I'm doing some digging around ultra-diffused galaxies and would like to know what is the faintest visible light (biggest apparent magnitude) JWST can see. I have not been able to find it online.
Thank you!
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2$\begingroup$ Wikipedia mentions magnitude 34, but does not provide a reference. $\endgroup$– Pierre PaquetteCommented Dec 3, 2023 at 7:24
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2$\begingroup$ The answer (also) depends on integration time of the images. $\endgroup$– planetmakerCommented Dec 3, 2023 at 10:13
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$\begingroup$ The size of the object also matters. A single pixel would easily get lost in the noise. But a large object of the same surface brightness could stand out. $\endgroup$– Greg MillerCommented Dec 3, 2023 at 22:52
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1 Answer
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I went kind of far afield with this answer, so the super-short and straightforward response is: "The faintest object it can view is limited only by the amount of time that we're willing to devote to staring at the same spot in the sky and how long the observatory continues to operate."
A Simple Answer
The answer really isn't simple, but just to have a number, if we look at Hubble's Ultra Deep Field image, it was taken over 11.3 days with 800 separate images of ~20 minutes each. That allowed us to see objects which were magnitude 31 and brighter in its field of view. JWST's Deep Field was 12.5 hours and I haven't been able to find the number of exposures/exposure time. It resulted in us being able to see objects which have magnitudes 34 and brighter. If JWST observed the same region of space for longer, it would be able to see dimmer objects still, with the only real limit being total observation time. Unfortunately it's not as simple as saying, "JWST was 1/20th of Hubble's exposure time, so we could see things that are 20 times fainter still if the total times were equal." We can say, however, that if the times were equal, JWST's signal to noise ratio would improve by a factor of ~4.6. The image would become significantly sharper, fine details would be observable that are now simply blurs, and some extremely faint objects that we can't distinguish from noise would come into view.
Our Eyes
There is a bit of a problem with this question that is based on the fact that our vision works at a more or less fixed time scale as compared to a camera doing astrophotography. To see something our retinas need to absorb enough photons in the same spot in a short enough time to activate the nerves running to our brains. (Numbers and rates vary wildly depending on whether we're talking about the absolute minimum detectable flash of light vs normal low light vision). If we don't get a high enough rate of incoming photons we just don't see anything, no matter how long we stare.
As a result, we can say that with X telescope the limiting magnitude for visual observation is Y, given some standard visual acuity and observation conditions. There are simplistic calculators to help hobbyists figure out limiting magnitudes for telescopes based on aperture, so if you could plonk down the JWST in your back yard and somehow look through it, you could see objects at magnitude ~22.
A Camera
A digital camera, particularly a research grade camera (or similar sensor) that is cooled to extremely low temperatures, is extraordinarily good at capturing photons. In the best detectors, nearly every photon that hits is counted. They are also able to capture photons for an arbitrary length of time (anywhere from seconds to half an hour or longer, depending on equipment and the purpose of the image). The way that astrophotographers (whether it's hobbyists or JWST) take images is by exposing a sensor for a period of time, recording that data (called a sub-exposure, or sub), then repeating. Once a number of subs are gathered, any with problems can be thrown out and the remaining subs are averaged (aka stacked) together. Each sub has real data, called signal, and false data, called noise. Because the signal is always in the same place but the noise varies randomly, the stacking process improves the signal to noise ratio as more and more subs are added. In general, to improve the signal to noise ratio by a factor of N, you need N² subs to be stacked, so there are diminishing returns but no real upper limit to the extent to which you can increase this by stacking.
TL;DR: Astrophotography cameras/sensors gather light over a period of time, then use software to average many short exposures into a low noise image. The length of a single exposure would change based on what was being observed, why it was being observed, and the instrument that was doing the observation. NASA offers aspiring JWST researchers a set of tools and information to calculate the exposure length that best suits their research goals.
What That All Means
When a telescope like JWST images the sky, there really is no theoretical limit to the faintness of objects it can observe. With thousands of long sub-exposures of the same area, it could detect far, far fainter objects than it ever actually will due to time budgeting among researchers. That said, if JWST took a series of images equivalent to Hubble's Ultra Deep Field, its larger mirror and improved instrumentation would allow it to collect significantly more light and would result in us seeing much fainter objects than were visible in the JWST Deep Field.
In Hubble's Ultra Deep Field, the faintest objects were around magnitude 31. In the much shorter duration JWST Deep Field of the same area of the sky, objects of magnitude 34 were visible. We don't know if the sub-exposure lengths for these images were equal or not, which matters, but we do know that more sub-exposures and longer sub-exposures would result in seeing objects fainter still. In the end, the faintest object it can view is limited only by the amount of time that we're willing to devote to staring at the same spot in the sky and how long the observatory continues to operate.
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$\begingroup$ JWST performs station keeping every ~25 days or so. Is it able to use its instruments while the engines are burning? I would assume not, or at least, that the vibrations this would cause would blur any image beyond use. This might therefore be a bound on the longest duration of any single exposure. Given an expected life time of ~20 years, a maximum of ~250 such exposures could be taken. $\endgroup$ Commented Feb 24 at 16:46
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$\begingroup$ @ScienceSnake Usually, a telescope would rarely expose for more than ~30 minutes (in my experience), because the risk of cosmic rays (or bright objects) saturating pixels becomes too high. Long exposures are therefore divided in multiple shorter exposures, subsequently taking the median of all images, so that pixels with high count rates are ignored. Between each exposure, you also dither the telescope a little, to avoid objects falling on the same pixels during each exposure, in case some pixels are bad. $\endgroup$– pelaCommented Feb 24 at 22:17
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1$\begingroup$ @pela Indeed, and usually it's less than that. JWST uses nondestructive photon counting methods though, so it's very hard to get a saturated pixel. I was trying to make the point that there are objects that emit so few photons per second that very short exposures will never see them as a practical matter, but I didn't want to get into a huge discussion of digital astrophotography in general. $\endgroup$ Commented Feb 25 at 1:05
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$\begingroup$ @ScienceSnake (Accidentally @ -ed you in my earlier comment, apologies if you get confusing notifications.) Yeah, and I would be stunned if it could observe reliably for that long. A major benefit of doing 6 10 minute subs instead of a single hour long exposure is that if something goes wrong at minute 23, you lose 10 minutes of data instead of 60. There are other problems with very long exposures, but even with research grade observatories in space bad frames are a fact of life. $\endgroup$ Commented Feb 25 at 1:12
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$\begingroup$ There will always be noise of some sort, either instrumental or in the radiation field itself, so the magnitude of detectable objects is certainly not without limits. $\endgroup$– ThomasCommented Mar 5 at 18:59