<p>The MIRI uses Si:As doped detectors which I believe are just about destruction-proof.  The spectral dispersion is done with Ge and ZnS material, also tough things.  There's a wealth of info on the cooling system <a href="https://www.jwst.nasa.gov/content/about/innovations/cryocooler.html" rel="noreferrer">here</a>, which suggests that the  only failure mechanisms are He leakage or mechanical wear.<br />
A detailed <a href="https://jwst-docs.stsci.edu/jwst-mid-infrared-instrument/miri-observing-modes/miri-imaging" rel="noreferrer">&quot;user manual&quot;</a> specifically points out that there are subarrays dedicated to bright(er) targets, avoiding saturation by shortening frame times.<br />
Given that, my interpretation is that a bright (in the IR) source might cause a heat load on MIRI that exceeds the cooling capability, causing the sensor to stop functioning until it can once again reach the necessary cryotemps.  There doesn't seem to be any element which could suffer physical (non-reversible) damage from a bright source.</p>

CC BY-SA 4.0
2022-01-27T14:01:06.523Z
2022-01-27T14:01:06.523Z
Carl Witthoft
<p><a href="https://astronomy.stackexchange.com/a/48317/11841">This answer by pela to question #48317</a> at one point states</p>
<blockquote>
<p>The star should be bright, but probably not too bright (like Betelgeuse), since we don't want to burn MIRI off from the beginning</p>
</blockquote>
<p><a href="https://astronomy.stackexchange.com/questions/48316/why-hd-84406-will-be-chosen-as-the-first-target-for-testing-jwst#comment105617_48317">A comment by Carl Kevinson</a> points out this statement, and asks a reasonable question:</p>
<blockquote>
<p>&quot;but probably not too bright (like Betelgeuse), since we don't want to burn MIRI off&quot; Is damage to the instrument a risk when observing a bright star?</p>
</blockquote>
<p>I am generally aware of the sunshield in place to protect it from the Sun's comparatively close and high radiation. It seems that maintaining temperature is the key issue this is concerned with. The whole craft is aimed to be at most 40 K; MIRI has two more systems that reduce it specifically down to 7 K.</p>
<p>This is presumably a noise-reduction strategy, given that the instruments are generally trying to image in the IR spectrum, and any excess heat is going to be emitted primarily as IR. MIRI may be more susceptible to such noise, given its ability to see longer wavelengths (i.e. lower energy, and more prominent from colder things than shorter wavelengths, as far as I understand).</p>
<p>However, it does not seem that a distant star should be able to do much affecting the instruments in the same way as heat from the Sun, otherwise there would be additional shielding on the &quot;outside&quot; of the craft as well (which might largely defeat the purpose).</p>
<p>What risks exist to the JWST instrumentation, that come specifically from looking at a star that is &quot;too bright&quot; as opposed to the risks of being unshielded from the Sun and/or from getting too hot?</p>
<p>What kind(s) of stars would be counted as &quot;too bright&quot;? Examples would be helpful (though please do not include the Sun).</p>

CC BY-SA 4.0
2022-01-27T14:14:39.980Z
A similar question is asked [here](https://astronomy.stackexchange.com/q/48319/5264). As I commented there, you can definitely damage the detector _temporarily_, so that you see a "ghost image" of the star on the subsequent exposures (I did that myself with an Earth-based 1.5 m telescope, and JWST's area is ~20 larger and has a ~10 times higher resolution so of the order 1000 more photons per pixel for point sources). However, damaging MIRI permanently is _probably_ not something you should worry about (according to my colleague whose expertise is detectors and electronics).
pela
<infrared><james-webb-space-telescope>
What exactly is the risk to any JWST instrumentation when looking at objects that are "too bright"?
Nij
# What exactly is the risk to any JWST instrumentation when looking at objects that are "too bright"?

(Asked by: username_0 on 2022-01-26)

[This answer by username_1 to question #48317](https://astronomy.stackexchange.com/a/48317/11841) at one point states

> The star should be bright, but probably not too bright (like Betelgeuse), since we don't want to burn MIRI off from the beginning

[A comment by Carl Kevinson](https://astronomy.stackexchange.com/questions/48316/why-hd-84406-will-be-chosen-as-the-first-target-for-testing-jwst#comment105617_48317) points out this statement, and asks a reasonable question:

> "but probably not too bright (like Betelgeuse), since we don't want to burn MIRI off" Is damage to the instrument a risk when observing a bright star?

I am generally aware of the sunshield in place to protect it from the Sun's comparatively close and high radiation. It seems that maintaining temperature is the key issue this is concerned with. The whole craft is aimed to be at most 40 K; MIRI has two more systems that reduce it specifically down to 7 K.

This is presumably a noise-reduction strategy, given that the instruments are generally trying to image in the IR spectrum, and any excess heat is going to be emitted primarily as IR. MIRI may be more susceptible to such noise, given its ability to see longer wavelengths (i.e. lower energy, and more prominent from colder things than shorter wavelengths, as far as I understand).

However, it does not seem that a distant star should be able to do much affecting the instruments in the same way as heat from the Sun, otherwise there would be additional shielding on the "outside" of the craft as well (which might largely defeat the purpose).

What risks exist to the JWST instrumentation, that come specifically from looking at a star that is "too bright" as opposed to the risks of being unshielded from the Sun and/or from getting too hot?

What kind(s) of stars would be counted as "too bright"? Examples would be helpful (though please do not include the Sun).

**Tags:** infrared, james-webb-space-telescope

> **Comments:**
> **username_1**: A similar question is asked [here](https://astronomy.stackexchange.com/q/48319/5264). As I commented there, you can definitely damage the detector _temporarily_, so that you see a "ghost image" of the star on the subsequent exposures (I did that myself with an Earth-based 1.5 m telescope, and JWST's area is ~20 larger and has a ~10 times higher resolution so of the order 1000 more photons per pixel for point sources). However, damaging MIRI permanently is _probably_ not something you should worry about (according to my colleague whose expertise is detectors and electronics).  

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## 1 answers

### Answer by username_2 on 2022-01-27. Score: 5 | Accepted answer

The MIRI uses Si:As doped detectors which I believe are just about destruction-proof. The spectral dispersion is done with Ge and ZnS material, also tough things. There's a wealth of info on the cooling system [here](https://www.jwst.nasa.gov/content/about/innovations/cryocooler.html), which suggests that the only failure mechanisms are He leakage or mechanical wear.  
A detailed ["user manual"](https://jwst-docs.stsci.edu/jwst-mid-infrared-instrument/miri-observing-modes/miri-imaging) specifically points out that there are subarrays dedicated to bright(er) targets, avoiding saturation by shortening frame times.  
Given that, my interpretation is that a bright (in the IR) source might cause a heat load on MIRI that exceeds the cooling capability, causing the sensor to stop functioning until it can once again reach the necessary cryotemps. There doesn't seem to be any element which could suffer physical (non-reversible) damage from a bright source.


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