<p>This <a href="https://blogs.nasa.gov/webb/2022/01/27/the-webb-team-looks-back-on-successful-deployments/" rel="nofollow noreferrer">hot off the press</a>, from Bill Ochs, Webb project manager:</p>
<blockquote>
<p>This is just the first step; HD 84406 will be too bright to study with Webb once the telescope starts to come into focus.</p>
</blockquote>

CC BY-SA 4.0
2022-01-28T14:09:41.787Z
Hi TonyK, thanks, this is an important comment, but not really an answer to the question, so it should probably be converted to a comment.
pela
CC BY-SA 4.0
2022-01-28T14:31:51.217Z
@pela: My feeling is that it contributes relevant additional information to your nice answer. So I put it where it would  be more likely to get noticed.
TonyK
CC BY-SA 4.0
2022-01-28T15:20:37.237Z
It does, but it's still a comment, not an actual, stand-alone answer to the question, so my guess is that it will be closed/converted. The moderators tend to be quite strict about these rules (for a reason).
pela
CC BY-SA 4.0
2022-01-28T16:14:18.040Z
Oh for heaven's sake.
TonyK
CC BY-SA 4.0
2022-01-28T00:34:47.097Z
2022-01-28T00:34:47.097Z
TonyK
<p>The most important selection criterion is that the star should be available for observation for a prolonged time.</p>
<p>Because James Webb observes in the infrared, it must hide the Sun (and Earth and the Moon) behind its sunshield, and can hence only observe some 39% of the sky at any given time (source: <a href="https://jwst-docs.stsci.edu/jwst-observatory-characteristics/jwst-observatory-coordinate-system-and-field-of-regard/jwst-target-viewing-constraints" rel="nofollow noreferrer">NASA</a>). Webb needs around three months for its optical alignment, so we need a star that has just entered its field of view<span class="math-container">$^\dagger$</span>.</p>
<p>In addition to this, we also don't want a star in a field that is too crowded, and HD 84406 is located in a rather isolated region in the NW part of Ursa Major:</p>
<p><a href="https://i.sstatic.net/QwSlC.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/QwSlC.jpg" alt="HD84406-position" /></a>
<sup>Credit: <a href="https://en.wikipedia.org/wiki/Ursa_Major#/media/File:Ursa_Major_IAU.svg" rel="nofollow noreferrer">IAU/S&amp;T/Roger Sinnott/Rick Fienberg</a> with my own annotations.</sup></p>
<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. <strong>EDIT</strong>: After discussing with several colleagues, damaging MIRI permanently is <em>probably</em> not going to be an issue. Bright sources <em>can</em> however damage the detector temporarily, showing a &quot;ghost image&quot; of the star in subsequent exposures. I've done that myself on a 1.5 m Earth-based telescope, and JWST has a ~20 times larger area, and ~10 times higher resolution, meaning several 1000 times more photons per pixel.</p>
<p>Anyway, as you see there are some important constraints, but many other stars would fit as well; there are quite a lot to choose from.</p>
<p>And the best part: HD 84406 is a magnitude 6.9 star, so while you can't see it with your naked eye, it should be visible in a pair of regular binoculars.</p>
<hr />
<p><span class="math-container">$\dagger$</span><sub>Stars too close to the ecliptic are only available for a more limited time, so a star that is close to the poles is preferred.</sub></p>

CC BY-SA 4.0
2022-01-26T16:22:02.883Z
"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?
Carl Kevinson
CC BY-SA 4.0
2022-01-26T20:24:25.727Z
for those interested in reading further about JWST's FoR (Field of Regard) see for example [Why does JWST have such a big Blind Spot?](https://space.stackexchange.com/q/56076/12102) and [How much of the sky can the JWST see?](https://space.stackexchange.com/q/14551/12102)
uhoh
CC BY-SA 4.0
2022-01-26T21:51:06.927Z
@CarlKevinson Perhaps not permanently, but temporarily I think yes. I've personally observed a too bright star (with a 2.5 m telescope) where pixels over-saturated to the point that succeeding exposures has suspiciously high values in those pixels. That was a CDD though, maybe JWST's IR detectors works differently.
pela
CC BY-SA 4.0
2022-01-26T21:55:41.263Z
But even if you don't _damage_ the detector, imaging of a too bright star is difficult. You'll need extremely low exposure times, probably impossibly small, or else you saturate a large area on the detector, making it very hard to find the centroid of the star (which you want to in order to align the mirror segments).
pela
CC BY-SA 4.0
2022-01-27T14:23:22.533Z
@CarlKevinson So, my conclusion after discussing with several colleagues is that we probably shouldn't worry about damaging MIRI permanently, only temporarily. I edited my text accordingly. Thanks for questioning this!
pela
CC BY-SA 4.0
2022-01-27T15:00:14.393Z
@pela Thank you for the follow up!  I was just curious and I'm happy to learn something!
Carl Kevinson
CC BY-SA 4.0
2022-01-26T09:53:15.370Z
2022-01-28T14:07:13.507Z
2022-01-28T14:07:13.507Z
pela
<blockquote>
<p><a href="https://en.wikipedia.org/wiki/HD_84406" rel="noreferrer">HD 84406</a>, is a star approximately 241 light-years away in the constellation of Ursa Major. HD 84406 will be the first star to be imaged by the James Webb Space Telescope in order to test the focus of the telescope. The star is a spectral type G star and has a high proper motion.</p>
</blockquote>
<p>Why aren't the other more popular stars like Alpha Centauri or Betelgeuse chosen for the first light instead?</p>
<p>Which properties of HD 84406 make it beneficial to the telescope mirror testing? Spectral type? Magnitude? Distance? Does the star have any interesting stuff? Or is it just for the reason that the position of the star is the most convenient for JWST to locate during the test process?</p>

CC BY-SA 4.0
2022-01-26T09:03:00.300Z
because it can never point at [HDE 226868](https://astronomy.stackexchange.com/users/2153/hde-226868)?
uhoh
CC BY-SA 4.0
2022-01-26T10:32:15.807Z
Nice question..
Kavin Ishwaran
CC BY-SA 4.0
2022-01-26T17:55:54.087Z
@uhoh - kind of depends where HDE226868's career takes them. A trans-Jupiter observatory might be a fine place to do astronomy from...
Jon Custer
<star><observational-astronomy><james-webb-space-telescope>
Why will HD 84406 be chosen as the first target for testing JWST?
NeutronCat
# Why will HD 84406 be chosen as the first target for testing JWST?

