Limits to telescope size

Just seen some pictures of Hubble, and The "Very Large Telescope"
(original name) in Chile. It made me think.

With a reasonable space based industry, moon mining, metal working,
aluminium and glass production, precision engineering, how big an
optical telescope could be built in zero-g?

What are the limits? Could a 100m diameter optical telescope be built?
what would it see?

As for radio telescopes, what are the limits are baseline inferometry?
what would a few telescopes, each say 3km across, in solar orbit, say 1
billion km apart be able to achieve?

On 20 Feb 2005 15:24:38 -0800, in a place far, far away,
made the phosphor on my monitor glow in such a
way as to indicate that:

Just seen some pictures of Hubble, and The "Very Large Telescope"
(original name) in Chile. It made me think.

With a reasonable space based industry, moon mining, metal working,
aluminium and glass production, precision engineering, how big an
optical telescope could be built in zero-g?

What are the limits? Could a 100m diameter optical telescope be built?

Possibly, but there'd be little point, since you can get equivalent
resolution with multiple-mirror systems.

In article ,
Rand Simberg wrote:
What are the limits? Could a 100m diameter optical telescope be built?

Possibly, but there'd be little point, since you can get equivalent
resolution with multiple-mirror systems.

Actually, there are groups working on concepts for a 100m ground-based
telescope, notably the European OWL project. The main mirror *is*
segmented, but apparently there are practical advantages in having a
single filled aperture rather than a wide scattering of smaller mirrors.

There's no reason why you couldn't build an OWL in space, although it
would be an expensive project if it used current infrastructure.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

Henry Spencer wrote:
Actually, there are groups working on concepts for a 100m ground-based
telescope, notably the European OWL project. The main mirror *is*
segmented, but apparently there are practical advantages in having a
single filled aperture rather than a wide scattering of smaller mirrors.

Light buckets on Earth have historically been spectrometry
workhorses. Two excellent cases in point being extra-solar
planet hunting and high-Z supernova searches (both of which
have produced ground breaking science within the last
decade). For that you very much want a whole heck of a lot
of light gathering area and you really don't want to mess
around with futzy issues like nulling and whatnot. With
adaptive optics and the bleeding edge of interferometry it
has become possible to compete at the high end (with space
based observatories) in more than just spectrometry, but
that's still their bread and butter.


There's no reason why you couldn't build an OWL in space, although it
would be an expensive project if it used current infrastructure.

However, there is a realm of space access cost where
constructing something like a 100m telescope in space
would actually be cheaper than doing so on Earth.
Probably somewhere around an order of magnitude cheaper
than today's launch costs, though that's just a SWAG.

Henry Spencer wrote:

In article ,
apparently there are practical advantages in having a
single filled aperture rather than a wide scattering of smaller mirrors.

I suspect that apodisation is very much easier with the filled aperture.

Henry Spencer wrote:
There's no reason why you couldn't build an OWL in space, although it
would be an expensive project if it used current infrastructure.

Speaking of big telescopes and space, would operation in space simplify
(if nothing else) the optical design of an OWL-scale segmented
telescope?

Mike Miller

In sci.space.policy wrote:

Henry Spencer wrote:
There's no reason why you couldn't build an OWL in space, although it
would be an expensive project if it used current infrastructure.

Speaking of big telescopes and space, would operation in space simplify
(if nothing else) the optical design of an OWL-scale segmented
telescope?

Yes, for one thing, you can relax the specs on the deformable mirror bits
of the system, as they now only have to cope with surface inaccuracies and
thermal movement, not the atmospheric.
However.
While what's needed of most of the mechanics would be a lot simpler, as
there is less weight load, ..., the overall system would not be.

On an earth based telescope, if mirror positioner 34 dies, you say oh ****,
as you've lost observations for the night, and have to send Harry up to fix
it.

How about this: putting 3 or 4 Hubble-sized (for redundancy & time-use
issues) craft in Jovian Trojan orbits would give you a 10 1/2
AU-equivalent instrument! That would almost see planets in Andromeda!

wrote:

Just seen some pictures of Hubble, and The "Very Large Telescope"
(original name) in Chile. It made me think.

With a reasonable space based industry, moon mining, metal working,
aluminium and glass production, precision engineering, how big an
optical telescope could be built in zero-g?

What are the limits? Could a 100m diameter optical telescope be built?
what would it see?

I don't see any reason in principle (as opposed to the actual
engineering) why you couldn't make a parabolic mirror of almost any
size.

Of course, where any planetary surface is involved, sooner or later
there will be deformity and sag issues. And a sufficently massive mirror
(with associated framework and/or tube up to the focal point) in free
fall would be difficult to steer and track. Espically if orbiting deep
in some gravity well, where there would be a tendency to line itself up
with the planet's center of mass.

But you'll likely have observed plenty of neat new stuff (including,
I would think, direct imaging of extrasolar planets) well before
reaching those limits...


As for radio telescopes, what are the limits are baseline inferometry?
what would a few telescopes, each say 3km across, in solar orbit, say 1
billion km apart be able to achieve?

Problems here are somewhat similar to those in optical, except that
minute deformations of the parabolic surface are less critical at these
wavelengths.

--

You know what to remove, to reply....

wrote:
Just seen some pictures of Hubble, and The "Very Large Telescope"
(original name) in Chile. It made me think.

With a reasonable space based industry, moon mining, metal working,
aluminium and glass production, precision engineering, how big an
optical telescope could be built in zero-g?

What are the limits? Could a 100m diameter optical telescope be
built?
what would it see?

Heck, they are seriously looking at doing this on the ground, including
exactly the studies you describe:

http://www.eso.org/projects/owl/publ..._Messenger.htm

Several times larger, at the very least, should be possible in space.

As for radio telescopes, what are the limits are baseline
inferometry?
what would a few telescopes, each say 3km across, in solar orbit, say
1
billion km apart be able to achieve?

Good question. We know from pulsar studies that coherence is
maintained at the microsecond level all the way around the earth's
orbit. We can't measure any better now, but there is no obvious reason
why a humongous interferometer would not give resolution roughly equal
to wavelength/spacing (in radians).

We have done correlation from high orbit to earth, and it works as
expected, so single dishes up to 36,000 km diameter (!) would work.
Two of these on opposite sides of the sun would be a rather powerful
facility.

Lou Scheffer