<p>It's true that James Webb carries fuel, and you're right that it is not used for positioning, at least not directly (see below). Fuel is used for maintaining its orbit around L2, and was also used three times on its journey to L2, as &quot;corrections burns&quot;. Note that the distance to the target is irrelevant. A nearby exoplanet and a distant galaxy are both &quot;infinitely far away&quot; for observing purposes (although of course in general more distant object are fainter).</p>
<h3>Reaction wheels</h3>
<p>To acquire a target, James Webb (and other space telescopes) uses a number of <a href="https://en.wikipedia.org/wiki/Reaction_wheel" rel="noreferrer">reaction wheels</a>, one for each &quot;axis&quot;. At least three are needed, but James Webb has six; more allows for easier control, but are also heavier. These wheels rotate constantly, thus storing a large amount of angular momentum to keep the telescope steady. Changing the angular speed of one of the action wheels causes Webb to change its direction along that wheel's axis.</p>
<p>Edit thanks to @KarlKastor: While James Webb observes, the photon pressure of the Sun's light exerts a torque on the telescope. To maintain its position, this is counteracted by adjusting the spin of the reaction wheels. This causes angular momentum to build up, which must occasionally be dumped by firings Webb's thrusters once per week or so (<a href="https://jwst-docs.stsci.edu/jwst-observatory-hardware/jwst-momentum-management" rel="noreferrer">JWST Momentum Management</a>).</p>
<h3>Gyroscopes</h3>
<p>Additionally, Webb has six gyroscopes which tell the telescope which direction it's currently pointing, and how fast it's turning. Unlike Hubble's gyroscopes, however, which are mechanical, Webb uses <a href="https://en.wikipedia.org/wiki/Hemispherical_resonator_gyroscope" rel="noreferrer">Hemispherical Resonator Gyroscopes</a>, which have no moving parts susceptible to wear, instead measuring the precession of vibration patterns in a crystal.</p>
<h3>Star tracking</h3>
<p>Finally, to ensure a perfect pointing, one of Webb's four instruments, <a href="https://en.wikipedia.org/wiki/Fine_Guidance_Sensor_and_Near_Infrared_Imager_and_Slitless_Spectrograph" rel="noreferrer">NIRISS</a>, is equipped with a &quot;Fine Guidance Sensor&quot; which &quot;locks&quot; the telescope on a target by observing the exact position of a star in its field of view.</p>
<p>Edit thanks to @David Hammen: In addition to the Fine Guidance Sensor, the JWST also has a few regular star trackers.</p>
<p>To power its various moving parts, James Webb has its solar array, capable of providing 2 kW, twice the needed amount.</p>
<p>You can read more about the positioning system at the <a href="https://www.jwst.nasa.gov/content/about/faqs/faq.html#pointing" rel="noreferrer">NASA FAQ</a>.</p>

