<p>Depends largely on the targets elevation/altitude/latitude above the solar ecliptic.</p>
<p>Best case is for targets that are within 5° of the poles of the solar ecliptic. These targets never leave the Field of Regard for JWST.</p>
<blockquote>
<p>JWST operates in an ecliptic coordinate framework, there are 2 small continuous viewing zones (CVZs) centered at each of the ecliptic poles (see Figure 6). The 85° solar exclusion zone then determines the radius of the allowed CVZs to be essentially 5°, although any observation approaching the 85° limit will have additional limitations due to safety considerations. <a href="https://jwst-docs.stsci.edu/jwst-observatory-characteristics/jwst-observatory-coordinate-system-and-field-of-regard#JWSTObservatoryCoordinateSystemandFieldofRegard-CVZContinuousviewingzone(CVZ)" rel="nofollow noreferrer">1</a></p>
</blockquote>
<p>Blind time increases for targets farther from the poles of the solar ecliptic.</p>
<p>Worst case is for targets that are directly on the solar ecliptic.</p>
<pre><code>Field of Regard &quot;Width&quot;: 50° (135° - 85°)
Field of Regard period: 1 sidereal year
</code></pre>
<p>The target will pass through the Field of Regard twice in a given period.</p>
<p><a href="https://www.google.com/search?q=50%2F360%20*%201%20sidereal%20year" rel="nofollow noreferrer">50° / 360° * 1 sidereal year</a> = 50.7300504 days</p>
<p>By Angle:</p>
<pre><code> In:  50°
Out:  90° anti-solar blind spot
 In:  50°
Out: 170° sun shield blind spot
Repeat...
</code></pre>
<p>By Time:</p>
<pre><code> In:  50.7300504 days
Out:  91.3140908 days anti-solar blind spot
 In:  50.7300504 days
Out: 172.482171  days sun shield blind spot
Repeat...
</code></pre>
<p>I guess this isn't the precise function you are asking for, but maybe it provides someone else insights to discover the function. If I manage to do it I will update this answer.</p>
<p>Update: Here is a plot of said function against ecliptic latitude.
<a href="https://jwst-docs.stsci.edu/jwst-observatory-characteristics/jwst-observatory-coordinate-system-and-field-of-regard/jwst-target-viewing-constraints#JWSTTargetViewingConstraints-Targetobservability" rel="nofollow noreferrer"><img src="https://i.sstatic.net/B9Hrd.png" alt="Graph of the function against ecliptic latitude" /></a></p>

CC BY-SA 4.0
2022-03-04T21:06:38.277Z
I just realized that JWST is in the ecliptic coordinate system and not in the equatorial coordinate system like all other telescopes. I suppose astronomers are already used to translating between all the astronomical coordinate systems.
Gabriel
CC BY-SA 4.0
2022-03-04T21:06:08.420Z
2022-03-04T22:55:03.470Z
2022-03-04T22:55:03.470Z
Gabriel
<p>In <a href="https://space.stackexchange.com/a/56077/12102">this answer to <em>Why does JWST have such a big Blind Spot?</em></a> I argue that this space telescope primary relationship with time is that it strives to look way back in it and so as long as a given direction is visible sometime during the year and it doesn't matter that at any moment only about 40% of <span class="math-container">$4 \pi$</span> SR is viewable (polar angles 85° to 135° wrt the Sun, see below).</p>
<p>But now I'm wondering</p>
<ol>
<li>If there are some types of transient events where an infrared telescope like this (0.6 to 28 microns I think?) could offer something that Hubble or ground observatories couldn't. For that I've already asked <a href="https://astronomy.stackexchange.com/q/47726/7982">For what types of transient events would the JWST be ideal or even uniquely-suited for observing? (If it could look in time)</a></li>
</ol>
<p>But here I'd like to ask:</p>
<p><strong>Question:</strong> What is the fraction of the time that the JWST could view a short transient event on-demand as a function of position on the celestial sphere?&quot;</p>
<p>The author of that question gives a few examples just to illustrate the idea of transient events</p>
<blockquote>
<p>Over a 6 month half-orbit of the sun, the entire sky is observable. But what happens if an interesting event occurs in the “blind spot”? Like Shoemaker-Levy or the arrival of Rama? A gravitational wave event?</p>
</blockquote>
<p>and (at least) one of those really did &quot;shine&quot; in the thermal infrared!</p>
<hr />
<p><strong>JWST's field of regard (FOR)</strong> from the linked question and originally from <a href="https://jwst-docs.stsci.edu/jwst-observatory-characteristics/jwst-observatory-coordinate-system-and-field-of-regard#JWSTObservatoryCoordinateSystemandFieldofRegard-forJWSTfieldofregard(FOR)" rel="nofollow noreferrer">JWST field of regard (FOR)</a> Also see <a href="https://space.stackexchange.com/q/14551/12102">How much of the sky can the JWST see?</a>.</p>
<p><a href="https://i.sstatic.net/KxP3t.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/KxP3t.png" alt="JWST's field of regard (FOR)" /></a></p>

