At a distance of 1.5e6 km, the Hubble's ACS instrument gets a resolution of 364 meters/pixel, so JWST would be a dimensionless point.

I hope that the images that JWST will actually take can attract as much enthusiasm as some have for a hypothetical photo of JWST.

Of the Cycle 1 plans, one of the most imaginative campaigns will image the Galileans while they're in Jupiter's shadow for the purpose not of imaging them (they will be ~black in the given wavelengths) but for seeing the silhouettes of them against the background radiation from the galaxy. These two brightness measurements will tell us the brightness of the zodiacal light, which is the reflection of sunlight from dust between Earth and Jupiter, and then the brightness of the galaxy, which will be the brightness of the area around the Galileans minus the brightness of their silhouettes.

Then we have many exoplanet campaigns, many solar system campaigns (of course, mainly outer solar system and small body targets) and many observations of black holes, stellar objects, and objects of cosmological interest.

Out of curiosity does the public know the actual first target planned?

I can't seem to find much about the first public photos planned.

An analyst who put their mind to it might be able to make some guesses, but it's not possible to know the exact schedule of observations until it's known when commissioning will end and science observations begin. Some of the planned observations are time-limited and if, say, the start of science were delayed a week, it might mean that one observation would have to be shifted from, eg, August 2022 to August 2023.

Basically, targets on the ecliptic are limited to about 100 days of accessibility each year. Targets far from the ecliptic are limited little or zero. Solar system targets have their own complex timing constraints. And exoplanet transits have their own complex periodicity (particularly if the period is relatively longer). So it's impossible to fully schedule the first year until the precise start of science observations is known.

Very intriguing idea - can you point to any details about this?

It sounds really hard. The zodiacal light away from the sun is extremely faint - even the Gegenschein you'd be looking through when Jupiter was near opposition is very faint. So any light scattered onto the eclipsed moons could easily be important. Eg, you'd expect some light scattered around the limb of Jupiter. This is analagous to the orange glow of our moon during total lunar eclipse, but probably fainter for a deeper Galilean eclipse. Also light scattered from the other Galileans, and even illumination from the inner solar system zodiacal light.

Jupiter's well off the Galactic plane for the next while, so the background would also be extremely faint.

Given all that I could imagine the eclipsed discs to be brighter than the background galaxy glow. And all of this imaging while crazy-bright Jupiter is nearby!

Here is the proposal. As you note, Jupiter is well off the galactic plane; the study is looking at extra-galactic background luminosity, and must be made before Jupiter returns to the galactic plane.

If the signal is strong enough, the complicating sources of noise can be accounted for. Eg, light from Jupiter will occur above and below the moon's silhouette as well. The thermal contribution of each moon should be a known quantity. Europa, Ganymede, and Callisto will all be observed.

https://www.stsci.edu/jwst/phase2-public/2134.pdf

Yes the background Milky Way galaxy glow is something that would be interesting to map around different galactic lat/lons. Even ground based deep imaging (both professional and amateur communities) can show faint galactic cirrus when looking away from the galactic plane. A related key question is what is the brightness of extragalactic glow (as noted in the post/proposal just above). This would ostensibly be from unresolved galaxies that wouldn't be seen in something like the Hubble Deep Field. Can this proposal really discriminate between galactic cirrus (DGL) and extragalactic background glow (EBL)? They are both of comparable brightness.

Thanks for that.

They do discuss (though non-quantitatively) stray light from Jupiter in the optical system, but they don't mention any of the sources of light that will illuminate the eclipsed discs of the moons. With Jupiter subtending only a few degrees from the outer Galileans, solar corona and zodiacal light would seem to be potentially important.

New horizons made some interesting observations from well outside the zodiacal glow, though they were limited by the instrumentation which wasn't ideal for that application:

https://arxiv.org/abs/2011.03052

And in a somewhat unrelated thought, one wonders if Webb will be able to see to the point where the Opaque Universe become the Clear Universe.

--Bill

Was wondering to what extent Jupiter being relatively hot in the infrared could impart 'backshine' brightening the moon's silhouette, but the proposal seems to suggest otherwise:

"...However, Jupiter is dark at F140M owing to the methane absorption in the Jovian atmosphere, thus stray light from Jupiter is also reduced..."

It cannot. The light from that event, which is called recombination, is at z = 1100. I.e., it's been redshifted 1100-fold,
from near infrared at the time of recombination to microwave now. That's what the cosmic microwave background is.

So this will indeed take us to the edge of the optical Universe.
Next step: VLB radio space telescopes to go microwave and beyond.

--Bill

For context, Planck has already mapped the microwave background, seeing back further than JWST will see – but at very low resolution.

Spitzer imaged at IR wavelengths comparable to JWST – but at less resolution and with far, far less light gathering.

HST images at about the same resolution as JWST – but different wavelengths.

JWST isn't so much extending capabilities in some single dimension that hasn't been investigated before, but in a combination of dimensions, a corner of the space so to speak, that will be entirely new.

Yeah, Planck (and others) have mapped the cosmic microwave background (CMB) at much lower resolution than JWST is capable of, but there very likely isn't much to see in the CMB at JWST resolutions, since the CMB was emitted well before the first stars and galaxies formed, when the Universe was extremely smooth.

JWST should, for the first time, be able to see back to the first stars, which is one of its main goals. UV light from those stars reionized the gas, which was neutral and transparent since recombination when the CMB was emitted. The time of reionization is considered one of the half dozen or so parameters that describe the Universe in bulk, so this will be a big thing for cosmology.