The performance of the James Webb Space Telescope

NASA, ESA and CSA:

During the six-month commissioning period of JWST, the mission team worked with dedication and focus to prepare the observatory for science operations. A key part of commissioning activities was characterizing the on-orbit performance of the observatory. This document summarizes those results, drawn from many activities and analyses over the past six months.

The design and architecture of JWST, and pre-launch predicted performance, are described elsewhere. This document summarizes what was actually delivered, and how the actual performance differs from pre-launch expectations. We summarize in turn the performance of the spacecraft, telescope, science instruments, and ground system. Further details will appear in a planned series of papers in a PASP special issue on JWST.

The key outcome of six months of commissioning is this: JWST is fully capable of achieving the discoveries for which it was built. JWST was envisioned “to enable fundamental breakthroughs in our understanding of the formation and evolution of galaxies, stars, and planetary systems” (Gardner et al. 2006) — we now know with certainty that it will. The telescope and instrument suite have demonstrated the sensitivity, stability, image quality, and spectral range that are necessary to transform our understanding of the cosmos through observations spanning from near-earth asteroids to the most distant galaxies.

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.