CC BY-SA 4.0
2021-12-07T17:18:19.303Z
Note the timescale for Target of Opportunity (ToO) observations for both HST and JWST are long for a substantial fraction of transient science. Non disruptive ToOs, which already require significant additional justification in the proposal, take at least 2 weeks before execution. Disruptive ToOs, of which there can only be 8 triggers per cycle, take 48 hours (Source: [JWST ToO restrictions](https://jwst-docs.stsci.edu/jwst-opportunities-and-policies/jwst-call-for-proposals-for-cycle-1/jwst-cycle-1-observation-types-and-restrictions/target-of-opportunity-observations))
astrosnapper
CC BY-SA 4.0
2022-08-23T14:38:19.197Z
Space Telescope Science Institute video [Webb's Field of Regard](https://www.youtube.com/watch?v=y0bOi3kVIBs)
uhoh
CC BY-SA 4.0
2022-12-28T19:45:47.987Z
@astrosnapper That link is now dead. But there's general ToO info on https://jwst-docs.stsci.edu/methods-and-roadmaps/jwst-target-of-opportunity-observations
PM 2Ring
CC BY-SA 4.0
2022-12-28T23:05:35.547Z
There's now a new [link](https://jwst-docs.stsci.edu/jwst-opportunities-and-policies/jwst-call-for-proposals-for-cycle-2/jwst-observation-types#JWSTObservationTypes-TOOobservationsTarget-of-Opportunity(ToO)Observations) for Cycle 2 but your link looks to be the better and more general/stable one @PM2Ring
astrosnapper
<observational-astronomy><time-domain-astronomy><james-webb-space-telescope>
What is the fraction of the time that the JWST could view a short transient event on-demand as a function of position on the celestial sphere?
uhoh
# What is the fraction of the time that the JWST could view a short transient event on-demand as a function of position on the celestial sphere?

(Asked by: username_0 on 2021-12-07)

In [this answer to *Why does JWST have such a big Blind Spot?*](https://space.stackexchange.com/a/56077/12102) I argue that this space telescope primary relationship with time is that it strives to look way back in it and so as long as a given direction is visible sometime during the year and it doesn't matter that at any moment only about 40% of $4 \pi$ SR is viewable (polar angles 85° to 135° wrt the Sun, see below).

But now I'm wondering

1. If there are some types of transient events where an infrared telescope like this (0.6 to 28 microns I think?) could offer something that Hubble or ground observatories couldn't. For that I've already asked [For what types of transient events would the JWST be ideal or even uniquely-suited for observing? (If it could look in time)](https://astronomy.stackexchange.com/q/47726/7982)

But here I'd like to ask:

**Question:** What is the fraction of the time that the JWST could view a short transient event on-demand as a function of position on the celestial sphere?"

The author of that question gives a few examples just to illustrate the idea of transient events

> Over a 6 month half-orbit of the sun, the entire sky is observable. But what happens if an interesting event occurs in the “blind spot”? Like Shoemaker-Levy or the arrival of Rama? A gravitational wave event?

and (at least) one of those really did "shine" in the thermal infrared!

