What are the farthest distances JWST can see? I was just wondering a simple question: what is the farthest distance (equivalently the least time after the Big Bang) that the JWST could theorically see? Can it be estimated somehow WITH SOME formula? When should we expect that measurements done?
Bonus (related): are theorists expecting to find out galaxies and/or Pop III star at one concrete distance (time after the Big Bang)?
P. S. : It seems than there are some consensus telling the maximum redshift JWST can see is ~20, but can it theoretically guessed with the telescope parameters or is it just a pure theoretical guess with no JWST instruments?
 A: In principle, it could see the surface of last scatter, where the CMB comes from. In practice, that would be obscured by the cosmic infrared background and JWST probably isn't sensitive enough for that, even if there were no CIB. At any rate, the redshift of the CMB is about 1,100. Drop that in to Ned Wright's Cosmology Calculator and that gets you 13.7 Gyr light travel time, a and a comoving distance of 45.5 billion light years.
So, you can guarantee that everything Webb sees will be closer than that.
At a guess, the farthest thing Webb is likely to see will be around a redshift of 20, which is 13.5 Gyr light travel time, and 35.9 billion light-years away. Webb has already seen a galaxy that may be at a redshift of 13 (13.4 Gyr, 33.3 billion light-years away).
The reason why I expect Webb to have a hard time seeing much further is because: galaxies were smaller (thus dimmer) back then, the universe was more neutral, and thus more opaque to UV light, back then, and we currently think that things hadn't lit up, generally, at some point.
That said, if the black holes that became supermassive black holes in the centers of galaxies were somehow formed during the big bang, then JWST could see further than expected, especially with the help of gravitational lenses.
