[lamontcg]

JWST is driven by the size of the mirror, the need for an IR instrument and the temperature you need to keep the instrument at to do the observations. Spamming a bunch of hubble-sized instruments up into orbit won't accomplish the same thing. And I don't think you can do space VLBI in the optical/IR or it would have been done already (but I don't know why?).

It is kind of like asking why 5 Ford Rangers can't replace one Lamborghini or something.

[petschge]

The reason why we are barely getting started into VLBI in the IR on the ground (and nothing that I have heard of yet in space) is that the different apertures need to be stable relative to each other with a precision better than a small fraction of a wavelength. (One tenth, one twelves and one twenties are often used, depending basically on which performance drop relative to the theoretical optimum you are willing to live with.)

For radio astronomy, where we do VLBI everyday, we have to handle waves of wavelength 1 cm and position antennas to a precision better than a millimeter. Not easy when the antennas are scattered across the country, but something we can pull off.

For IR astronomy we are talking wavelength in the range of 1000 nanometers to 30 microns. So at the easiest end of the spectrum you would have to position satellites to a precision better than 3 microns relative to each other, while flying on orbit and being pulled and pushed by tidal forces, gradients in the graviational fields and solar wind pressure (which contains turbulent fluctuations). For it to actually work in near IR you would have to get the positioning right to within 100nm.

For comparison: The mirrors of JWST itself are flat to within about 25nm. And in some sense we ARE doing IR VLBI with JWST since we have separate mirror segments that we all position correctly relatively to each other. But doing so we separate freeflying satellites is something we just aren't capable of yet.

PS: Yes, LISA Pathfinder has demonstrated measurements of spacecraft separation down to a few picometer, so we are slowly getting there.

[lamontcg]

That's kinda what I guessed. So on the ground you can measure the distance between two observatories that are a few hundred meters apart with incredibly high precision and track that over time as the ground under them slowly deforms and moves.

[petschge]

Geodesy is indeed one of the side products of VLBI observations on Earth.

[CamperBob2]

Spamming a bunch of hubble-sized instruments up into orbit won't accomplish the same thing.

It will, and then some, once we get optical interferometry nailed down. JWST is great, and needed to be done, and I'm glad it's finally getting deployed. But if I had an argument to make against it, it would be, "Let's wait until we know how to do this properly. We're not there yet."

That's a weak argument and should almost never be heeded, but it's also not wrong.

[lamontcg]

> That's a weak argument and should almost never be heeded

Well that's an incredibly strong opinion with almost nothing to back it up.

Maybe it all goes boom tomorrow on launch or the deployment is ultimately completely FUBAR'd and you can feel satisfied with yourself, but I don't buy that your principle is what we should always follow.

Hopefully we start getting data from JWST in a few months and then I invite the scientific community to figure out how to spamcraft optical and IR instruments into LEO and achieve VLBI with them in the future, but we won't have to keep holding our breath for the technical breakthroughs there.

And even as a jobs program and Keynesian stimulus the JWST beats digging holes and filling them up again, even if it all goes boom. Better than building yet another weapons program as well. And the Senate just tacked on an extra $23B to the 2022 Defense Appropriations Act for one year of spending. The $10B that JWST cost over 25 years of the program doesn't really matter at all.

[CamperBob2]

True, I don't disagree with any of that, but:

Maybe it all goes boom tomorrow on launch or the deployment is ultimately completely FUBAR'd and you can feel satisfied with yourself

A more charitable interpretation of my post would read something like this: "If it does go boom, we can either start building a new one now and try again in 20 years, or we can wait 15 years and build multiple better, cheaper ones in 5 more. Meanwhile, maybe a few hundred other researchers can get funded with the same money."

[aargh_aargh]

The reason we can't achieve the same resolution as JWST with interferometry has been mentioned in a recent startalk podcast. The distance between the telescopes would have to be coordinated to a precision somewhere on the order of nanometers.

[CamperBob2]

The distance between the telescopes would have to be coordinated to a precision somewhere on the order of nanometers.

That'll happen. It's fundamentally a timing problem, and we can already build clocks that will tell you what floor of the building they're on.

Less hand-wavingly, it's fundamentally a data-acquisition and correlation problem of the sort that was solved long ago for microwave VLBI. Back in the day, the individual stations had their own maser clocks, but now I imagine they're all GNSS-based. It is by no means trivial to go from RF interferometry to optical, or to move the antenna elements from earth to space, but that's the basic approach that will ultimately be used.