a significant portion of the celestial sphere in the anti-sun direction. See JWST field of regard (FOR) (source of the image below) Also see How much of the sky can the JWST see?


The spacecraft can’t pitch up to align the telescope boresight to within 45* of the anti-sun direction because the telescope would move out from behind the fixed sunshields.

enter image description here

Observatory within “Penumbra” of sunshield

Over a 6 month half-orbit of the sun, the entire sky is observable. But what happens if an interesting event occurs in the “blind spot”? Like Shoemaker-Levy or the arrival of Rama? A gravitational wave event?

If the observatory and sunshield were articulated to move 22.5* with respect to each other, the entire celestial hemisphere would be included in the field of regard.

It is not necessary to violate the single, rigid structure of the observatory to point it in a different direction. It is only necessary to ensure it is shaded in the new direction. This could be done by re-designing the sun shield to provide more coverage. The increased coverage need not be in all axes. Adding another 25* of rotation in the V2 axis would extend the field of regard to the entire celestial hemisphere. This would have no effect on reaction wheels, fuel consumption, failure risk or lifespan

Was this capability not seen as a design priority? I can’t imagine it would be a technical challenge.

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    $\begingroup$ JWST is not a telescope designed with transient events in mind. There is more than enough interesting science to be done in the sphere of the fixed stars. The main design challenge is to keep the telescope cool and stable to allow high-precision measurements, not change its angle all the time, which might induce tumbling motion. $\endgroup$ Commented Dec 6, 2021 at 23:18
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    $\begingroup$ "I can’t imagine it would be a technical challenge." Haha, technical challenge is James Webb's middle name! $\endgroup$ Commented Dec 7, 2021 at 3:27

2 Answers 2



Why does JWST have such a big Blind Spot?

Short answer: Because for a measly US \$500 million price tag and a launch in 2007, how could it not have one? (okay, now \$9,700 million and ca. 2022)

For near-all-sky performance behind that big sunshield, you'd need a much more complicated articulation mechanism.

Hubble gave us thirty years of science primarily between 100 and 800 nm (near UV to very near IR) and the new telescope will now cover a totally different wavelength range red to thermal IR (about 600 nm to 28 microns?) for a totally different set of astronomical observations.

The new telescope will look back in time by observing very distant and very strongly red-shifted objects from early in the universe's history.

It will also look at infrared light from closer objects at wavelengths we can't observe through Earth's atmosphere, including exoplanet phenomena among others.

None of these generally fall into the category of . It's already worth every bit of its \$500 million \$9,700 million price tag.

Why that would be hard and expensive:

The infrared telescope needs constant, stable exposure to the "cold of space" to keep all its optics and instruments at a stable cold temperature.

That means it needs constant, stable and reliable complete shielding from the Sun.

The current observatory is a single rigid body. The telescope and sunshield move together. The sunshield is large enough to allow +/- 25° tilting with respect to the Sun-telescope axis (thus the 85° to 135°) and can rotate anywhere around that axis without any moving parts except its spinning internal reaction wheels and occasional use of its thrusters. It does this in part by literally leveraging the photon pressure from the Sun.

Once operational, it doesn't have to use any major moving parts. There is no telescope mount.

A design that could point to a much larger fraction of 4π would have to have some degree of mechanical separation between the shield part and the telescope part, so that they could be pointed independently.

But they couldn't be completely separated, the way that the observatory conserves precious fuel is by taking full advantage of the Sun's photon pressure on the sunshield to maintain its orbit and adjust its attitude and to do momentum unloading of its reaction wheels.

Adding a new articulation feature like this would have taken extra years and added substantially to its \$500 million \$9,700 million price tag.

It also may have been much harder to fit in the payload fairing of the rocket capable of holding this already-huge-when-folded observatory.

It also adds to the risk of failure or shorter life.

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    $\begingroup$ @Woody that's easy to say in a comment but I don't yet believe it. The sunshield needs to remain flat for several reasons; each layer of the multilayer reflector needs to be stretched flat, so larger angular coverage means much larger size, weight, and problems fitting into the fairing of the rocket, and the additional photon pressure may become more of a liability than an advantage. Draw it up, simulate it in terms of shielding and photon pressure, show how it fits in the launch vehicle and keeps those reflective films safely deployed and flat, under tension and... $\endgroup$
    – uhoh
    Commented Dec 7, 2021 at 3:17
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    $\begingroup$ ...not in contact with each other and then maybe a "Why didn't they do it this way?" question can follow. $\endgroup$
    – uhoh
    Commented Dec 7, 2021 at 3:17
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    $\begingroup$ Also the only thing JWST as designed can't do is look at any arbitrary point immediately. It can however look at any point after a few months of waiting, as the Sun-JWST axis sweeps a full ~360° every year, thus yielding two observation windows a year for any point on the celestial sphere. $\endgroup$
    – TooTea
    Commented Dec 7, 2021 at 8:29
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    $\begingroup$ @uhoh I agree, my point was that many time-domain observations are still possible. Even points close to the ecliptic get two months-long observation windows just a few months apart. Adding coverage for that relatively small blind spherical cap wouldn't help too much, just turn say 2x2 months into 7 months straight (too lazy to do actual calculations). There's still the whole other hemisphere that's unobservable and going to stay that way. $\endgroup$
    – TooTea
    Commented Dec 7, 2021 at 9:48
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    $\begingroup$ @uhoh Great video link! Exciting times for astronomy and cosmology. $\endgroup$
    – Woody
    Commented Dec 24, 2021 at 20:59

The obvious answer is: because it is not required.

The relevant requirements are given in the official Mission Requirement Document JWST-RQMT-00634, Oct 2007

Especially Section (Celestial Sphere Coverage).

Note that in a complex system engineering, a guiding principle called KISS (Keep It Stupid & Simple) is often used. This is because if a feature is added while it is not required by the mission, analyzing it for possible adverse interactions with other functions and documenting it, both take time (and money). Recall that the documentations must cover not only the design and implementation phases but also the future operational phases.

A valid and interesting question is in fact why this is not required by the scientific goals. @uhoh questions in Astronomy SE may provide useful insightful answers.

  • $\begingroup$ "both take time (and money)" We can all be grateful that such rigid controls were effected on the cost and schedule of the JWST. $\endgroup$ Commented Dec 7, 2021 at 14:00
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    $\begingroup$ @Organic Marble, it would be fair to say that the overruns are not due to "features creeping syndrome". AFAIK, they are due to the complexities of Test & Integration. $\endgroup$
    – Ng Ph
    Commented Dec 7, 2021 at 14:12
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    $\begingroup$ The applicable paragraph is " Field of Regard MR-104 The Observatory Field of Regard shall be at least 35% of the celestial sphere" This is minimum, not optimum. $\endgroup$
    – Woody
    Commented Dec 7, 2021 at 17:03
  • $\begingroup$ @Woody, off course, and this is how the entire Mission Requirement document is formulated (as any requirement document should). Your "What if" question would not be taken onboard by the JWST's Project Manager (assuming you were part of the design team). However, the Project Manager may ask: what is the constraint that (optimally) limits the achievable Field of Regard to 50° in actual operation? I assume that somebody has made a technical note to address such a question. It is likely to be a multi-disciplinary task. $\endgroup$
    – Ng Ph
    Commented Dec 7, 2021 at 18:56

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