<p>The Bright object observation by JWST is still being studied. The known problems that arises with Bright object observation is image persistence. Although it is said that James Webb space Telescope can observe near bright planets. This has limitations, The telescope cannot observe near earth object, Moon, Mercury, Venus, Comets near the Sun, Since the thermal design does not allow us to do so. Jupiter, Saturn, the asteroid belt, Neptune, Venus and Mars (which could be a challenge) can be observed (obviously not the Sun)</p>
<blockquote>
<p>Anything more than 85° from the Sun as viewed from L2, which includes Mars, Jupiter, Saturn, their satellites, the asteroid belt, and all outer Solar System objects. JWST also can't look farther from the Sun than 135° (i.e. within 45° of the anti-sun position) but outer solar system objects will all be observable some of the time. The field of regard limitations are a fundamental consequence of the observatory thermal design and the sunshield design that keeps the telescope and instruments cold. This means that the Sun, Earth, Moon, Mercury, and Venus, and of course sun-grazing comets and many known NEOs cannot be observed. We are not aware that the near-IR or mid-IR detectors can be damaged by bright light from any celestial source (except the Sun, which will never be in the JWST field of view). However, the detectors have some image persistence after bright sources are observed, so we have to minimize bright target exposures and allow time for recovery before observing fainter objects after bright ones. This subject is still being studied and quantified. Can Webb observe near bright planets?
Yes, unless the planet would fall into the Fine Guidance Sensor (FGS) field of view and prevent locating and tracking the guide star. Webb guide stars are generally in the range 17th to 12th magnitude (J), which is not that faint for a big space telescope. The FGS field of view is located several arc minutes from the science instruments. If the desired instrument and detector would saturate during a normal exposures, the observer can specify a subarray of the detector for more rapid readout. The fewer the pixels, the more samples each can get per second. The smallest field for this method for NIRCam is 128x128 pixels, which allows sampling up to 5 per second and a minimum exposure time of 0.18 seconds. We think only Mars will be a real challenge.</p>
</blockquote>
<p>Source: <a href="https://jwst.nasa.gov/content/forScientists/faqSolarsystem.html" rel="noreferrer">NASA</a></p>

CC BY-SA 4.0
2022-01-27T22:05:33.460Z
Near Earth Objects *can* be tracked with JWST but the rate is limited to 30mas/second (1.8"/min; maximum speed of Mars) which limits how close they can be observed ([Reference](https://jwst-docs.stsci.edu/methods-and-roadmaps/jwst-moving-target-observations/jwst-moving-target-proposal-planning/moving-target-acquisition-and-tracking))
astrosnapper
CC BY-SA 4.0
2022-01-28T06:40:13.847Z
related: [Which JWST instrument modes are compatible with observations of the bright trans-Earth planets Mars, Jupiter, and Saturn? Which aren't?](https://astronomy.stackexchange.com/q/48131/7982)
uhoh
CC BY-SA 4.0
2022-12-14T04:14:00.127Z
The brightest objects have negative magnitudes. So I think you mean the minimum magnitudes, though you may prefer a different phrasing. I'm actually interested in the faintest (largest magnitude) objects that JWST can detect over an expected observing run.
nealmcb
CC BY-SA 4.0
2022-01-27T17:18:57.553Z
2022-01-27T17:18:57.553Z
Kavin Ishwaran
<p>Similar to <a href="https://astronomy.stackexchange.com/q/48319/31410">Can JWST be damaged by looking at a bright star?</a>. I'd like to know the brightest magnitude that the James Webb Space Telescope can safely observe without damaging any of its instruments.</p>
<p><strong>Edit:</strong> Two proposed duplicates to my question are the one that I've linked and <a href="https://astronomy.stackexchange.com/questions/48320/31410">What exactly is the risk to any JWST instrumentation when looking at objects that are &quot;too bright&quot;?</a>. These are <em>not</em> duplicates, as the first is a yes/no question regarding if the JWST can be damaged from looking at a bright object, whlie the other asks for the risk involved.</p>

CC BY-SA 4.0
2022-01-27T17:12:52.727Z
Hi. How is this question different from the question you linked to?
JohnHoltz
CC BY-SA 4.0
2022-01-28T12:15:07.770Z
Does this answer your question? [What exactly is the risk to any JWST instrumentation when looking at objects that are "too bright"?](https://astronomy.stackexchange.com/questions/48320/what-exactly-is-the-risk-to-any-jwst-instrumentation-when-looking-at-objects-tha)
Carl Witthoft
CC BY-SA 4.0
2022-01-28T13:36:39.947Z
@CarlWitthoft I don't think it's a duplicate to the question that you've linked, nor to the one that I linked, as I am asking for the limiting magnitude of the JWST, compared to describing the risks of the JWST when observing such objects and a yes/no question, respectively.
WarpPrime
CC BY-SA 4.0
2022-01-28T16:34:29.897Z
I don't feel this question is duplicate. The OP's argument is reasonable. Hence I am ***voting to leave this question open***
Kavin Ishwaran
CC BY-SA 4.0
2022-02-02T20:48:56.277Z
agreed, **voting to leave open** and hoping we stop *non cogitas* votes to close as duplicate based nothing more than "hmm... looks similar"
uhoh
CC BY-SA 4.0
2022-08-23T14:40:33.080Z
Space Telescope Science Institute video [Webb's Field of Regard](https://www.youtube.com/watch?v=y0bOi3kVIBs)
uhoh
<space-telescope><apparent-magnitude><james-webb-space-telescope>
What is the smallest (brightest) magnitude that the JWST can safely observe?
WarpPrime
# What is the smallest (brightest) magnitude that the JWST can safely observe?

