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Once the JWST is in orbit around L2, would it be possible to view it using any ground based telescopes? I’m mostly thinking of equipment available to amateurs, since I don’t have a 10m reflector. But would this be feasible for anything we have built or could build on the ground?

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  • $\begingroup$ I imaged it with a 14 inch SCT and a CMOS video camera. No problem. $\endgroup$ Jan 6 at 20:28
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    $\begingroup$ Any chance you would want to share it? $\endgroup$
    – user287095
    Jan 6 at 22:21

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At 1.5 million km JWST will be roughly 4,000 times farther than the ISS when it's overhead, and very roughly a tenth of the cross-section.

So if the ISS can be as bright as -4 magnitude, then JWST will be very roughly +16. That's well within the range of small observatories and large amateur telescopes.

But there is going to be quite a lot of variability in this!

Since we're in the same direction as the Sun, JWST's 20 x 15 meter sunshield is going to act like a diffuse mirror.

A good quality mirror in the sun still looks almost black if it's clean and reflecting a dark area of the sky, and super-bright if reflecting the Sun. But the heat-shield while reflective will have some amount of diffuseness.

And as the telescope points in different direction, the normal to the heatshield's surface can point almost directly towards us or actually tens of degrees away.

So I expect that once in orbit and under operation, JWST's brightness will vary a lot depending on where it's looking.


All the answers are helpful but @astrosnapper's answer provides links to the data showing that a 1 meter telescope tracked SpaceX Starman/Roadster from magnitude +16 all the way down past +21.

So amateurs with good telescope should be able to track it in deep space on its several-month trip out to its Sun-Earth L2 halo orbit and follow it there.

An object associated with the Sun-Earth Lagrange points is really in orbit around the Sun, but because of Earth's nudging gravity has become in a resonant orbit with Earth as well.

As seen from Earth, the Sun-Earth L2 point itself travels the ecliptic at a point exactly opposite the Sun. But JWST's big halo orbit will cause it to appear to wiggle up and down by about 10 or 15 degrees from the ecliptic roughly twice a year.

Lower your volume before watching the video:

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    $\begingroup$ The Ground-based Optical Tracking of Gaia (GBOT) project (GBOT site) regularly tracks the Gaia satellite out at L2 using ~2m telescopes. Its brightness is somewhat wavelength dependent but around 20. This was fainter than expected based on tests done with WMAP and Planck which were also out at L2, despite the similar spacecraft sizes and shapes. I don't know of any pre-launch modelling of the JWST brightness so we'll have to see once it gets to orbit and into the operational configuration. $\endgroup$ Sep 16, 2021 at 15:49
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    $\begingroup$ The exact magnitude of JWST is hard to calculate, because while the sun shield will reflect almost specualarly, its shape is not flat but somewhat saddle shaped. From discussions in the astronomer FB group, it seems mag 17-18 is the best we can hope for. Still visible in a relatively cheap telescope though. $\endgroup$
    – pela
    Dec 30, 2021 at 2:10
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    $\begingroup$ No I think we probably agree on what is cheap, but I hadn't thought it through — to see a 17th magnitude object, you would need a 1-m class telescope, which is definitely not cheap. But with a cheap telescope and a camera, it shouldn't be too difficult (although you might have to do some sky background subtraction). $\endgroup$
    – pela
    Dec 30, 2021 at 19:33
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    $\begingroup$ Current status on the visibility of JWST: My colleague took this picture through his 11.5 cm telescope with 2 min's exposure. A rough estimate based on the surrounding stars is that JWST is currently V ~ 16, although it fluctuates somewhat. Since it's currently 1e6 km away, in L2 it will be 1.5 more distant, so 1.5² times fainter, or roughly one mag. $\endgroup$
    – pela
    Jan 6 at 16:05
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    $\begingroup$ The ephemeris can be found here: unistellaroptics.com/observe-jwst. I don't know how easy it was, though. $\endgroup$
    – pela
    Jan 10 at 14:15
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We can use the same expression that is commonly used to estimate the apparent magnitude of a planet or asteroid in the Solar System*:

$$\boxed{m=5 \log \frac{1329}{d \cdot \sqrt p}+5 \log (D_s \cdot D_e)-2.5\log f(F)}$$

Where:

m is the apparent magnitude

log is the decimal logarithm

d is the diameter in km

p is the albedo

Ds is the distance to the Sun in Astronomical Units (AU)

De is the distance to Earth in Astronomical Units (AU)

f(F) is the phase function

If we want to apply this expression to the James Webb Space Telescope, the values to use are as follows.

As the surface area of the JWST solar shield is $S=21\cdot 14=294$ square meters:

$$d=\sqrt{\frac{4\cdot S}{\pi}}$$

Equivalent diameter "d"

d = 0.01935 km

The satellite hosting the Webb Telescope will be placed in a halo orbit around Lagrange L2 Sun-Earth, a point located 1.5 million kilometers from the Earth (see drawing). The radius of the halo orbit is 0.8 million km.

$$D_e=\sqrt{1.5^2+0.8^2}=1.7$$

De = 0.01136 AU

Ds = 1.01004 AU

As a phase function we will use the usual one:

$$f(F)=\dfrac{1+\cos F}2$$

The phase angle "F" is calculated using basic trigonometry from the drawing

Webb position

It is obtained:

F = 27.8º

And we see that the contribution of the corresponding summand is almost negligible:

$$-2.5 \log f(F)=+0.06$$

The albedo "p" of the heat shield will be high, to reflect the maximum amount of heat possible. But since the albedo of the solar shield is unknown to me, I add the table below showing the estimated apparent magnitude value, for different albedo values.

magnitude vs albedo table

The table tells us that the apparent magnitude of the JWST when it reaches its halo orbit around L2 will be of the order of +15 and therefore this should be visible with some powerful telescopes of amateur astronomers.

*Reference: Binary asteroid population. 1. Angular momentum content, by P. Pravec and A.W. Harris

Best regards.

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I have been imaging the JWST with a Celestron C11 almost every clear night since January 4th. After it arrived at L2 I estimate the magnitude around 17 or 18 based on asteroids about that magnitude that I have imaged. I caught a significant brief brightening that can be seen in this set of images https://www.flickr.com/photos/mjzuraw/51845428333/in/photostream/ It should be possible to predict these brighting or flaring events much like they do for iridium satellites.

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One of the problems with imaging GAIA is that the satellite is moving fairly rapidly with respect to the stars so increase the exposure time does not make the image any brighter. Since JWST is in a larger halo orbit it may have a higher apparent motion so is more difficult to image. If you can track JWST it may be easier to image but of course the stars will trail.

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