# What percentage of visible stars will JWST be able to survey over the next 10 years?

The James Webb telescope is projected to stay operational over the next 10 years. During this time, what percentage of the visible universe/stars will it be able to survey? Of course, technically the answer is 100% if it just takes wide-angle photos of the sky, so I'm curious about "surveying" at the highest possible resolution available on the satellite.

• JWST != Gaia. It isn't doing surveying. It's doing deliberate studies of particular locations with a carefully planned (and costed) booking system.So the question is rather odd. It's like asking "What percentage of the worlds roads will the new Ferrari F1 car be able to drive along? Jul 20, 2022 at 21:58
• @JamesK if NASA had 10 clones of JSWT would it still only survey the currently planned locations in space? I assume the answer is no, so now I want to understand what percentage of locations could be possibly explored over all the planned bookings. Jul 20, 2022 at 22:04
• I think you're confusing "quantity" with "quality". Jul 20, 2022 at 23:53
• We don't have 10 copies of JWST and note that the targets for JWST are prioritized, so you'd expect that to mean the most important knowledge will be for what are targeted by the one JWST we have. More targets, with all due respect to those on the waiting list, won't necessarily be as good or as likely to get the desired information or may simply be providing confirmation of earlier results. If they could afford another JWST they'd probably build a different telescope to cover different kinds of target. Jul 21, 2022 at 0:38
• The widest field camera on JWST has a field of view about 132 arcseconds square. An arcsecond is 1/3600 of a degree. So you would need over 700 images to capture an area of one degree in the sky. A 360 degree by 1 degree stripe of the sky would require 262,000 images. A full survey of the sky would require roughly 95 million images. Depending how deep you want to image, an exposure could be mimutes to hours. Call it 1 hour each. It would take about 10,000 years to complete the survey. With 10 JWSTs, hey it's only 1,000 years. That's why you don't do surveys with it. Jul 21, 2022 at 15:26

## Not Very Many - It Isn't a Survey Scope

The JWST is designed as a narrow field of view telescope for examining very small targets. It is not suitable for sky surveys.

But let's go through some numbers:

Field of View

Field of view is how much of the sky JWST can see in a single image.

The widest field camera on JWST is NIRCam, and it has a field of view about 132 arcseconds square. An arcsecond is 1/3600 of a degree. So you would need over 700 images to capture an area of one degree in the sky.

A 360 degree by 1 degree stripe of the sky would therefore require 262,000 images. A full survey of the sky would require roughly 95 million images.

https://jwst-docs.stsci.edu/jwst-observatory-characteristics/jwst-field-of-view

Exposure time

So once you know how many images are needed to cover the sky, the next question is how long you want each exposure to be.
Depending how deep you want to image and what filters and instruments are in use, an exposure could be minutes to many hours.

We can get an idea of exposure times by looking at the planned Cycle 1 Observations. Drilling into status reports for planned NIRCam imaging of stars in the Milky Way, exposure times are on the order of 1-3 hours.

Call it 1 hour each, to give JWST the best shot at its survey. We'll also ignore repositioning and setup time for each image.

It would then take about 10,000 years to complete the survey. With 10 JWSTs, hey it's only 1,000 years. The scope has a lifespan of 5-20 years.

In comparison, a scope designed for all-sky survey like the upcoming Vera Rubin observatory will capture the entire sky every 3 days. And the Gaia all-sky astrometry telescope has already captured the 3d positions, distances, motion, color and brightness of over a billion stars in our galaxy, down to the 20th magnitude.

Dan Hanson has it right in terms of survey speed, but your question admits the possibility that you could simply point JWST at each of the 6000 "visible" stars in turn. Leaving aside the problem that some of the brightest stars might damage JWST's sensors, it appears thst you could do that in a few years (using the same sort of survey speed figures quoted by Dan).

However, you wouldn't do that because there would be no scientific case for doing so and JWST has a finite lifetime. The bright stars that JWST points at will mostly be targeted because they are known to host exoplanets that are suitable for closer investigation by JWST.

JWST is not a wide-field survey instrument. It is designed to (a) do careful studies of individual, already interesting, stellar objects and (b) to take very deep images, over narrow fields, of extragalactic space in order to explore the formation and evolution of galaxies and the cosmos.

• So you're saying that even if we had 10 clones of JSWT, the other 9 would be pointless as we don't have enough known investigation areas for them? Jul 21, 2022 at 18:56
• I don't think he said that. The JWST team very much has to pick and choose which proposals for observation get prioritized. If we had ten of them, I'm sure all ten would be very busy. But JWST has already soaked up a whole lot of funding for astronomy, and there are other priorities. And rather than spend money to build and launch more JWSTs, it is much better to take what we learn from JWST and use it to inform the design of future telescopes such as Luvoir. Jul 21, 2022 at 19:16
• @DanHanson better to take what we learn from JWST and use it to inform the design of future telescopes such as Luvoir => is this a personal opinion or the conclusion of a cost-benefit analysis that someone did at NASA? Jul 21, 2022 at 20:48
• Every ten years astronomers get together for the Decadal Survey, where they decide the priorities for upcoming research and which telescopes should be funded. The latest one is here: nationalacademies.org/our-work/… You'll note that no one wanted a copy of JWST. You can also see a list of proposed observatories here: en.wikipedia.org/wiki/List_of_proposed_space_observatories Jul 21, 2022 at 21:25
• As an example of the "bright stars that JWST points at will mostly be targeted because they are known to host exoplanets that are suitable for closer investigation by JWST" : The JWST recently spent almost two days observing Gliese 1214 because that star system has an exoplanet of interest. A telescope that spends almost two days observing one star system is not going to do a very good job as a survey telescope. Jul 22, 2022 at 10:36