<p>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 &quot;visible&quot; 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).</p>
<p>However, you wouldn't do that <strong>because there would be no scientific case for doing so</strong> and JWST has a finite lifetime. The bright stars that JWST points at will mostly be targeted because they are <em>known</em> to host exoplanets that are suitable for closer investigation by JWST.</p>
<p>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.</p>

CC BY-SA 4.0
2022-07-21T18:56:22.873Z
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?
JonathanReez
CC BY-SA 4.0
2022-07-21T19:16:14.083Z
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.
Dan Hanson
CC BY-SA 4.0
2022-07-21T20:48:14.157Z
@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?
JonathanReez
CC BY-SA 4.0
2022-07-21T21:25:50.970Z
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:  https://www.nationalacademies.org/our-work/decadal-survey-on-astronomy-and-astrophysics-2020-astro2020  You'll note that no one wanted a copy of JWST.   You can also see a list of proposed observatories here:  https://en.wikipedia.org/wiki/List_of_proposed_space_observatories
Dan Hanson
CC BY-SA 4.0
2022-07-22T10:36:51.540Z
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.
David Hammen
CC BY-SA 4.0
2022-07-21T18:43:45.963Z
2022-07-21T18:43:45.963Z
ProfRob
<h2>Not Very Many - It Isn't a Survey Scope</h2>
<p>The JWST is designed as a narrow field of view telescope for examining very small targets.  It is not suitable for sky surveys.</p>
<p>But let's go through some numbers:</p>
<p><strong>Field of View</strong></p>
<p>Field of view is how much of the sky JWST can see in a single image.</p>
<p>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.</p>
<p>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.</p>
<p><a href="https://jwst-docs.stsci.edu/jwst-observatory-characteristics/jwst-field-of-view" rel="noreferrer">https://jwst-docs.stsci.edu/jwst-observatory-characteristics/jwst-field-of-view</a></p>
<p><strong>Exposure time</strong></p>
<p>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.<br />
Depending how deep you want to image and what filters and instruments are in use, an exposure could be minutes to many hours.</p>
<p>We can get an idea of exposure times by looking at the planned <a href="https://www.stsci.edu/jwst/science-execution/approved-programs/cycle-1-go" rel="noreferrer">Cycle 1 Observations</a>.  Drilling into status reports for planned NIRCam imaging of stars in the Milky Way,  exposure times are on the order of 1-3 hours.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>

CC BY-SA 4.0
2022-07-21T18:15:14.853Z
2022-07-21T19:17:39.670Z
2022-07-21T19:17:39.670Z
Dan Hanson
<p>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 &quot;surveying&quot; at the highest possible resolution available on the satellite.</p>

CC BY-SA 4.0
2022-07-20T21:58:50.217Z
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?
James K
CC BY-SA 4.0
2022-07-20T22:04:13.733Z
@JamesK if NASA [had 10 clones of JSWT](https://astronomy.stackexchange.com/questions/49977/why-not-launch-a-few-more-copies-of-jwst?noredirect=1&lq=1) 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.
JonathanReez
CC BY-SA 4.0
2022-07-20T23:53:37.390Z
I think you're confusing "quantity" with "quality".
StephenG - Help Ukraine
CC BY-SA 4.0
2022-07-20T23:54:33.440Z
@StephenG-HelpUkraine so no quality would be gained if NASA had 10 copies of JSWT?
JonathanReez
CC BY-SA 4.0
2022-07-21T00:23:29.940Z
This particular instrument of science is intended to *answer questions, discover unexpected things, and raise further questions*. It's not meant to survey or even primarily focus on stars. An instrument like the [Vera C. Rubin Observatory](https://en.wikipedia.org/wiki/Vera_C._Rubin_Observatory) is designed to do surveys for example. See https://astronomy.stackexchange.com/questions/tagged/vera-c-rubin-observatory
uhoh
CC BY-SA 4.0
2022-07-21T00:38:36.410Z
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.
StephenG - Help Ukraine
CC BY-SA 4.0
2022-07-21T02:34:28.273Z
@StephenG-HelpUkraine wait, are you saying that out of the *entire observable universe* we’ve already managed to pick out all the interesting targets and having 9 extra copies of the same satellite would be pointless?
JonathanReez
CC BY-SA 4.0
2022-07-21T10:12:04.810Z
I was quite clear in what I said.  Frankly I consider these sort of hypothetical questions that have no relationship to the real world (where we're lucky to have one JWST) silly.
StephenG - Help Ukraine
CC BY-SA 4.0
2022-07-21T14:36:06.000Z
@JonathanReez The answer to this kind of question are always the same: resources are limited. It really is silly to ask "Should we have more X?"
uhoh
CC BY-SA 4.0
2022-07-21T15:26:20.303Z
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.
Dan Hanson
CC BY-SA 4.0
2022-07-21T15:34:01.127Z
I guess to sort of answer the question,  if you decided to use JWST as a survey scope,  in 10 years you could cover about 1/1000 of the sky.  But if you wanted each image to see all the way back to the earliest stars/galaxies,  you would need about 100 hours for each image.  In that case, you could only 'deep field image' 1/100,000 of the sky in 10 years.  That's .001% of stars.
Dan Hanson
CC BY-SA 4.0
2022-07-21T15:59:18.407Z
@DanHanson thanks, want to add an answer? And I'm surprised that the idea of 10 JSWTs is taken with hostility rather than excitement!
JonathanReez
CC BY-SA 4.0
2022-07-21T21:31:12.360Z
I don't see what the downvotes are for.  It's obviously not JWST's mission for those that know, but still a perfectly sensible question.
Greg Miller
CC BY-SA 4.0
2022-07-21T21:44:51.613Z
@GregMiller and the question isn't even suggesting JSWT *should* survey the full sky, it just asks how much of the universe won't be explored even though we'd like to if we had more clones of JSWT.
JonathanReez
CC BY-SA 4.0
2022-08-04T02:36:28.633Z
Besides buying 9 more Webb telescopes, you'll need to buy lots more capacity for the Deep Space Network, and lots more capacity for the data archives, and most of all, lots more capacity for the scientists dreaming up, proposing and analyzing the results. As others have noted, you'd be much better off using those resources to support different missions which would provide complementary observations of the universe. That's why the Decadal Survey didn't say "launch 10 JWSTs".
nealmcb
<observational-astronomy><space-telescope><james-webb-space-telescope>
What percentage of visible stars will JWST be able to survey over the next 10 years?
JonathanReez
# What percentage of visible stars will JWST be able to survey over the next 10 years?