CC BY-SA 4.0
2022-08-01T12:02:32.247Z
It's not entirely accurate to say no fuel is used for attitude control: the reaction wheels have to be desaturated with thrusters several times per month: "[...], angular momentum accumulates in the reaction wheels. [...] Each momentum unload activity takes a few hours.  The observatory slews to a particular orientation to minimize the impact on the orbit, then fires thrusters to allow the spin rates of the reaction wheels to be adjusted." https://jwst-docs.stsci.edu/jwst-observatory-hardware/jwst-momentum-management
KarlKastor
CC BY-SA 4.0
2022-08-01T12:33:01.143Z
Just regarding "focus". (I picture a huge "focus" knob :) )  I *assume* that if the JWST was pointed at one of our planets (say Mars) the large knob would still be on the infinity symbol.  Correct?  But what about if it was used to take a photo of the surface of our moon?  Still keep the big focus knob on the infinity symbol??
Fattie
CC BY-SA 4.0
2022-08-01T12:46:56.870Z
@Fattie From JWST’s location, our moon (and the Earth itself) are too close to the Sun to be viable observational targets. The only target in JWST’s current programme that is within the Solar System is Jupiter and its moons.
gandalf61
CC BY-SA 4.0
2022-08-01T12:52:00.413Z
@gandalf61 true, I realize that it can't point towards the sun and should have mentioned that. I was wondering about the "focus" issue in general.  In short, given that the JWST is "set on infinity", what then is it's closest focus point, just in theory.  Is it "5 meters"?  A mile?  A million miles?
Fattie
CC BY-SA 4.0
2022-08-01T13:22:00.427Z
In addition to the HRGs and fine guidance sensor, the JWST also has star trackers. A rate gyro measures angular rate rather than angular position. Without an occasional update from an angular position sensor such as a star tracker, even the very best rate gyro will eventually succumb to the dead reckoning problem. The fine guidance sensor is essentially an extremely sensitive kind of star tracker. In addition to the fine guidance sensor, the JWST also has two or three (the references I found are in conflict) plain old vanilla star trackers.
David Hammen
CC BY-SA 4.0
2022-08-01T14:19:15.543Z
@Fattie the big focus knob stays on infinity always.  Its nearest focus point is about 3 km.  The math is here: https://space.stackexchange.com/a/58766/1235
Camille Goudeseune
CC BY-SA 4.0
2022-08-01T17:13:41.017Z
3km !!  incredible.  thanks!
Fattie
CC BY-SA 4.0
2022-08-01T19:11:47.020Z
@KarlKastor Yes, thanks! I had forgotten about that part! Edited.
pela
CC BY-SA 4.0
2022-08-01T19:13:35.040Z
@DavidHammen Thanks, I knew it had some extra star trackers, but don't know enough about them to comment on the differences, so I let them out. But I added a line about them, and you're welcome to add more, if you feel like it :)
pela
CC BY-SA 4.0
2022-08-01T21:03:23.133Z
*reaction wheels* I had to use those in Kerbal! That's the big piece I was missing. Thank you for a perfect answer.
joeyfb
CC BY-SA 4.0
2022-08-01T21:19:06.553Z
@Fattie, 3 km isn't that incredible.  I've got an ordinary camera lens with a hyperfocal distance of around 1500 meters.
Mark
CC BY-SA 4.0
2022-08-01T21:27:20.243Z
@Mark I meant it's incredible it's so *close* !
Fattie
CC BY-SA 4.0
2022-08-01T21:39:30.820Z
“ Its nearest focus point is about 3 km. The math is here: space.stackexchange.com/…” Using stackexchange to answer stackexchange, lord have mercy on jeebus. Anything within roughly the orbit of not-quite-Mars will be smeared due to relative motion, and insufficient panning authority. At 3km, jitter might even become relevant.
caInstrument
CC BY-SA 4.0
2022-08-01T21:45:09.593Z
“The only target in JWST’s current programme that is within the Solar System is Jupiter and its moons.“ False consensus bias. I sat in a meeting with a Small Body scientist, running JW tests of tracking/panning performance. Since JW is- it appears- exceeding many design specifications, Mars-Crossers will likely be observed, and comet activity followed that far.
caInstrument
CC BY-SA 4.0
2022-08-01T21:49:35.913Z
“The only target in JWST’s current programme that is within the Solar System is Jupiter and its moons.” I already nixed this in a previous question. JW obtained funding from the Outer Planets budget, to help bridge its cost overruns. JW is now obligated to do e. g., cloud and ring studies of Uranus/Neptune. Jupiter is close enough that JW imaging is a bit redundant (though not spectrally). It’s certainly less relevant now that Juno is surviving, and Juno has complementary instruments for staggering breadth and depth (metaphorically and literally).
caInstrument
CC BY-SA 4.0
2022-08-02T08:18:49.267Z
@2012rcampion Yes, you're right, I'll edit. Thanks!
pela
CC BY-SA 4.0
2022-08-03T12:54:00.727Z
I find it strange to use the phrase "no moving parts" when talking about an instrument with a vibrating part. Is vibration not movement?
Arthur
CC BY-SA 4.0
2022-08-03T19:09:29.417Z
@Arthur Yes, you’re right of course, but there isn’t anything moving relative to each other causing friction. So it doesn’t really wear down.
pela
CC BY-SA 4.0
2022-08-01T05:43:04.887Z
2022-08-02T08:54:50.030Z
2022-08-02T08:54:50.030Z
pela
<p>Let's say the James Webb Space Telescope wants to move from observing the Andromeda galaxy millions of light years away to looking at the Trappist-1e planet some dozens of light years away, what actions are need to change targets and find and resolve that target?</p>
<p>I think I read it has jet propulsion for movement, but does it have a different system for fine movement? It seems like jetting around would be extremely finicky.</p>
<p>I imagine Hubble has to do something similar but I never heard how that works either.</p>

