It is in all the news that the first images of James Webb Telescope "exceed expectations", but in what features? resolution?

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    $\begingroup$ Is it worth adding a link into the question to a news story such as James Webb First Images Explained as Telescope Exceeds NASA's Expectations? $\endgroup$ Mar 25, 2022 at 10:17
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    $\begingroup$ It has to "exceed expectations" for the US-american taxpayer. The ESO and ESA partners have a much more stable funding system for their project parts and therefore no need to keep the public and a congress convinced about how over-the-top-world-class-awesome they are. $\endgroup$ Mar 25, 2022 at 10:39
  • $\begingroup$ I think a reference is needed for the quote "exceed expectations". At this point with one or two diagnostiic images, there is no basis for that comment with respect to its research goals. Perhaps after 25 years of development it could be said that jwst "exceeds expections" in that all of the startup/setup mechanisms worked to get it started. $\endgroup$
    – tckosvic
    Mar 26, 2022 at 20:31
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    $\begingroup$ Maybe the launch was better than might have been expected. Everyone seemed pleased by how much propellant was left and that the expected lifetime for JWST was a lot longer than the earlier conservative estimates. $\endgroup$
    – Roger Wood
    Mar 27, 2022 at 0:53
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    $\begingroup$ One way it exceeded expectations is in fuel available. The launch vehicle itself exceeded expectations for accuracy in placing JWST into its transfer orbit to L2, meaning JWST used less of its own fuel for correction. The result was a doubling of its expected fuel lifespan from approximately 10 years to as much as 20 years. $\endgroup$
    – Dan Hanson
    Aug 6, 2022 at 17:07

3 Answers 3


The launch

James Webb did indeed exceed expectations in several ways. Firstly, as Dan Hanson comments, the launch itself was "nominal", i.e. a near-perfect trajectory, resulting in the telescope needing to spend a minimal amount of fuel during the three correction boosts on its journey to L2. The result was that Webb will have fuel enough to maintain its orbit around L2 for up to 20 years, instead of the expected 10 years.

Whether or not the instruments will last so long is not known, though; they have been designed to last at least five years, with the goal of ten. But if we're lucky that the instruments last longer, we have a telescope for 20 years.


After the telescope had deployed, it spent several weeks aligning the mirrors, nanometer by nanometer. Marshall Perrin from STScI gave a talk at my department last year where he described this process as "watching grass grow". The requirement was that Webb should be diffraction-limited down to 2 µm (McElwain et al. 2023), but it has actually turned out to be diffraction-limited down to 1.1 µm.

The requirement for the unobscured collecting area of the mirror was $25\,\mathrm{m}^2$, but was measured (using NIRCam's pupil lens) to be $25.44\,\mathrm{m}^2$.

Moving parts of titanium, carbon fiber, etc., build up different amounts of stress, and are released as somewhat unpredictable moments. Luckily, tests show that it's not bad. But sometimes the wing mirrors suddenly move by ~½ µm! Hopefully these events will become rarer over time.


A recent report by Jonathan Gardner, the Deputy Senior Project Scientist for James Webb, and 1000+ other authors (Gardner et al. 2023), describe several additional factors in which James Webb has exceeded expectations. In most cases, the throughput of the instrument is higher than pre-launch expectations. For NIRSpec's integral field unit, the throughput is slightly lower in the red, but higher in the blue. NIRCams's wide-field slitless spectroscopy is 20–40% higher than pre-launch expectations, and NIRSpec's is also better.

For NIRCam, the imaging sensitivity exceeds the pre-launch expectations in almost all filters. The requirements were 11.4 nJy and 13.8 nJy at 2.0 μm and 3.5 μm, respectively, for a point-source, but were measured to be 7.3 nJy and 8.8 nJy.

The instruments (and the telescope) has turned out to be immaculately clean, with no signs of any ice deposition or other contaminants.

Guiding, pointing, and optical stability

MIRI's cryocooler and the reaction wheels do in principle cause some vibration of the telescope, but this has turned out to be insignificant. Tests (Lallo & Hartig 2022) show that the jitter is only ~1 milli-arcsec (mas), which is significantly better than the expectations, and the requirement of 7 mas.

It has also turned out that the telescope is able to track moving targets at 3× the speed it was designed to: The tracking requirement was 30 mas/s, but when DART slammed into the asteroid P/Didymos, Webb tracked the impact at 105 mas/s.

Other super-expectation performances are discussed in the papers cited above.


Target tracking, for one, and corresponding slew (panning) capability appear to be exceeding the design specifications. If nothing else then, JW will have a more efficient observing cycle, spending less time pointing from target to target. The r capability (target-to-Sun distance) will likely move inward a bit.

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    $\begingroup$ Can you link to a source that confirms this? $\endgroup$
    – James K
    Aug 1, 2022 at 22:21
  • $\begingroup$ The JW staff, as I stated in a similar SE question. She came to our meeting and gave us a status update. $\endgroup$ Aug 1, 2022 at 23:10
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    $\begingroup$ Nice, can you link to the similar SE question? $\endgroup$
    – James K
    Aug 1, 2022 at 23:17
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    $\begingroup$ @caInstrument ‘As I stated in a similar SE’ answers should be self contained without having to dig around other posts $\endgroup$
    – user438383
    Aug 2, 2022 at 18:05
  • $\begingroup$ That other SE question is actually redundant, no further info there that isn’t already here. Just pointing out that this isn’t the first time it came up. Yes, in hindsight it’s a bit ‘garden-path sentence.’ Lots of people are asking similar questions, maybe some of them should search for prior questions before posting. $\endgroup$ Aug 4, 2022 at 14:26

About the James Webb Telescope

The James Webb telescope pushes the limits farther than any telescope in known history. The 10 billion dollar telescope was designed to do more than any telescope or human being has ever been capable of.

Goals of the James Webb Telescope

It has been said that the "goals" for the James Webb telescope are (1.) Trying to observe/study the first light in the universe, or in other words learning more about the creation of the universe. (2.) How galaxies formed in the early creation of the universe. (3.) The birth of stars and other planetary systems such as our own solar system.

The answer to your question: How exactly does the James Webb Telescope exceed expectations, and what are these expectations? Of course, it is no surprise that the telescope is exceeding some sort of expectation. The original budget of the James Webb Telescope was planned to be around $1 billion, until the creators found that with more funding / money, their original goals could be far beyond what they had ever planned. It was originally supposed to explore the cosmos, researching more on the solar system and beyond, but now with it's 10 billion dollar budget, they can do more than just that such as gathering information on the original light from the universe and how it was created.

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    $\begingroup$ I don't see how does it answer the question. $\endgroup$
    – Heopps
    Apr 4, 2022 at 10:25
  • $\begingroup$ Apparently, you didn't see the part about how more money would expand the goals and research from what they were originally planning to do. $\endgroup$
    – Mintvbz
    Apr 4, 2022 at 17:45
  • $\begingroup$ I think the question was referring to the expectation of the final project, the pre-launch expectations, after the 10 G$ were already spent, versus the actual telescope capabilities. I don't think it asked about the change in expectation between the first project drafts 15 years ago and the final product $\endgroup$
    – Prallax
    Aug 2, 2022 at 7:19

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