---

**JWST's field of regard (FOR)** from the linked question and originally from [JWST field of regard (FOR)](https://jwst-docs.stsci.edu/jwst-observatory-characteristics/jwst-observatory-coordinate-system-and-field-of-regard#JWSTObservatoryCoordinateSystemandFieldofRegard-forJWSTfieldofregard(FOR)) Also see [How much of the sky can the JWST see?](https://space.stackexchange.com/q/14551/12102).

[![JWST's field of regard (FOR)](https://i.sstatic.net/KxP3t.png)](https://i.sstatic.net/KxP3t.png)

**Tags:** observational-astronomy, time-domain-astronomy, james-webb-space-telescope

> **Comments:**
> **username_1**: Note the timescale for Target of Opportunity (ToO) observations for both HST and JWST are long for a substantial fraction of transient science. Non disruptive ToOs, which already require significant additional justification in the proposal, take at least 2 weeks before execution. Disruptive ToOs, of which there can only be 8 triggers per cycle, take 48 hours (Source: [JWST ToO restrictions](https://jwst-docs.stsci.edu/jwst-opportunities-and-policies/jwst-call-for-proposals-for-cycle-1/jwst-cycle-1-observation-types-and-restrictions/target-of-opportunity-observations))  
> **username_0**: Space Telescope Science Institute video [Webb's Field of Regard](https://www.youtube.com/watch?v=y0bOi3kVIBs)  
> **username_2**: @username_1 That link is now dead. But there's general ToO info on https://jwst-docs.stsci.edu/methods-and-roadmaps/jwst-target-of-opportunity-observations  
> **username_1**: There's now a new [link](https://jwst-docs.stsci.edu/jwst-opportunities-and-policies/jwst-call-for-proposals-for-cycle-2/jwst-observation-types#JWSTObservationTypes-TOOobservationsTarget-of-Opportunity(ToO)Observations) for Cycle 2 but your link looks to be the better and more general/stable one @username_2  

---

## 1 answers

### Answer by username_3 on 2022-03-04. Score: 1

Depends largely on the targets elevation/altitude/latitude above the solar ecliptic.

Best case is for targets that are within 5° of the poles of the solar ecliptic. These targets never leave the Field of Regard for JWST.

> JWST operates in an ecliptic coordinate framework, there are 2 small continuous viewing zones (CVZs) centered at each of the ecliptic poles (see Figure 6). The 85° solar exclusion zone then determines the radius of the allowed CVZs to be essentially 5°, although any observation approaching the 85° limit will have additional limitations due to safety considerations. [1](https://jwst-docs.stsci.edu/jwst-observatory-characteristics/jwst-observatory-coordinate-system-and-field-of-regard#JWSTObservatoryCoordinateSystemandFieldofRegard-CVZContinuousviewingzone(CVZ))

Blind time increases for targets farther from the poles of the solar ecliptic.

Worst case is for targets that are directly on the solar ecliptic.

```
Field of Regard "Width": 50° (135° - 85°)
Field of Regard period: 1 sidereal year

```

The target will pass through the Field of Regard twice in a given period.

[50° / 360° \* 1 sidereal year](https://www.google.com/search?q=50%2F360%20*%201%20sidereal%20year) = 50.7300504 days

By Angle:

```
 In:  50°
Out:  90° anti-solar blind spot
 In:  50°
Out: 170° sun shield blind spot
Repeat...

```

By Time:

```
 In:  50.7300504 days
Out:  91.3140908 days anti-solar blind spot
 In:  50.7300504 days
Out: 172.482171  days sun shield blind spot
Repeat...

```

I guess this isn't the precise function you are asking for, but maybe it provides someone else insights to discover the function. If I manage to do it I will update this answer.

Update: Here is a plot of said function against ecliptic latitude.
[![Graph of the function against ecliptic latitude](https://i.sstatic.net/B9Hrd.png)](https://jwst-docs.stsci.edu/jwst-observatory-characteristics/jwst-observatory-coordinate-system-and-field-of-regard/jwst-target-viewing-constraints#JWSTTargetViewingConstraints-Targetobservability)

> **Comments:**
> **username_3**: I just realized that JWST is in the ecliptic coordinate system and not in the equatorial coordinate system like all other telescopes. I suppose astronomers are already used to translating between all the astronomical coordinate systems.  

---

