But that is exactly the case that some are making, either build a neutered/descoped JWST OR spend 2 more billion to make it be what it was intended to be. My point is that a descoped JWST isn’t worth doing, AND neither is spending 2 more billion.

Bottom line for me is: Doing it cheap doesn’t give us a worthwhile scope, and doing it right may cost too much. So, MAYBE the answer is not do it at all?

The reason is simple. Ground-based telescopes, no matter what their power or image compensation, are both wavelength and noise limited. To some degree, the latter problem can and has been mittigated. There is not a thing you can do about the former except go above the atmosphere.

On the commercial space front, human tended space telescopes are a market for launch vehicle suppliers. As I’ve argued with Greg, I do believe that we should concentrate our scientific resources on places we’re foreseeably able to visit. That doesn’t mean we should not build space telescopes when the resources are there to do so.

— Donald

— Donald

I’m not trying to make the case that a dramatically descoped JWST is worth it. In fact, the descope option being talked about now involves removing MIRI. But the OP seemed to be referring to JWST in general in the quote “JWST wouldn’t be able to compete scientifically with the next generation of giant ground-based telescopes except in small portions of the infrared.”

Put the word “descoped” at the beginning of that sentence (as the anonymous member of the science community quoted in S&T did), and we might actually agree.

Bottom line — the baseline JWST design offers huge advantages and scientific value (science/$) over any possible contemporary ground-based telescope. Descoped option might not.

Well, I would have to say quite possibly either. That is not what any of the community strategic planning committees have concluded, and these committees are tasked with evaluating science value. I could turn your statement around and ask about getting more money for large ground based telescopes. Different orifice, maybe.

JWST with MIRI would operate out to 30 microns. It is a simple fact that most (and I mean the dominant fraction) of the wavelength regime that JWST will look at with MIRI is inaccessible from the ground. Small portions of the infrared, my foot. A cold JWST beats warm 30m ground based telescopes at these wavelengths by orders of magnitude.

As Kathryn says, it is by no means clear that a greatly descoped JWST would offer similar value, but there is no question in the science community about the value of the full-up version.

At the risk of trivializing it, let me give you one of many examples. JWST will see the fundamental rotational transition of molecular hydrogen at 28 microns. A profoundly important transition for astrophysics of the galaxy. Completely invisible from the ground.

Pure fantasy. Get real.

I’ll deal with this comment first. If you read the article, you will note that it refers to “one astronomer,” not several. There is not yet a consensus opinion on whether a de-scoped JWST would be useless. In fact, there cannot be until the detailed review is conducted and the team determines how much telescope they can get for how much cost.

Let me remind you that a similar thing happened with the Spitzer telescope. It originally started out both bigger and more ambitious. However, it was unaffordable. The primary mirror was reduced to one meter and the cost was cut significantly. The general consensus is that Spitzer now does about 80% of the science they originally planned for at significantly less cost. And no astronomer would claim that Spitzer is not doing important work.

and:

“That is, for all intents and purposes, an admission of the fact that space based telescopes are rapidly becoming less and less cost effective with respect to ground based scopes that can do almost everything the space based scopes can. ”

This is not really true. There are some wavelengths that simply do not penetrate to the ground, such as UV. So no ground-based telescope, no matter how advanced, is going to see that wavelength. The same is true for X-rays. It also might surprise you to learn that many of the targets imaged by the Chandra X-Ray observatory are made for an entire day. You cannot do things like that from a rotating planet.

JWST is also intended to be launched to a distant orbit and has a large sunshield. These things are intended to reduce the influence of temperature and gravity on the detector array. There are certain requirements that can only be met in space.

Take for example, observations of very dim, distant objects. You simply cannot do that from the ground. The reason is that the atmosphere spreads around photons and for many targets there will be more photons bouncing around from atmospheric dispersion than come in from the target. So you have to get above the atmosphere with a powerful detector.

Ground-based telescopes have improved dramatically in recent years. But it is important to understand that their improvement has only been in certain areas and there are many things that they will never be able to do. In addition, their costs have also risen. They are not cheap.

Finally, I might add that it is worthwhile to read up on some of the dramatic discoveries made in cosmology and astronomy in the past decade, many of them as a result of the Hubble. For instance, we have now discovered the existence of dark energy. We don’t know what it is, but we have observed its effects and it is startling. Astronomy can fundamentally change our understanding of the laws of physics, even if we do not see any practical applications for the discoveries at the moment.

If you are interested in learning more about this, including what ground-based telescopes can and will never do, I can recommend some good astronomy textbooks.

OK Doug, tell that to those scientists who S&T quoted: “JWST wouldn’t be able to compete scientifically with the next generation of giant ground-based telescopes except in small portions of the infrared.”

So are those scientists just ignorant, or are they blowing smoke up an orfice (lying) in attempt to get more money for JWST?

That is, for all intents and purposes, pure fantasy. (I resist the temptation to use stronger language here). Ground based telescopes can perform efficiently in a sliver of the electromagnetic spectrum, while space telescopes have no such constraint. In other parts of the spectrum, the cost effectiveness of ground-based telescopes is pretty near zero, because that’s how many photons get through our atmosphere. Even within that visual sliver of the spectrum, large ground based telescopes have an advantage over JWST only at high spectral resolution.

For those few wavelengths at which they can detect celestial photons, ground based telescopes can offer higher spatial resolution than what we can presently achieve in space, but strategies for doing this over even modest fields of view are by no means proven.

Get real.

Well, maybe so and maybe not. Evidently those scientists quoted by S&T thought that without an extra billion or so the JWST would be just “little better” than up coming ground based scopes. You used the phrase “a lot of things”, but I think it may be more accurate to say space scopes can do “a few things” better than “advanced” ground scopes. Are those “few things” worth the enormous extra expense? I don’t know, but I doubt it.

Its been said here before by others and bears repeating; I am much more interested in things within our solar system since these are places we can visit soon, we’re not likly to be visiting the Crab Nebula in any living persons lifetime. That’s why I place more emphisis on Moon/Mars and manned voyages to those places as well as NEAs rather than multi billion dollar space scopes that gaze at places we’ll never visit in a hundred years if ever.

I do agree with you that if/when space access becomes more affordable space scopes and the “few things” they’re better at will become much more attractive.

The saddest thing is that it really doesn’t have to
be that way. Space based telescopes (particularly
big ones) can do a lot of things that can’t really
be done well with a ground-based telescope. The
real key seems to be lowering the cost of doing the
space-based telescope. Once again we see another
problem that more or less requires inexpensive and
frequent access to space to be solved. It’ll be
interesting to see:

a) if any group working on the problem (like
SpaceX for instance) can actually make a major
reduction in the cost of accessing space.
b) if that cheap access becomes available, how the
whole space economics situation changes. Ie
what secondary effects will that lower cost and
more frequent access have to the systems and
services being offered on-orbit……

~Jon