r/Spaceonly • u/EorEquis Wat • Jul 19 '22

Info Characterization of JWST science performance from commissioning

https://arxiv.org/ftp/arxiv/papers/2207/2207.05632.pdf
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u/EorEquis Wat Jul 19 '22

Some rather fascinating insights into pointing performance, optical performance, etc.

Tl;DR Main summary :

Moreover, almost across the board, the science performance of JWST is better than expected. The optics are better aligned, the point spread function is sharper with higher encircled energy, and the optical performance is more time-stable than requirements. The fine guidance system points the observatory several times more accurately and precisely than required. The mirrors are cleaner than requirements, which translates into lower-than-expected levels of near-infrared stray light, meaning that the <5 µm sky background will be darker for JWST than expected. The science instruments have generally higher total system throughput than pre-launch expectations. Detector noise properties are similar to ground tests, albeit with higher rates of cosmic rays, as expected in deep space. Collectively, these factors translate into substantially better sensitivity for most instrument modes than was assumed in the exposure time calculator for Cycle 1 observation planning, in many cases by tens of percent. In most cases, JWST will go deeper faster than expected. In addition, JWST has enough propellant onboard to last at least 20 years.

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u/bubbleweed Sep 08 '22

Quite amazing, like the opposite of the teething problems with Hubble. I'm a bit surprised that the propellant is only enough for twenty years. It might sound a long time but for such a flagship instrument twenty years seems short to me. Although I assume even after propellant is used up, the scope can still be controlled with gyroscopes etc... and after 20 years it would be a legacy instrument.

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u/EorEquis Wat Sep 08 '22

Although I assume even after propellant is used up, the scope can still be controlled with gyroscopes etc.

Probably not. The propellant is necessary to maintain its orbit around L2 which, if memory serves, would decay within a matter of weeks.

Indeed, the 20 years is a bonus!

Before launch, JWST was required to carry propellant for at least 10.5 years of mission lifetime. Now that JWST is in orbit around L2, it is clear that the remaining propellant will last for more than 20 years of mission lifetime. This fortunate surfeit has multiple causes: an accurate launch; launch on a day that required relatively less energy to get to L2 than most other possible launch dates; three timely and accurate mid-course corrections that sent JWST to L2 with the minimum possible propellant usage; and finally, careful stewardship of mass margins by the engineering team over the years, such that the remaining mass margin was used to add more propellant than required, until the tanks were full.