In The Observatory @Donald.McLean linked to Scientists will peer at first galaxies with James Webb telescope which says in part:

Many of the proposed tasks for the Webb telescope were planned and approved in the 1990s as the observatory was under initial development, said Klaus Pontoppidan, an astronomer and Webb project scientist at the institute.

"We've just added the final projects that will be included during the first year or 13 months," Pontoppidan said.

The institute, which solicits proposals each year for Hubble, will do so for Webb, as well, and each of the space telescopes attract more than 1,000 such ideas each year, he said.

The Webb telescope will take about three months to power up, extend its massive mirror and begin observations. After that, scientists based at institutions around the world have planned more than 10,000 hours of observations with it.

Hundreds of hours

It takes an average of 25 hours to observe a single planet, but longer observations -- such as detecting the oldest and farthest galaxies -- will require about 200 to 300 hours, Pontoppidan said.

"The first galaxies in the universe, those are really faint objects, so you have to stare at them for a long time," he said.

Another big astronomy question for the telescope is to determine which came first -- galaxies or the black holes that reside at the heart of most large galaxies, said Rogier Windhorst, an astronomer and physics professor at Arizona State University.

JWST is a very different telescope and suite of instruments than Hubble and is designed to address several different questions that Hubble can't. We should not make an apples-to-apples comparison between the two.

But is a generalization on the expected durations of observations possible? Even though HST does have some famous long duration deep field exposures, many individual exposures at least have durations of only minutes.

Question: Will JWST have fewer observing sessions of much longer duration compared to Hubble?

  • $\begingroup$ One might suppose that orbital geometry alone would allow JWST much longer uninterrupted observing windows around Earth's L2 point, while Hubble's field-of-view in LEO is constantly interrupted by the Earth. $\endgroup$
    – Connor Garcia
    Commented May 19, 2021 at 22:03
  • 1
    $\begingroup$ @ConnorGarcia okay so the "famous long duration deep field exposures" are broken up into many exposures both for CCD reasons and because the Earth might get in the way regularly (does that depend on the the particular object's direction?) But I'm just counting that as one observation at least from the point of view of a proposal's "total hours" or however it's defined. Here's an example of a 5.5 hour observation with exposure times of order 5 minutes each. Will JWST have a lot of tasks that short? $\endgroup$
    – uhoh
    Commented May 19, 2021 at 22:08
  • $\begingroup$ So, you are asking about the cumulative "time-on-target" totals for observations for JWST rather then the individual durations of the shorter snapshots, correct? $\endgroup$
    – Connor Garcia
    Commented May 19, 2021 at 22:59
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    $\begingroup$ Hubble has two Continuous Viewing Zones (CVZ) near the poles and the deep field observations are all from one of those. $\endgroup$ Commented May 21, 2021 at 18:08


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