CC BY-SA 4.0
2022-08-01T14:33:32.770Z
["Space is big"](https://www.goodreads.com/quotes/14434-space-is-big-you-just-won-t-believe-how-vastly-hugely)
Barmar
<space-telescope><james-webb-space-telescope>
How does JWST position itself to see and resolve an exact target?
joeyfb
# How does JWST position itself to see and resolve an exact target?

(Asked by: username_0 on 2022-08-01)

Let's say the James Webb Space Telescope wants to move from observing the Andromeda galaxy millions of light years away to looking at the Trappist-1e planet some dozens of light years away, what actions are need to change targets and find and resolve that target?

I think I read it has jet propulsion for movement, but does it have a different system for fine movement? It seems like jetting around would be extremely finicky.

I imagine Hubble has to do something similar but I never heard how that works either.

**Tags:** space-telescope, james-webb-space-telescope
> **Comments:**
> **username_1**: ["Space is big"](https://www.goodreads.com/quotes/14434-space-is-big-you-just-won-t-believe-how-vastly-hugely)  

---

## 1 answers

### Answer by username_2 on 2022-08-01. Score: 35 | Accepted answer

It's true that James Webb carries fuel, and you're right that it is not used for positioning, at least not directly (see below). Fuel is used for maintaining its orbit around L2, and was also used three times on its journey to L2, as "corrections burns". Note that the distance to the target is irrelevant. A nearby exoplanet and a distant galaxy are both "infinitely far away" for observing purposes (although of course in general more distant object are fainter).

### Reaction wheels

To acquire a target, James Webb (and other space telescopes) uses a number of [reaction wheels](https://en.wikipedia.org/wiki/Reaction_wheel), one for each "axis". At least three are needed, but James Webb has six; more allows for easier control, but are also heavier. These wheels rotate constantly, thus storing a large amount of angular momentum to keep the telescope steady. Changing the angular speed of one of the action wheels causes Webb to change its direction along that wheel's axis.

Edit thanks to @KarlKastor: While James Webb observes, the photon pressure of the Sun's light exerts a torque on the telescope. To maintain its position, this is counteracted by adjusting the spin of the reaction wheels. This causes angular momentum to build up, which must occasionally be dumped by firings Webb's thrusters once per week or so ([JWST Momentum Management](https://jwst-docs.stsci.edu/jwst-observatory-hardware/jwst-momentum-management)).

### Gyroscopes

Additionally, Webb has six gyroscopes which tell the telescope which direction it's currently pointing, and how fast it's turning. Unlike Hubble's gyroscopes, however, which are mechanical, Webb uses [Hemispherical Resonator Gyroscopes](https://en.wikipedia.org/wiki/Hemispherical_resonator_gyroscope), which have no moving parts susceptible to wear, instead measuring the precession of vibration patterns in a crystal.

### Star tracking

Finally, to ensure a perfect pointing, one of Webb's four instruments, [NIRISS](https://en.wikipedia.org/wiki/Fine_Guidance_Sensor_and_Near_Infrared_Imager_and_Slitless_Spectrograph), is equipped with a "Fine Guidance Sensor" which "locks" the telescope on a target by observing the exact position of a star in its field of view.

Edit thanks to @David Hammen: In addition to the Fine Guidance Sensor, the JWST also has a few regular star trackers.

To power its various moving parts, James Webb has its solar array, capable of providing 2 kW, twice the needed amount.

You can read more about the positioning system at the [NASA FAQ](https://www.jwst.nasa.gov/content/about/faqs/faq.html#pointing).

