Limits to telescope size

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!

In article .com,
wrote:
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!

Only if you could hold the distance between them stable to within a
fraction of a wavelength of light, *and* beam the light gathered by one
to another across that distance without losing much of it. That...
presents problems, to put it mildly.

People are still struggling to make imaging interferometry work well at
distances of a hundred *meters* with both telescopes resting on solid rock.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

In article ,
Henry Spencer wrote:
In article .com,
wrote:
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!

Only if you could hold the distance between them stable to within a
fraction of a wavelength of light, *and* beam the light gathered by one
to another across that distance without losing much of it. That...
presents problems, to put it mildly.

In principle, there's another option, analogous to the way radio
astronomers do inteferometry with telescopes thousands of miles
apart; it only requires that you be able to record phase information
as well as brightness information.

But while this means recording data at a few gigahertz for 20cm radio
work, this would mean doing so at about 10^14 Hz, which also...
presents problems, to put it mildly. But this particular problem
may be more solvable in the long run.

cheers,

Steven

--


"M-Theory is the unifying pachyderm of the five string theories."
- Brian Greene, _The Elegant Universe_

(Henry Spencer) :

In article .com,
wrote:
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!

Only if you could hold the distance between them stable to within a
fraction of a wavelength of light, *and* beam the light gathered by one
to another across that distance without losing much of it. That...
presents problems, to put it mildly.

People are still struggling to make imaging interferometry work well at
distances of a hundred *meters* with both telescopes resting on solid rock.

Henry, correct me if I am wrong. But I thought with present day computers
you don't need to hold the distance steady between the scopes. Rather you
needed to know the distance to a fraction of a wavelenght between the scopes
at the time the signals are recorded.

I seem to remember that this has already been done with radio telescopes on
earth where two telescopes used a single atomic clock to timestamp the
recording.

Opps, just realized with long radiowaves even the phase shift can be measured
- if you need the phase of visible light to be recorded at both scopes at the
same time I don't think we have reach that tech yet.

Earl Colby Pottinger

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