(Asked by: username_0 on 2022-07-20)

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.

**Tags:** observational-astronomy, space-telescope, james-webb-space-telescope
> **Comments:**
> **username_1**: 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?  
> **username_0**: @JamesK if NASA [had 10 clones of JSWT](https://astronomy.stackexchange.com/questions/49977/why-not-launch-a-few-more-copies-of-jwst?noredirect=1&lq=1) 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.  
> **username_2**: I think you're confusing "quantity" with "quality".  
> **username_0**: @StephenG-HelpUkraine so no quality would be gained if NASA had 10 copies of JSWT?  
> **username_3**: This particular instrument of science is intended to *answer questions, discover unexpected things, and raise further questions*. It's not meant to survey or even primarily focus on stars. An instrument like the [Vera C. Rubin Observatory](https://en.wikipedia.org/wiki/Vera_C._Rubin_Observatory) is designed to do surveys for example. See https://astronomy.stackexchange.com/questions/tagged/vera-c-rubin-observatory  
> **username_2**: 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.  
> **username_0**: @StephenG-HelpUkraine wait, are you saying that out of the *entire observable universe* we’ve already managed to pick out all the interesting targets and having 9 extra copies of the same satellite would be pointless?  
> **username_2**: I was quite clear in what I said.  Frankly I consider these sort of hypothetical questions that have no relationship to the real world (where we're lucky to have one JWST) silly.  
> **username_3**: @JonathanReez The answer to this kind of question are always the same: resources are limited. It really is silly to ask "Should we have more X?"  
> **username_4**: 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.  
> **username_4**: I guess to sort of answer the question,  if you decided to use JWST as a survey scope,  in 10 years you could cover about 1/1000 of the sky.  But if you wanted each image to see all the way back to the earliest stars/galaxies,  you would need about 100 hours for each image.  In that case, you could only 'deep field image' 1/100,000 of the sky in 10 years.  That's .001% of stars.  
> **username_0**: @DanHanson thanks, want to add an answer? And I'm surprised that the idea of 10 JSWTs is taken with hostility rather than excitement!  
> **username_5**: I don't see what the downvotes are for.  It's obviously not JWST's mission for those that know, but still a perfectly sensible question.  
> **username_0**: @GregMiller and the question isn't even suggesting JSWT *should* survey the full sky, it just asks how much of the universe won't be explored even though we'd like to if we had more clones of JSWT.  
> **username_6**: Besides buying 9 more Webb telescopes, you'll need to buy lots more capacity for the Deep Space Network, and lots more capacity for the data archives, and most of all, lots more capacity for the scientists dreaming up, proposing and analyzing the results. As others have noted, you'd be much better off using those resources to support different missions which would provide complementary observations of the universe. That's why the Decadal Survey didn't say "launch 10 JWSTs".  

---

## 2 answers

### Answer by username_4 on 2022-07-21. Score: 10 | Accepted answer

## 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](https://www.stsci.edu/jwst/science-execution/approved-programs/cycle-1-go). 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.


---

### Answer by username_7 on 2022-07-21. Score: 7

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.

> **Comments:**
> **username_0**: 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?  
> **username_4**: 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.  
> **username_0**: @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?  
> **username_4**: 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:  https://www.nationalacademies.org/our-work/decadal-survey-on-astronomy-and-astrophysics-2020-astro2020  You'll note that no one wanted a copy of JWST.   You can also see a list of proposed observatories here:  https://en.wikipedia.org/wiki/List_of_proposed_space_observatories  
> **username_8**: 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.  

---