(Asked by: username_0 on 2022-01-27)

Similar to [Can JWST be damaged by looking at a bright star?](https://astronomy.stackexchange.com/q/48319/31410). I'd like to know the brightest magnitude that the James Webb Space Telescope can safely observe without damaging any of its instruments.

**Edit:** Two proposed duplicates to my question are the one that I've linked and [What exactly is the risk to any JWST instrumentation when looking at objects that are "too bright"?](https://astronomy.stackexchange.com/questions/48320/31410). These are *not* duplicates, as the first is a yes/no question regarding if the JWST can be damaged from looking at a bright object, whlie the other asks for the risk involved.

**Tags:** space-telescope, apparent-magnitude, james-webb-space-telescope

> **Comments:**
> **username_1**: Hi. How is this question different from the question you linked to?  
> **username_2**: Does this answer your question? [What exactly is the risk to any JWST instrumentation when looking at objects that are "too bright"?](https://astronomy.stackexchange.com/questions/48320/what-exactly-is-the-risk-to-any-jwst-instrumentation-when-looking-at-objects-tha)  
> **username_0**: @username_2 I don't think it's a duplicate to the question that you've linked, nor to the one that I linked, as I am asking for the limiting magnitude of the JWST, compared to describing the risks of the JWST when observing such objects and a yes/no question, respectively.  
> **username_3**: I don't feel this question is duplicate. The OP's argument is reasonable. Hence I am ***voting to leave this question open***  
> **username_4**: agreed, **voting to leave open** and hoping we stop *non cogitas* votes to close as duplicate based nothing more than "hmm... looks similar"  
> **username_4**: Space Telescope Science Institute video [Webb's Field of Regard](https://www.youtube.com/watch?v=y0bOi3kVIBs)  

---

## 1 answers

### Answer by username_3 on 2022-01-27. Score: 6

The Bright object observation by JWST is still being studied. The known problems that arises with Bright object observation is image persistence. Although it is said that James Webb space Telescope can observe near bright planets. This has limitations, The telescope cannot observe near earth object, Moon, Mercury, Venus, Comets near the Sun, Since the thermal design does not allow us to do so. Jupiter, Saturn, the asteroid belt, Neptune, Venus and Mars (which could be a challenge) can be observed (obviously not the Sun)

> Anything more than 85° from the Sun as viewed from L2, which includes Mars, Jupiter, Saturn, their satellites, the asteroid belt, and all outer Solar System objects. JWST also can't look farther from the Sun than 135° (i.e. within 45° of the anti-sun position) but outer solar system objects will all be observable some of the time. The field of regard limitations are a fundamental consequence of the observatory thermal design and the sunshield design that keeps the telescope and instruments cold. This means that the Sun, Earth, Moon, Mercury, and Venus, and of course sun-grazing comets and many known NEOs cannot be observed. We are not aware that the near-IR or mid-IR detectors can be damaged by bright light from any celestial source (except the Sun, which will never be in the JWST field of view). However, the detectors have some image persistence after bright sources are observed, so we have to minimize bright target exposures and allow time for recovery before observing fainter objects after bright ones. This subject is still being studied and quantified. Can Webb observe near bright planets?
> Yes, unless the planet would fall into the Fine Guidance Sensor (FGS) field of view and prevent locating and tracking the guide star. Webb guide stars are generally in the range 17th to 12th magnitude (J), which is not that faint for a big space telescope. The FGS field of view is located several arc minutes from the science instruments. If the desired instrument and detector would saturate during a normal exposures, the observer can specify a subarray of the detector for more rapid readout. The fewer the pixels, the more samples each can get per second. The smallest field for this method for NIRCam is 128x128 pixels, which allows sampling up to 5 per second and a minimum exposure time of 0.18 seconds. We think only Mars will be a real challenge.

Source: [NASA](https://jwst.nasa.gov/content/forScientists/faqSolarsystem.html)

> **Comments:**
> **username_5**: Near Earth Objects *can* be tracked with JWST but the rate is limited to 30mas/second (1.8"/min; maximum speed of Mars) which limits how close they can be observed ([Reference](https://jwst-docs.stsci.edu/methods-and-roadmaps/jwst-moving-target-observations/jwst-moving-target-proposal-planning/moving-target-acquisition-and-tracking))  
> **username_4**: related: [Which JWST instrument modes are compatible with observations of the bright trans-Earth planets Mars, Jupiter, and Saturn? Which aren't?](https://astronomy.stackexchange.com/q/48131/7982)  
> **username_6**: The brightest objects have negative magnitudes. So I think you mean the minimum magnitudes, though you may prefer a different phrasing. I'm actually interested in the faintest (largest magnitude) objects that JWST can detect over an expected observing run.  

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