> **Comments:**
> **username_3**: It's not entirely accurate to say no fuel is used for attitude control: the reaction wheels have to be desaturated with thrusters several times per month: "[...], angular momentum accumulates in the reaction wheels. [...] Each momentum unload activity takes a few hours.  The observatory slews to a particular orientation to minimize the impact on the orbit, then fires thrusters to allow the spin rates of the reaction wheels to be adjusted." https://jwst-docs.stsci.edu/jwst-observatory-hardware/jwst-momentum-management  
> **username_4**: Just regarding "focus". (I picture a huge "focus" knob :) )  I *assume* that if the JWST was pointed at one of our planets (say Mars) the large knob would still be on the infinity symbol.  Correct?  But what about if it was used to take a photo of the surface of our moon?  Still keep the big focus knob on the infinity symbol??  
> **username_5**: @Fattie From JWST’s location, our moon (and the Earth itself) are too close to the Sun to be viable observational targets. The only target in JWST’s current programme that is within the Solar System is Jupiter and its moons.  
> **username_4**: @gandalf61 true, I realize that it can't point towards the sun and should have mentioned that. I was wondering about the "focus" issue in general.  In short, given that the JWST is "set on infinity", what then is it's closest focus point, just in theory.  Is it "5 meters"?  A mile?  A million miles?  
> **username_6**: In addition to the HRGs and fine guidance sensor, the JWST also has star trackers. A rate gyro measures angular rate rather than angular position. Without an occasional update from an angular position sensor such as a star tracker, even the very best rate gyro will eventually succumb to the dead reckoning problem. The fine guidance sensor is essentially an extremely sensitive kind of star tracker. In addition to the fine guidance sensor, the JWST also has two or three (the references I found are in conflict) plain old vanilla star trackers.  
> **username_7**: @Fattie the big focus knob stays on infinity always.  Its nearest focus point is about 3 km.  The math is here: https://space.stackexchange.com/a/58766/1235  
> **username_4**: 3km !!  incredible.  thanks!  
> **username_2**: @KarlKastor Yes, thanks! I had forgotten about that part! Edited.  
> **username_2**: @DavidHammen Thanks, I knew it had some extra star trackers, but don't know enough about them to comment on the differences, so I let them out. But I added a line about them, and you're welcome to add more, if you feel like it :)  
> **username_0**: *reaction wheels* I had to use those in Kerbal! That's the big piece I was missing. Thank you for a perfect answer.  
> **username_8**: @Fattie, 3 km isn't that incredible.  I've got an ordinary camera lens with a hyperfocal distance of around 1500 meters.  
> **username_4**: @Mark I meant it's incredible it's so *close* !  
> **username_9**: “ Its nearest focus point is about 3 km. The math is here: space.stackexchange.com/…” Using stackexchange to answer stackexchange, lord have mercy on jeebus. Anything within roughly the orbit of not-quite-Mars will be smeared due to relative motion, and insufficient panning authority. At 3km, jitter might even become relevant.  
> **username_9**: “The only target in JWST’s current programme that is within the Solar System is Jupiter and its moons.“ False consensus bias. I sat in a meeting with a Small Body scientist, running JW tests of tracking/panning performance. Since JW is- it appears- exceeding many design specifications, Mars-Crossers will likely be observed, and comet activity followed that far.  
> **username_9**: “The only target in JWST’s current programme that is within the Solar System is Jupiter and its moons.” I already nixed this in a previous question. JW obtained funding from the Outer Planets budget, to help bridge its cost overruns. JW is now obligated to do e. g., cloud and ring studies of Uranus/Neptune. Jupiter is close enough that JW imaging is a bit redundant (though not spectrally). It’s certainly less relevant now that Juno is surviving, and Juno has complementary instruments for staggering breadth and depth (metaphorically and literally).  
> **username_2**: @2012rcampion Yes, you're right, I'll edit. Thanks!  
> **username_10**: I find it strange to use the phrase "no moving parts" when talking about an instrument with a vibrating part. Is vibration not movement?  
> **username_2**: @Arthur Yes, you’re right of course, but there isn’t anything moving relative to each other causing friction. So it doesn’t really wear down.  

---