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

> [HD 84406](https://en.wikipedia.org/wiki/HD_84406), is a star approximately 241 light-years away in the constellation of Ursa Major. HD 84406 will be the first star to be imaged by the James Webb Space Telescope in order to test the focus of the telescope. The star is a spectral type G star and has a high proper motion.

Why aren't the other more popular stars like Alpha Centauri or Betelgeuse chosen for the first light instead?

Which properties of HD 84406 make it beneficial to the telescope mirror testing? Spectral type? Magnitude? Distance? Does the star have any interesting stuff? Or is it just for the reason that the position of the star is the most convenient for JWST to locate during the test process?

**Tags:** star, observational-astronomy, james-webb-space-telescope
> **Comments:**
> **username_1**: because it can never point at [HDE 226868](https://astronomy.stackexchange.com/users/2153/hde-226868)?  
> **username_2**: Nice question..  
> **username_3**: @uhoh - kind of depends where HDE226868's career takes them. A trans-Jupiter observatory might be a fine place to do astronomy from...  

---

## 2 answers

### Answer by username_5 on 2022-01-26. Score: 22 | Accepted answer

The most important selection criterion is that the star should be available for observation for a prolonged time.

Because James Webb observes in the infrared, it must hide the Sun (and Earth and the Moon) behind its sunshield, and can hence only observe some 39% of the sky at any given time (source: [NASA](https://jwst-docs.stsci.edu/jwst-observatory-characteristics/jwst-observatory-coordinate-system-and-field-of-regard/jwst-target-viewing-constraints)). Webb needs around three months for its optical alignment, so we need a star that has just entered its field of view$^\dagger$.

In addition to this, we also don't want a star in a field that is too crowded, and HD 84406 is located in a rather isolated region in the NW part of Ursa Major:

[![HD84406-position](https://i.sstatic.net/QwSlC.jpg)](https://i.sstatic.net/QwSlC.jpg)
Credit: [IAU/S&T/Roger Sinnott/Rick Fienberg](https://en.wikipedia.org/wiki/Ursa_Major#/media/File:Ursa_Major_IAU.svg) with my own annotations.

The star should be bright, but probably not too bright (like Betelgeuse), since we don't want to burn MIRI off from the beginning. **EDIT**: After discussing with several colleagues, damaging MIRI permanently is *probably* not going to be an issue. Bright sources *can* however damage the detector temporarily, showing a "ghost image" of the star in subsequent exposures. I've done that myself on a 1.5 m Earth-based telescope, and JWST has a ~20 times larger area, and ~10 times higher resolution, meaning several 1000 times more photons per pixel.

Anyway, as you see there are some important constraints, but many other stars would fit as well; there are quite a lot to choose from.

And the best part: HD 84406 is a magnitude 6.9 star, so while you can't see it with your naked eye, it should be visible in a pair of regular binoculars.

---

$\dagger$Stars too close to the ecliptic are only available for a more limited time, so a star that is close to the poles is preferred.

> **Comments:**
> **username_6**: "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?  
> **username_1**: for those interested in reading further about JWST's FoR (Field of Regard) see for example [Why does JWST have such a big Blind Spot?](https://space.stackexchange.com/q/56076/12102) and [How much of the sky can the JWST see?](https://space.stackexchange.com/q/14551/12102)  
> **username_5**: @CarlKevinson Perhaps not permanently, but temporarily I think yes. I've personally observed a too bright star (with a 2.5 m telescope) where pixels over-saturated to the point that succeeding exposures has suspiciously high values in those pixels. That was a CDD though, maybe JWST's IR detectors works differently.  
> **username_5**: But even if you don't _damage_ the detector, imaging of a too bright star is difficult. You'll need extremely low exposure times, probably impossibly small, or else you saturate a large area on the detector, making it very hard to find the centroid of the star (which you want to in order to align the mirror segments).  
> **username_5**: @CarlKevinson So, my conclusion after discussing with several colleagues is that we probably shouldn't worry about damaging MIRI permanently, only temporarily. I edited my text accordingly. Thanks for questioning this!  
> **username_6**: @pela Thank you for the follow up!  I was just curious and I'm happy to learn something!  

---

### Answer by username_4 on 2022-01-28. Score: 4

This [hot off the press](https://blogs.nasa.gov/webb/2022/01/27/the-webb-team-looks-back-on-successful-deployments/), from Bill Ochs, Webb project manager:

> This is just the first step; HD 84406 will be too bright to study with Webb once the telescope starts to come into focus.

> **Comments:**
> **username_5**: Hi TonyK, thanks, this is an important comment, but not really an answer to the question, so it should probably be converted to a comment.  
> **username_4**: @pela: My feeling is that it contributes relevant additional information to your nice answer. So I put it where it would  be more likely to get noticed.  
> **username_5**: It does, but it's still a comment, not an actual, stand-alone answer to the question, so my guess is that it will be closed/converted. The moderators tend to be quite strict about these rules (for a reason).  
> **username_4**: Oh for heaven's sake.  

---

