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The deep field exploration is quite fascinating, first the Hubble Deep Field (HDF) showing a small piece of the Univers one billion year after the Big-Bang, next the Hubble Ultra-Deep Field (HUDF), and now the Hubble eXtreme Deep Field (XDF) showing galaxies 450 million years after the Big-Bang.

Question: what is the limit of deepness we can expect to observe (in term of million years after the Big-Bang)?

Note that by "limit" I do not mean the theoretical limit but the practical limit.
By "practical limit" I mean using a visible-light telescope (not necessarily Hubble).
We also allow infrared telescope for correcting the redshift phenomenon.
In particular, I'm not asking about the CMB.

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The practical limit in terms of sky coverage is probably given by funding. The HDF, according to wikipedia, covers a 24-millionth of the entire sky. We would have to launch thousands of Hubble-like instruments to cover the entire sky at that resolution and sensitivity. Much of that area is covered by the Milky Way and and gas clouds, anyway, so not all deep exposures will reveal as much information as the HDF. Will we do this? Probably not, however, there is a lot to be learned from very precise whole sky maps because galactic shapes are being distorted by the irregular gravitational fields in the universe. This leads is to weak gravitational lensing. Proposals like the Euclid mission will use specially designed space telescopes with visible and near infrared capabilities that are similar (not quite as high resolution) as that of Hubble to map areas of sky that are hundreds of times larger than those that Hubble's very small angle cameras can take.

If we want to go earlier in time, then we have to use near- and mid-IR and eventually deep-IR telescopes. The first of these will be the new James Webb Space Telescope, which is the mission that will augment Hubble with the necessary IR capability. If you have been following the JWT in the news, you may have heard that in terms of funding it is very close to breaking the bank... it's a very ambitious and expensive instrument that eats into the budget of a lot of other potentially equally interesting science missions. For the foreseeable future it's the largest space telescope investment that we will be making, which brings us back to the funding limits...

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  • $\begingroup$ I clarify my question: in term of million years after the Big-Bang, what is the limit of deepness we can expect to observe? $\endgroup$ – Sebastien Palcoux Jan 13 '16 at 20:31
  • $\begingroup$ You can observe all the way back to the CMB, which we are working on, right now. The hunt for the earliest galaxies is on and that's why instruments in the mid- and even deep-IR are necessary. Sorry if I misunderstood your question... I kind of excluded the deep searches because you said that you are not asking about the CMB... but it's really a continuum search from 300,000 years after the big bang all the way to today's galaxies. $\endgroup$ – CuriousOne Jan 13 '16 at 20:35
  • $\begingroup$ But in practice I don't think you can go up to 300,000 years with a visible-light telescope, moreover CMB is microwave (and so non-visible). The question is: we can go up to what? $\endgroup$ – Sebastien Palcoux Jan 13 '16 at 20:41
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    $\begingroup$ Exactly. If you look at the James Webb Space Telescope, it's not built for the visible but primarily for the IR. Once we go beyond low redshift galaxies, the information we are interested in is at ever longer wavelengths. In other words... the Hubble was the largest visible space telescope, for now. That science is mostly done. What scientists care about is beyond the human visual range. That doesn't mean there won't be large visual space telescopes in the future... it's just not a scientific preference, for now. Eventually we will have exoplanet surface imagers... 50 years from now? $\endgroup$ – CuriousOne Jan 13 '16 at 20:48
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    $\begingroup$ What do mean by "Is it a joke"? Are we well on our way to building interferometric arrays with such resolution? Yes. $\endgroup$ – CuriousOne Jan 14 '16 at 17:57

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